From mscott@sacadia.com Wed Nov 15 14:50:19 2000 Received: from goldengate.kojoworldwide.com. ([216.133.4.130]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id OAA11583 for ; Wed, 15 Nov 2000 14:50:13 -0500 (EST) Received: from localhost (localhost [127.0.0.1]) by goldengate.kojoworldwide.com. (8.9.1b+Sun/8.9.2) with ESMTP id LAA09998; Wed, 15 Nov 2000 11:35:33 -0800 (PST) Date: Wed, 15 Nov 2000 11:35:33 -0800 (PST) From: Myron Scott X-Sender: mscott@goldengate.kojoworldwide.com. To: "Mikheev, Vadim" , Bruce Momjian , Tom Lane Subject: Please help with some advice Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Status: ORr Dear Sirs, I have been lurking on the PostgreSQL hackers list for about 3 months now and your names comes up more than any with helpful info about the project so I was hoping you could help me. Let me cut to the chase. I have been experimenting with 7.0.2 source to see if I could create a mutlti-threaded version of the backend so I could link directly from java ( I have a fe<->be protocol that I use for my apps). Needless to say I got into much more than I bargained for. I now have a version that works and it has some nice benefits that are very helpful to a project that I am working on. What I gained was prepared statements outside of spi batched commits (fsync) one connection per thread multiple threads per process multiple processes per installation I never really intended for anyone else to see the work so I drifted pretty far from the original code. I also ended up using Solaris threads rather than pthreads, I did my own implementation of the bufmgr.c and gram.y, and used Solaris implementation of mutex in place of S_LOCK and TAS. I grabbed all global variables and put them in an environment variable that is thread local. I also did some really stupid things like making TransactionId uint64 and making all my inserts use the same oid. My question is this. I would like to get some critical feedback and suggestions about the work from others. What is the best way to go about this? I thought about trying to create a project on greatbridge.org but I am rather new to open source and the code needs commented properly and cleaned up before too many try and look at it. Any suggestions would be greatly appreciated. Thanks in advance, Myron Scott From mscott@sacadia.com Thu Nov 16 17:19:45 2000 Received: from goldengate.kojoworldwide.com. ([216.133.4.130]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id RAA04315 for ; Thu, 16 Nov 2000 17:19:43 -0500 (EST) Received: from localhost (localhost [127.0.0.1]) by goldengate.kojoworldwide.com. (8.9.1b+Sun/8.9.2) with ESMTP id OAA11449; Thu, 16 Nov 2000 14:05:15 -0800 (PST) Date: Thu, 16 Nov 2000 14:05:15 -0800 (PST) From: Myron Scott X-Sender: mscott@goldengate.kojoworldwide.com. To: Bruce Momjian cc: "Mikheev, Vadim" , Tom Lane Subject: Re: Please help with some advice In-Reply-To: <200011160533.AAA27886@candle.pha.pa.us> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Status: OR Bruce Momjian wrote: >I am curious how you isolated each thread. It seems we pretty much >assume all our memory is controlled by a single query in the process. I moved all global variables to a thread global variable which is accessed by the method GetEnv(). Which looks like this Env* GetEnv(void) { Env* env; thr_getspecific(*envkey,(void*)&env); return env; } The Env struct includes the CurrentMemoryContext, TopMemoryContext, PortalHeapMemory for each instance of a connection (one thread per connection). So, for example, EndPortalAllocMode uses GetEnv()->CurrentMemoryContext void EndPortalAllocMode() { PortalHeapMemory context; AssertState(PortalManagerEnabled); AssertState(IsA(GetEnv()->CurrentMemoryContext, PortalHeapMemory)); context = (PortalHeapMemory) GetEnv()->CurrentMemoryContext; AssertState(PointerIsValid(context->block)); /* XXX Trap(...) */ /* free current mode */ AllocSetReset(&HEAPMEMBLOCK(context)->setData); MemoryContextFree((MemoryContext) PortalHeapMemoryGetVariableMemory(context), context->block); /* restore previous mode */ context->block = FixedStackPop(&context->stackData); } From vmikheev@SECTORBASE.COM Thu Nov 16 17:23:22 2000 Received: from sectorbase2.sectorbase.com ([208.48.122.131]) by candle.pha.pa.us (8.9.0/8.9.0) with SMTP id RAA04562 for ; Thu, 16 Nov 2000 17:23:21 -0500 (EST) Received: by sectorbase2.sectorbase.com with Internet Mail Service (5.5.2650.21) id ; Thu, 16 Nov 2000 14:05:24 -0800 Message-ID: <8F4C99C66D04D4118F580090272A7A234D318D@sectorbase1.sectorbase.com> From: "Mikheev, Vadim" To: "'Myron Scott'" , Bruce Momjian Cc: Tom Lane Subject: RE: Please help with some advice Date: Thu, 16 Nov 2000 14:09:30 -0800 MIME-Version: 1.0 X-Mailer: Internet Mail Service (5.5.2650.21) Content-Type: text/plain; charset="iso-8859-1" Status: ORr I think the question do we want to make backend multy-threaded should be discussed in hackers. Vadim > -----Original Message----- > From: Myron Scott [mailto:mscott@sacadia.com] > Sent: Thursday, November 16, 2000 2:05 PM > To: Bruce Momjian > Cc: Mikheev, Vadim; Tom Lane > Subject: Re: Please help with some advice > > > Bruce Momjian wrote: > > >I am curious how you isolated each thread. It seems we pretty much > >assume all our memory is controlled by a single query in the process. > > > > I moved all global variables to a thread global variable > which is accessed > by the method GetEnv(). Which looks like this > > Env* GetEnv(void) { > Env* env; > thr_getspecific(*envkey,(void*)&env); > return env; > } > > The Env struct includes the CurrentMemoryContext, TopMemoryContext, > PortalHeapMemory for each instance of a connection (one thread per > connection). So, for example, > EndPortalAllocMode uses GetEnv()->CurrentMemoryContext > > void > EndPortalAllocMode() > { > PortalHeapMemory context; > > AssertState(PortalManagerEnabled); > AssertState(IsA(GetEnv()->CurrentMemoryContext, > PortalHeapMemory)); > > context = (PortalHeapMemory) GetEnv()->CurrentMemoryContext; > AssertState(PointerIsValid(context->block)); /* XXX > Trap(...) */ > > /* free current mode */ > AllocSetReset(&HEAPMEMBLOCK(context)->setData); > MemoryContextFree((MemoryContext) > PortalHeapMemoryGetVariableMemory(context), > context->block); > > /* restore previous mode */ > context->block = FixedStackPop(&context->stackData); > } > > > From mscott@sacadia.com Thu Nov 16 22:16:38 2000 Received: from goldengate.kojoworldwide.com. ([216.133.4.130]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id WAA14638 for ; Thu, 16 Nov 2000 22:16:36 -0500 (EST) Received: from localhost (localhost [127.0.0.1]) by goldengate.kojoworldwide.com. (8.9.1b+Sun/8.9.2) with ESMTP id TAA11874; Thu, 16 Nov 2000 19:04:48 -0800 (PST) Date: Thu, 16 Nov 2000 19:04:48 -0800 (PST) From: Myron Scott X-Sender: mscott@goldengate.kojoworldwide.com. To: Bruce Momjian cc: "Mikheev, Vadim" , Tom Lane Subject: Re: Please help with some advice In-Reply-To: <200011170156.UAA11438@candle.pha.pa.us> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Status: ORr Thanks very much, I will post to hackers. Myron From pgsql-hackers-owner+M2691@postgresql.org Tue Jan 2 00:30:20 2001 Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id AAA08195 for ; Tue, 2 Jan 2001 00:30:19 -0500 (EST) Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by mail.postgresql.org (8.11.1/8.11.1) with SMTP id f025UjL33335; Tue, 2 Jan 2001 00:30:45 -0500 (EST) (envelope-from pgsql-hackers-owner+M2691@postgresql.org) Received: from mailsys01.intnet.net (tmail.wwc.com [198.252.32.143] (may be forged)) by mail.postgresql.org (8.11.1/8.11.1) with ESMTP id f025UTL33232 for ; Tue, 2 Jan 2001 00:30:32 -0500 (EST) (envelope-from mscott@sacadia.com) Received: from [206.112.108.0] (HELO sacadia.com) by mailsys01.intnet.net (CommuniGate Pro SMTP 3.3.2) with ESMTP id 2214231; Tue, 02 Jan 2001 00:29:47 -0500 Message-ID: <3A5167DB.3050807@sacadia.com> Date: Mon, 01 Jan 2001 21:32:11 -0800 From: Myron Scott Reply-To: mscott@sacadia.com User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/20001108 Netscape6/6.0 X-Accept-Language: en MIME-Version: 1.0 To: "Ross J. Reedstrom" CC: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] Using Threads? References: <004401c058fd$fd498d40$f2356880@tracy> <20001204113307.B5871@rice.edu> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR For anyone interested, I have posted my multi-threaded version of PostgreSQL here. http://www.sacadia.com/mtpg.html It is based on 7.0.2 and the TAO CORBA ORB which is here. http://www.cs.wustl.edu/~schmidt/TAO.html Myron Scott mkscott@sacadia.com From bright@fw.wintelcom.net Tue Jan 2 03:02:28 2001 Received: from fw.wintelcom.net (bright@ns1.wintelcom.net [209.1.153.20]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id DAA16169 for ; Tue, 2 Jan 2001 03:02:27 -0500 (EST) Received: (from bright@localhost) by fw.wintelcom.net (8.10.0/8.10.0) id f0282Vm10623; Tue, 2 Jan 2001 00:02:31 -0800 (PST) Date: Tue, 2 Jan 2001 00:02:31 -0800 From: Alfred Perlstein To: Bruce Momjian Cc: Tom Lane , pgsql-hackers@postgresql.org Subject: Re: [HACKERS] Assuming that TAS() will succeed the first time is verboten Message-ID: <20010102000230.C19572@fw.wintelcom.net> References: <9850.978067943@sss.pgh.pa.us> <200101020759.CAA15836@candle.pha.pa.us> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2.5i In-Reply-To: <200101020759.CAA15836@candle.pha.pa.us>; from pgman@candle.pha.pa.us on Tue, Jan 02, 2001 at 02:59:20AM -0500 Status: OR * Bruce Momjian [010101 23:59] wrote: > > Alfred Perlstein writes: > > > One trick that may help is calling sched_yield(2) on a lock miss, > > > it's a POSIX call and quite new so you'd need a 'configure' test > > > for it. > > > > The author of the current s_lock code seems to have thought that > > select() with a zero delay would do the equivalent of sched_yield(). > > I'm not sure if that's true on very many kernels, if indeed any... > > > > I doubt we could buy much by depending on sched_yield(); if you want > > to assume POSIX facilities, ISTM you might as well go for user-space > > semaphores and forget the whole TAS mechanism. > > > Another issue is that sched_yield brings in the pthreads library/hooks > on some OS's, which we certainly want to avoid. I know it's a major undertaking, but since the work is sort of done, have you guys considered the port to solaris threads and seeing about making a pthreads port of that? I know it would probably get you considerable gains under Windows at the expense of dropping some really really legacy system. Or you could do what apache (is rumored) does and have it do either threads or processes or both... -- -Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org] "I have the heart of a child; I keep it in a jar on my desk." From pgsql-hackers-owner+M4275@postgresql.org Mon Feb 5 21:45:00 2001 Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id VAA09262 for ; Mon, 5 Feb 2001 21:44:59 -0500 (EST) Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by mail.postgresql.org (8.11.1/8.11.1) with SMTP id f162ixx00920; Mon, 5 Feb 2001 21:44:59 -0500 (EST) (envelope-from pgsql-hackers-owner+M4275@postgresql.org) Received: from goldengate.kojoworldwide.com. ([216.133.4.130]) by mail.postgresql.org (8.11.1/8.11.1) with ESMTP id f162fSx00595 for ; Mon, 5 Feb 2001 21:41:29 -0500 (EST) (envelope-from mscott@sacadia.com) Received: from localhost (localhost [127.0.0.1]) by goldengate.kojoworldwide.com. (8.9.1b+Sun/8.9.2) with ESMTP id SAA03298 for ; Mon, 5 Feb 2001 18:25:05 -0800 (PST) Date: Mon, 5 Feb 2001 18:25:05 -0800 (PST) From: Myron Scott X-Sender: mscott@goldengate.kojoworldwide.com. To: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] Using Threads? Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR I have put a new version of my multi-threaded postgresql experiment at http://www.sacadia.com/mtpg.html This one actually works. I have added a server based on omniORB, a CORBA 2.3 ORB from ATT. It is much smaller than TAO and uses the thread per connection model. I haven't added the java side of the JNI interface yet but the C++ side is there. It's still not stable but it is much better than the last. Myron Scott mkscott@sacadia.com From pgsql-hackers-owner+M4304@postgresql.org Tue Feb 6 10:24:21 2001 Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id KAA22027 for ; Tue, 6 Feb 2001 10:24:20 -0500 (EST) Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by mail.postgresql.org (8.11.1/8.11.1) with SMTP id f16FOBx97182; Tue, 6 Feb 2001 10:24:11 -0500 (EST) (envelope-from pgsql-hackers-owner+M4304@postgresql.org) Received: from goldengate.kojoworldwide.com. ([216.133.4.130]) by mail.postgresql.org (8.11.1/8.11.1) with ESMTP id f16FLWx96814 for ; Tue, 6 Feb 2001 10:21:33 -0500 (EST) (envelope-from mscott@sacadia.com) Received: from localhost (localhost [127.0.0.1]) by goldengate.kojoworldwide.com. (8.9.1b+Sun/8.9.2) with ESMTP id HAA04170; Tue, 6 Feb 2001 07:05:04 -0800 (PST) Date: Tue, 6 Feb 2001 07:05:04 -0800 (PST) From: Myron Scott X-Sender: mscott@goldengate.kojoworldwide.com. To: Karel Zak cc: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] Using Threads In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR > > Sorry I haven't time to see and test your experiment, > but I have a question. How you solve memory management? > The current mmgr is based on global variable > CurrentMemoryContext that is very often changed and used. > Use you for this locks? If yes it is probably problematic > point for perfomance. > > Karel > There are many many globals I had to work around including all the memory management stuff. I basically threw everything into and "environment" variable which I stored in a thread specific using thr_setspecific. Performance is acually very good for what I am doing. I was able to batch commit transactions which cuts down on fsync calls, use prepared statements from my client using CORBA, and the various locking calls for the threads (cond_wait,mutex_lock, and sema_wait) seem pretty fast. I did some performance tests for inserts 20 clients, 900 inserts per client, 1 insert per transaction, 4 different tables. 7.0.2 About 10:52 average completion multi-threaded 2:42 average completion 7.1beta3 1:13 average completion If I increased the number of inserts per transaction, multi-threaded got closer to 7.1 for inserts. I haven't tested other other types of commands yet. Myron Scott mkscott@sacadia.com From pgsql-hackers-owner+M4313@postgresql.org Tue Feb 6 12:32:00 2001 Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id MAA29163 for ; Tue, 6 Feb 2001 12:31:59 -0500 (EST) Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by mail.postgresql.org (8.11.1/8.11.1) with SMTP id f16HVox17454; Tue, 6 Feb 2001 12:31:51 -0500 (EST) (envelope-from pgsql-hackers-owner+M4313@postgresql.org) Received: from ara.zf.jcu.cz (ara.zf.jcu.cz [160.217.161.4]) by mail.postgresql.org (8.11.1/8.11.1) with ESMTP id f16HV6x17323 for ; Tue, 6 Feb 2001 12:31:06 -0500 (EST) (envelope-from zakkr@zf.jcu.cz) Received: from localhost (zakkr@localhost) by ara.zf.jcu.cz (8.9.3/8.9.3/Debian 8.9.3-21) with SMTP id SAA03980; Tue, 6 Feb 2001 18:31:02 +0100 Date: Tue, 6 Feb 2001 18:31:02 +0100 (CET) From: Karel Zak To: Myron Scott cc: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] Using Threads In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR On Tue, 6 Feb 2001, Myron Scott wrote: > There are many many globals I had to work around including all the memory > management stuff. I basically threw everything into and "environment" > variable which I stored in a thread specific using thr_setspecific. Yes, it's good. I working on multi-thread application server (http://mape.jcu.cz) and I use for this project some things from PG (like mmgr), I planning use same solution. > Performance is acually very good for what I am doing. I was able to batch > commit transactions which cuts down on fsync calls, use prepared > statements from my client using CORBA, and the various locking calls for > the threads (cond_wait,mutex_lock, and sema_wait) seem pretty fast. I did > some performance tests for inserts > > 20 clients, 900 inserts per client, 1 insert per transaction, 4 different > tables. > > 7.0.2 About 10:52 average completion > multi-threaded 2:42 average completion > 7.1beta3 1:13 average completion It is very very good for time for 7.1, already look forward to 7.2! :-) BTW, I not sure if you anytime in future will see threads in official PostgreSQL and if you spending time on relevant things (IMHO). Karel From pgsql-hackers-owner+M4304@postgresql.org Tue Feb 6 10:24:21 2001 Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id KAA22027 for ; Tue, 6 Feb 2001 10:24:20 -0500 (EST) Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28]) by mail.postgresql.org (8.11.1/8.11.1) with SMTP id f16FOBx97182; Tue, 6 Feb 2001 10:24:11 -0500 (EST) (envelope-from pgsql-hackers-owner+M4304@postgresql.org) Received: from goldengate.kojoworldwide.com. ([216.133.4.130]) by mail.postgresql.org (8.11.1/8.11.1) with ESMTP id f16FLWx96814 for ; Tue, 6 Feb 2001 10:21:33 -0500 (EST) (envelope-from mscott@sacadia.com) Received: from localhost (localhost [127.0.0.1]) by goldengate.kojoworldwide.com. (8.9.1b+Sun/8.9.2) with ESMTP id HAA04170; Tue, 6 Feb 2001 07:05:04 -0800 (PST) Date: Tue, 6 Feb 2001 07:05:04 -0800 (PST) From: Myron Scott X-Sender: mscott@goldengate.kojoworldwide.com. To: Karel Zak cc: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] Using Threads In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR > > Sorry I haven't time to see and test your experiment, > but I have a question. How you solve memory management? > The current mmgr is based on global variable > CurrentMemoryContext that is very often changed and used. > Use you for this locks? If yes it is probably problematic > point for perfomance. > > Karel > There are many many globals I had to work around including all the memory management stuff. I basically threw everything into and "environment" variable which I stored in a thread specific using thr_setspecific. Performance is acually very good for what I am doing. I was able to batch commit transactions which cuts down on fsync calls, use prepared statements from my client using CORBA, and the various locking calls for the threads (cond_wait,mutex_lock, and sema_wait) seem pretty fast. I did some performance tests for inserts 20 clients, 900 inserts per client, 1 insert per transaction, 4 different tables. 7.0.2 About 10:52 average completion multi-threaded 2:42 average completion 7.1beta3 1:13 average completion If I increased the number of inserts per transaction, multi-threaded got closer to 7.1 for inserts. I haven't tested other other types of commands yet. Myron Scott mkscott@sacadia.com From lamar.owen@wgcr.org Thu Jun 28 11:14:10 2001 Return-path: Received: from www.wgcr.org (IDENT:root@www.wgcr.org [206.74.232.194]) by candle.pha.pa.us (8.10.1/8.10.1) with ESMTP id f5SFE9U18758 for ; Thu, 28 Jun 2001 11:14:09 -0400 (EDT) Received: from lowen.wgcr.org (IDENT:lowen@[10.1.2.3]) by www.wgcr.org (8.9.3/8.9.3/WGCR) with SMTP id LAA11879; Thu, 28 Jun 2001 11:14:14 -0400 Content-Type: text/plain; charset="iso-8859-1" From: Lamar Owen To: Bruce Momjian Subject: Process weight (was:Re: [GENERAL] Re: Red Hat to support PostgreSQL) Date: Thu, 28 Jun 2001 11:14:09 -0400 X-Mailer: KMail [version 1.2] References: <200106272258.f5RMwIb26959@candle.pha.pa.us> In-Reply-To: <200106272258.f5RMwIb26959@candle.pha.pa.us> MIME-Version: 1.0 Message-ID: <01062811140902.01118@lowen.wgcr.org> Content-Transfer-Encoding: 8bit Status: ORr On Wednesday 27 June 2001 18:58, Bruce Momjian wrote: > > I had almost given up on using Postgres for this system because under > > Solaris, it just couldn't cut it (MySQL could do the work with one CPU > > while Postgres took up even more CPU and required *both* CPUs to be > > enabled), but when we moved the system to a Linux box, things worked > > much better. > Ah, back to a PostgreSQL topic. :-) > My guess on this one is that Solaris is slower for PostgreSQL because > process switching is _much_ heavier on Solaris than other OS's. This is > because of the way they implemented processes in SVr4. They got quite > heavy, almost requiring kernel threads so you weren't switching > processes all the time. Now, the question of the week: Is supporting a thread model for an inefficient OS a desirable thing to do, when more efficient OS kernels are available such as FreeBSD 4.x and Linux 2.4? My opinion is that our existing model, when used with a connection-pooling frontend, is rather efficient. (Yes, I use a connection-pooling frontend. Performance is rather nice, and I don't have to have a full backend spawned for every page hit.) In fact, on a Linux box threads show as processes. While I know that the kernel actually supports themin a slightly different manner than processes, they have more similarities than differences. However, even on OS's where threads are supported, the mechanism to support those threads must be an efficient one -- not all pthreads libraries are created equal. Many are frontends (expensive ones, at that) for plain old processes. Does anyone know of a resource that details the 'weight' of processes for our supported platforms? [reply off-list -- I'll be glad to summarize responses to HACKERS, ADMIN, or PORTS, as appropriate, if desired.] -- Lamar Owen WGCR Internet Radio 1 Peter 4:11 From pgsql-hackers-owner+M13599=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 17:25:32 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8QLPWo07589 for ; Wed, 26 Sep 2001 17:25:32 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8QLPf405606 for ; Wed, 26 Sep 2001 16:25:41 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13599=candle.pha.pa.us=pgman@postgresql.org) Received: from gromit.dotclick.com (ipn9-f8366.net-resource.net [216.204.83.66]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8QKj3h82020 for ; Wed, 26 Sep 2001 16:45:03 -0400 (EDT) (envelope-from markw@mohawksoft.com) Received: from mohawksoft.com (IDENT:markw@localhost.localdomain [127.0.0.1]) by gromit.dotclick.com (8.9.3/8.9.3) with ESMTP id QAA23693; Wed, 26 Sep 2001 16:43:02 -0400 Message-ID: <3BB23DD6.E86AF327@mohawksoft.com> Date: Wed, 26 Sep 2001 16:43:02 -0400 From: mlw X-Mailer: Mozilla 4.75 [en] (X11; U; Linux 2.4.2 i686) X-Accept-Language: en MIME-Version: 1.0 To: "D. Hageman" , "pgsql-hackers@postgresql.org" Subject: Re: [HACKERS] Spinlock performance improvement proposal References: Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR "D. Hageman" wrote: > The plan for the new spinlocks does look like it has some potential. My > only comment in regards to permformance when we start looking at SMP > machines is ... it is my belief that getting a true threaded backend may > be the only way to get the full potential out of SMP machines. I see that > is one of the things to experiment with on the TODO list and I have seen > some people have messed around already with this using Solaris threads. > It should probably be attempted with pthreads if PostgreSQL is going to > keep some resemblance of cross-platform compatibility. At that time, it > would probably be easier to go in and clean up some stuff for the > implementation of other TODO items (put in the base framework for more > complex future items) as threading the backend would take a little bit of > ideology shift. I can only think of two objectives for threading. (1) running the various connections in their own thread instead of their own process. (2) running complex queries across multiple threads. For item (1) I see no value to this. It is a lot of work with no tangible benefit. If you have an old fashion pthreads implementation, it will hurt performance because are scheduled within the single process's time slice.. If you have a newer kernel scheduled implementation, then you will have the same scheduling as separate processes. The only thing you will need to do is switch your brain from figuring out how to share data, to trying to figure out how to isolate data. A multithreaded implementation lacks many of the benefits and robustness of a multiprocess implementation. For item (2) I can see how that could speed up queries in a low utilization system, and that would be cool, but in a server that is under load, threading the queries probably be less efficient. ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org From pgsql-hackers-owner+M13604=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 18:40:26 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8QMePo13437 for ; Wed, 26 Sep 2001 18:40:25 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8QMeZ417944 for ; Wed, 26 Sep 2001 17:40:35 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13604=candle.pha.pa.us=pgman@postgresql.org) Received: from foghorn.airs.com (foghorn.airs.com [63.201.54.26]) by postgresql.org (8.11.3/8.11.4) with SMTP id f8QM59h01247 for ; Wed, 26 Sep 2001 18:05:09 -0400 (EDT) (envelope-from ian@airs.com) Received: (qmail 10089 invoked by uid 10); 26 Sep 2001 22:04:49 -0000 Received: (qmail 6837 invoked by uid 269); 26 Sep 2001 22:04:41 -0000 Mail-Followup-To: markw@mohawksoft.com, pgsql-hackers@postgresql.org, dhageman@dracken.com To: "D. Hageman" cc: mlw , "pgsql-hackers@postgresql.org" Subject: Re: [HACKERS] Spinlock performance improvement proposal References: From: Ian Lance Taylor Date: 26 Sep 2001 15:04:41 -0700 In-Reply-To: Message-ID: Lines: 45 User-Agent: Gnus/5.0808 (Gnus v5.8.8) Emacs/20.7 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR "D. Hageman" writes: > > you have a newer kernel scheduled implementation, then you will have the same > > scheduling as separate processes. The only thing you will need to do is > > switch your brain from figuring out how to share data, to trying to figure > > out how to isolate data. A multithreaded implementation lacks many of the > > benefits and robustness of a multiprocess implementation. > > Save for the fact that the kernel can switch between threads faster then > it can switch processes considering threads share the same address space, > stack, code, etc. If need be sharing the data between threads is much > easier then sharing between processes. When using a kernel threading model, it's not obvious to me that the kernel will switch between threads much faster than it will switch between processes. As far as I can see, the only potential savings is not reloading the pointers to the page tables. That is not nothing, but it is also not a lot. > I can't comment on the "isolate data" line. I am still trying to figure > that one out. Sometimes you need data which is specific to a particular thread. Basically, you have to look at every global variable in the Postgres backend, and determine whether to share it among all threads or to make it thread-specific. In other words, you have to take extra steps to isolate the data within the thread. This is the reverse of the current situation, in which you have to take extra steps to share data among all backend processes. > That last line is a troll if I every saw it ;-) I will agree that threads > isn't for everything and that it has costs just like everything else. Let > me stress that last part - like everything else. Certain costs exist in > the present model, nothing is - how should we say ... perfect. When writing in C, threading inevitably loses robustness. Erratic behaviour by one thread, perhaps in a user defined function, can subtly corrupt the entire system, rather than just that thread. Part of defensive programming is building barriers between different parts of a system. Process boundaries are a powerful barrier. (Actually, though, Postgres is already vulnerable to erratic behaviour because any backend process can corrupt the shared buffer pool.) Ian ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org From pgsql-hackers-owner+M13605=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 18:54:58 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8QMsvo14061 for ; Wed, 26 Sep 2001 18:54:57 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8QMt7420740 for ; Wed, 26 Sep 2001 17:55:07 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13605=candle.pha.pa.us=pgman@postgresql.org) Received: from goldengate.kojoworldwide.com. ([216.133.4.130]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8QMOPh04333 for ; Wed, 26 Sep 2001 18:24:26 -0400 (EDT) (envelope-from mscott@sacadia.com) Received: from localhost (localhost [127.0.0.1]) by goldengate.kojoworldwide.com. (8.9.1b+Sun/8.9.2) with ESMTP id PAA00633 for ; Wed, 26 Sep 2001 15:03:00 -0700 (PDT) Date: Wed, 26 Sep 2001 15:03:00 -0700 (PDT) From: Myron Scott X-Sender: mscott@goldengate.kojoworldwide.com. To: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] Spinlock performance improvement proposal In-Reply-To: <3BB23DD6.E86AF327@mohawksoft.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR On Wed, 26 Sep 2001, mlw wrote: > I can only think of two objectives for threading. (1) running the various > connections in their own thread instead of their own process. (2) running > complex queries across multiple threads. > I did a multi-threaded version of 7.0.2 using Solaris threads about a year ago in order to try and get multiple backend connections working under one java process using jni. I used the thread per connection model. I eventually got it working, but it was/is very messy ( there were global variables everywhere! ). Anyway, I was able to get a pretty good speed up on inserts by scheduling buffer writes from multiple connections on one common writing thread. I also got some other features that were important to me at the time. 1. True prepared statements under java with bound input and output variables 2. Better system utilization a. fewer Solaris lightweight processes mapped to threads. b. Fewer open files per postgres installation 3. Automatic vacuums when system activity is low by a daemon thread. but there were some drawbacks... One rogue thread or bad user function could take down all connections for that process. This was and seems to still be the major drawback to using threads. Myron Scott mscott@sacadia.com ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/users-lounge/docs/faq.html From pgsql-hackers-owner+M13602=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 17:45:26 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8QLjQo08483 for ; Wed, 26 Sep 2001 17:45:26 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8QLjY409914 for ; Wed, 26 Sep 2001 16:45:35 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13602=candle.pha.pa.us=pgman@postgresql.org) Received: from typhon.dracken.com (dv07m61.lawrence.ks.us [24.124.61.35]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8QLGDh91021 for ; Wed, 26 Sep 2001 17:16:13 -0400 (EDT) (envelope-from dhageman@dracken.com) Received: from localhost (dhageman@localhost) by typhon.dracken.com (8.11.4/8.11.4) with ESMTP id f8QLEMY01973; Wed, 26 Sep 2001 16:14:22 -0500 X-Authentication-Warning: typhon.dracken.com: dhageman owned process doing -bs Date: Wed, 26 Sep 2001 16:14:22 -0500 (CDT) From: "D. Hageman" To: mlw cc: "pgsql-hackers@postgresql.org" Subject: Re: [HACKERS] Spinlock performance improvement proposal In-Reply-To: <3BB23DD6.E86AF327@mohawksoft.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: ORr On Wed, 26 Sep 2001, mlw wrote: > > I can only think of two objectives for threading. (1) running the various > connections in their own thread instead of their own process. (2) running > complex queries across multiple threads. > > For item (1) I see no value to this. It is a lot of work with no tangible > benefit. If you have an old fashion pthreads implementation, it will hurt > performance because are scheduled within the single process's time slice.. Old fashion ... as in a userland library that implements POSIX threads? Well, I would agree. However, most *modern* implementations are done in the kernel or kernel and userland coop model and don't have this limitation (as you mention later in your e-mail). You have kinda hit on one of my gripes about computers in general. At what point in time does one say something is obsolete or too old to support anymore - that it hinders progress instead of adding a "feature"? > you have a newer kernel scheduled implementation, then you will have the same > scheduling as separate processes. The only thing you will need to do is > switch your brain from figuring out how to share data, to trying to figure > out how to isolate data. A multithreaded implementation lacks many of the > benefits and robustness of a multiprocess implementation. Save for the fact that the kernel can switch between threads faster then it can switch processes considering threads share the same address space, stack, code, etc. If need be sharing the data between threads is much easier then sharing between processes. I can't comment on the "isolate data" line. I am still trying to figure that one out. That last line is a troll if I every saw it ;-) I will agree that threads isn't for everything and that it has costs just like everything else. Let me stress that last part - like everything else. Certain costs exist in the present model, nothing is - how should we say ... perfect. > For item (2) I can see how that could speed up queries in a low utilization > system, and that would be cool, but in a server that is under load, threading > the queries probably be less efficient. Well, I don't follow your logic and you didn't give any substance to back up your claim. I am willing to listen. Another thought ... Oracle uses threads doesn't it or at least it has a single processor and multi-processor version last time I knew ... which do they claim is better? (Not saying that Oracle's proclimation of what is good and what is not matters, but it is good for another view point). -- //========================================================\\ || D. Hageman || \\========================================================// ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org From pgsql-hackers-owner+M13607=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 19:14:59 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8QNExo15536 for ; Wed, 26 Sep 2001 19:14:59 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8QNF8423944 for ; Wed, 26 Sep 2001 18:15:09 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13607=candle.pha.pa.us=pgman@postgresql.org) Received: from belphigor.mcnaught.org ([216.151.155.121]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8QMe3h07256 for ; Wed, 26 Sep 2001 18:40:04 -0400 (EDT) (envelope-from doug@wireboard.com) Received: (from doug@localhost) by belphigor.mcnaught.org (8.11.6/8.9.3) id f8QMdkB05502; Wed, 26 Sep 2001 18:39:46 -0400 X-Authentication-Warning: belphigor.mcnaught.org: doug set sender to doug@wireboard.com using -f To: "D. Hageman" cc: mlw , "pgsql-hackers@postgresql.org" Subject: Re: [HACKERS] Spinlock performance improvement proposal References: From: Doug McNaught Date: 26 Sep 2001 18:39:44 -0400 In-Reply-To: "D. Hageman"'s message of "Wed, 26 Sep 2001 16:14:22 -0500 (CDT)" Message-ID: Lines: 26 User-Agent: Gnus/5.0806 (Gnus v5.8.6) XEmacs/21.1 (20 Minutes to Nikko) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR "D. Hageman" writes: > Save for the fact that the kernel can switch between threads faster then > it can switch processes considering threads share the same address space, > stack, code, etc. If need be sharing the data between threads is much > easier then sharing between processes. This depends on your system. Solaris has a huge difference between thread and process context switch times, whereas Linux has very little difference (and in fact a Linux process context switch is about as fast as a Solaris thread switch on the same hardware--Solaris is just a pig when it comes to process context switching). > I can't comment on the "isolate data" line. I am still trying to figure > that one out. I think his point is one of clarity and maintainability. When a task's data is explicitly shared (via shared memory of some sort) it's fairly clear when you're accessing shared data and need to worry about locking. Whereas when all data is shared by default (as with threads) it's very easy to miss places where threads can step on each other. -Doug -- In a world of steel-eyed death, and men who are fighting to be warm, Come in, she said, I'll give you shelter from the storm. -Dylan ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to majordomo@postgresql.org) From pgsql-hackers-owner+M13611=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 21:05:02 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8R152o22010 for ; Wed, 26 Sep 2001 21:05:02 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8R158430261 for ; Wed, 26 Sep 2001 20:05:08 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13611=candle.pha.pa.us=pgman@postgresql.org) Received: from sss.pgh.pa.us ([192.204.191.242]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8R0lgh29430 for ; Wed, 26 Sep 2001 20:47:42 -0400 (EDT) (envelope-from tgl@sss.pgh.pa.us) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss.pgh.pa.us (8.11.4/8.11.4) with ESMTP id f8R0kpK14707; Wed, 26 Sep 2001 20:46:51 -0400 (EDT) To: Ian Lance Taylor cc: "D. Hageman" , mlw , "pgsql-hackers@postgresql.org" Subject: Re: [HACKERS] Spinlock performance improvement proposal In-Reply-To: References: Comments: In-reply-to Ian Lance Taylor message dated "26 Sep 2001 15:04:41 -0700" Date: Wed, 26 Sep 2001 20:46:51 -0400 Message-ID: <14704.1001551611@sss.pgh.pa.us> From: Tom Lane Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR Ian Lance Taylor writes: > (Actually, though, Postgres is already vulnerable to erratic behaviour > because any backend process can corrupt the shared buffer pool.) Not to mention the other parts of shared memory. Nonetheless, our experience has been that cross-backend failures due to memory clobbers in shared memory are very infrequent --- certainly far less often than we see localized-to-a-backend crashes. Probably this is because the shared memory is (a) small compared to the rest of the address space and (b) only accessed by certain specific modules within Postgres. I'm convinced that switching to a thread model would result in a significant degradation in our ability to recover from coredump-type failures, even given the (implausible) assumption that we introduce no new bugs during the conversion. I'm also *un*convinced that such a conversion will yield significant performance benefits, unless we introduce additional cross-thread dependencies (and more fragility and lock contention) by tactics such as sharing catalog caches across threads. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@postgresql.org so that your message can get through to the mailing list cleanly From pgsql-hackers-owner+M13616=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 23:10:52 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8R3Aqo03180 for ; Wed, 26 Sep 2001 23:10:52 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8R3B3438816 for ; Wed, 26 Sep 2001 22:11:03 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13616=candle.pha.pa.us=pgman@postgresql.org) Received: from spider.pilosoft.com (p55-222.acedsl.com [160.79.55.222]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8R2vCh48923 for ; Wed, 26 Sep 2001 22:57:12 -0400 (EDT) (envelope-from alex@pilosoft.com) Received: from localhost (alexmail@localhost) by spider.pilosoft.com (8.9.3/8.9.3) with ESMTP id WAA27630; Wed, 26 Sep 2001 22:58:41 -0400 (EDT) Date: Wed, 26 Sep 2001 22:58:41 -0400 (EDT) From: Alex Pilosov To: "D. Hageman" cc: "pgsql-hackers@postgresql.org" Subject: Re: [HACKERS] Spinlock performance improvement proposal In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR On Wed, 26 Sep 2001, D. Hageman wrote: > > > Save for the fact that the kernel can switch between threads faster then > > > it can switch processes considering threads share the same address space, > > > stack, code, etc. If need be sharing the data between threads is much > > > easier then sharing between processes. > > > > When using a kernel threading model, it's not obvious to me that the > > kernel will switch between threads much faster than it will switch > > between processes. As far as I can see, the only potential savings is > > not reloading the pointers to the page tables. That is not nothing, > > but it is also > > > I can't comment on the "isolate data" line. I am still trying to figure > > > that one out. > > > > Sometimes you need data which is specific to a particular thread. > > When you need data that is specific to a thread you use a TSD (Thread > Specific Data). Which Linux does not support with a vengeance, to my knowledge. As a matter of fact, quote from Linus on the matter was something like "Solution to slow process switching is fast process switching, not another kernel abstraction [referring to threads and TSD]". TSDs make implementation of thread switching complex, and fork() complex. The question about threads boils down to: Is there far more data that is shared than unshared? If yes, threads are better, if not, you'll be abusing TSD and slowing things down. I believe right now, postgresql' model of sharing only things that need to be shared is pretty damn good. The only slight problem is overhead of forking another backend, but its still _fast_. IMHO, threads would not bring large improvement to postgresql. Actually, if I remember, there was someone who ported postgresql (I think it was 6.5) to be multithreaded with major pain, because the requirement was to integrate with CORBA. I believe that person posted some benchmarks which were essentially identical to non-threaded postgres... -alex ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org From pgsql-hackers-owner+M13619=candle.pha.pa.us=pgman@postgresql.org Thu Sep 27 00:32:55 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8R4Wto07075 for ; Thu, 27 Sep 2001 00:32:55 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8R4X7444942 for ; Wed, 26 Sep 2001 23:33:07 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13619=candle.pha.pa.us=pgman@postgresql.org) Received: from sss.pgh.pa.us ([192.204.191.242]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8R4Jsh61257 for ; Thu, 27 Sep 2001 00:19:54 -0400 (EDT) (envelope-from tgl@sss.pgh.pa.us) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss.pgh.pa.us (8.11.4/8.11.4) with ESMTP id f8R4JLK15406; Thu, 27 Sep 2001 00:19:21 -0400 (EDT) To: "D. Hageman" cc: Alex Pilosov , "pgsql-hackers@postgresql.org" Subject: Re: [HACKERS] Spinlock performance improvement proposal In-Reply-To: References: Comments: In-reply-to "D. Hageman" message dated "Wed, 26 Sep 2001 22:41:39 -0500" Date: Thu, 27 Sep 2001 00:19:20 -0400 Message-ID: <15403.1001564360@sss.pgh.pa.us> From: Tom Lane Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR "D. Hageman" writes: > If you look at Myron Scott's post today you will see that it had other > advantages going for it (like auto-vacuum!) and disadvantages ... rogue > thread corruption (already debated today). But note that Myron did a number of things that are (IMHO) orthogonal to process-to-thread conversion, such as adding prepared statements, a separate thread/process/whateveryoucallit for buffer writing, ditto for vacuuming, etc. I think his results cannot be taken as indicative of the benefits of threads per se --- these other things could be implemented in a pure process model too, and we have no data with which to estimate which change bought how much. Threading certainly should reduce the context switch time, but this comes at the price of increased overhead within each context (since access to thread-local variables is not free). It's by no means obvious that there's a net win there. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org From pgsql-hackers-owner+M13621=candle.pha.pa.us=pgman@postgresql.org Thu Sep 27 01:59:44 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8R5xio11898 for ; Thu, 27 Sep 2001 01:59:44 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8R5xi449748 for ; Thu, 27 Sep 2001 00:59:45 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13621=candle.pha.pa.us=pgman@postgresql.org) Received: from goldengate.kojoworldwide.com. ([216.133.4.130]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8R5joh75612 for ; Thu, 27 Sep 2001 01:45:50 -0400 (EDT) (envelope-from mscott@sacadia.com) Received: from localhost (localhost [127.0.0.1]) by goldengate.kojoworldwide.com. (8.9.1b+Sun/8.9.2) with ESMTP id WAA01144 for ; Wed, 26 Sep 2001 22:24:29 -0700 (PDT) Date: Wed, 26 Sep 2001 22:24:29 -0700 (PDT) From: Myron Scott X-Sender: mscott@goldengate.kojoworldwide.com. To: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] Spinlock performance improvement proposal In-Reply-To: <15403.1001564360@sss.pgh.pa.us> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR > But note that Myron did a number of things that are (IMHO) orthogonal yes, I did :) > to process-to-thread conversion, such as adding prepared statements, > a separate thread/process/whateveryoucallit for buffer writing, ditto > for vacuuming, etc. I think his results cannot be taken as indicative > of the benefits of threads per se --- these other things could be > implemented in a pure process model too, and we have no data with which > to estimate which change bought how much. > If you are comparing just process vs. thread, I really don't think I gained much for performance and ended up with some pretty unmanageable code. The one thing that led to most of the gains was scheduling all the writes to one thread which, as noted by Tom, you could do on the process model. Besides, Most of the advantage in doing this was taken away with the addition of WAL in 7.1. The other real gain that I saw with threading was limiting the number of open files but that led me to alter much of the file manager in order to synchronize access to the files which probably slowed things a bit. To be honest, I don't think I, personally, would try this again. I went pretty far off the beaten path with this thing. It works well for what I am doing ( a limited number of SQL statements run many times over ) but there probably was a better way. I'm thinking now that I should have tried to add a CORBA interface for connections. I would have been able to accomplish my original goals without creating a deadend for myself. Thanks all for a great project, Myron mscott@sacadia.com ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to majordomo@postgresql.org) From pgsql-hackers-owner+M13632=candle.pha.pa.us=pgman@postgresql.org Thu Sep 27 10:21:22 2001 Return-path: Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8RELLo08607 for ; Thu, 27 Sep 2001 10:21:21 -0400 (EDT) Received: from postgresql.org (webmail.postgresql.org [216.126.85.28]) by server1.pgsql.org (8.11.6/8.11.6) with ESMTP id f8RELP487000 for ; Thu, 27 Sep 2001 09:21:26 -0500 (CDT) (envelope-from pgsql-hackers-owner+M13632=candle.pha.pa.us=pgman@postgresql.org) Received: from gromit.dotclick.com (ipn9-f8366.net-resource.net [216.204.83.66]) by postgresql.org (8.11.3/8.11.4) with ESMTP id f8RE49h21870 for ; Thu, 27 Sep 2001 10:04:09 -0400 (EDT) (envelope-from markw@mohawksoft.com) Received: from mohawksoft.com (IDENT:markw@localhost.localdomain [127.0.0.1]) by gromit.dotclick.com (8.9.3/8.9.3) with ESMTP id KAA24417; Thu, 27 Sep 2001 10:02:06 -0400 Message-ID: <3BB3315D.EC99FF65@mohawksoft.com> Date: Thu, 27 Sep 2001 10:02:05 -0400 From: mlw X-Mailer: Mozilla 4.75 [en] (X11; U; Linux 2.4.2 i686) X-Accept-Language: en MIME-Version: 1.0 To: "D. Hageman" cc: Ian Lance Taylor , "pgsql-hackers@postgresql.org" Subject: Re: [HACKERS] Spinlock performance improvement proposal References: Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR "D. Hageman" wrote: > On 26 Sep 2001, Ian Lance Taylor wrote: > > > > > Save for the fact that the kernel can switch between threads faster then > > > it can switch processes considering threads share the same address space, > > > stack, code, etc. If need be sharing the data between threads is much > > > easier then sharing between processes. > > > > When using a kernel threading model, it's not obvious to me that the > > kernel will switch between threads much faster than it will switch > > between processes. As far as I can see, the only potential savings is > > not reloading the pointers to the page tables. That is not nothing, > > but it is also not a lot. > > It is my understanding that avoiding a full context switch of the > processor can be of a significant advantage. This is especially important > on processor architectures that can be kinda slow at doing it (x86). I > will admit that most modern kernels have features that assist software > packages utilizing the forking model (copy on write for instance). It is > also my impression that these do a good job. I am the kind of guy that > looks towards the future (as in a year, year and half or so) and say that > processors will hopefully get faster at context switching and more and > more kernels will implement these algorithms to speed up the forking > model. At the same time, I see more and more processors being shoved into > a single box and it appears that the threads model works better on these > type of systems. "context" switching happens all the time on a multitasking system. On the x86 processor, a context switch happens when you call into the kernel. You have to go through a call-gate to get to a lower privilege ring. "context" switching is very fast. The operating system dictates how heavy or light a process switch is. Under Linux (and I believe FreeBSD with Linux threads, or version 4.x ) threads and processes are virtually identical. The only difference is that the virtual memory pages are not "copy on write." Process vs thread scheduling is also virtually identical. If you look to the future, then you should accept that process switching should become more efficient as the operating systems improve. > > > > I can't comment on the "isolate data" line. I am still trying to figure > > > that one out. > > > > Sometimes you need data which is specific to a particular thread. > > When you need data that is specific to a thread you use a TSD (Thread > Specific Data). Yes, but Postgres has many global variables. The assumption has always been that it is a stand-alone process with an explicitly shared paradigm, not implicitly. > > > Basically, you have to look at every global variable in the Postgres > > backend, and determine whether to share it among all threads or to > > make it thread-specific. > > Yes, if one was to implement threads into PostgreSQL I would think that > some re-writing would be in order of several areas. Like I said before, > give a person a chance to restructure things so future TODO items wouldn't > be so hard to implement. Personally, I like to stay away from global > variables as much as possible. They just get you into trouble. In real live software, software which lives from year to year with active development, things do get messy. There are always global variables involved in a program. Efforts, of course, should be made to keep them to a minimum, but the reality is that they always happen. Also, the very structure of function calls may need to change when going from a process model to a threaded model. Functions never before reentrant are now be reentrant, think about that. That is a huge undertaking. Every single function may need to be examined for thread safety, with little benefit. > > > > That last line is a troll if I every saw it ;-) I will agree that threads > > > isn't for everything and that it has costs just like everything else. Let > > > me stress that last part - like everything else. Certain costs exist in > > > the present model, nothing is - how should we say ... perfect. > > > > When writing in C, threading inevitably loses robustness. Erratic > > behaviour by one thread, perhaps in a user defined function, can > > subtly corrupt the entire system, rather than just that thread. Part > > of defensive programming is building barriers between different parts > > of a system. Process boundaries are a powerful barrier. > > I agree with everything you wrote above except for the first line. My > only comment is that process boundaries are only *truely* a powerful > barrier if the processes are different pieces of code and are not > dependent on each other in crippling ways. Forking the same code with the > bug in it - and only 1 in 5 die - is still 4 copies of buggy code running > on your system ;-) This is simply not true. All software has bugs, it is an undeniable fact. Some bugs are more likely to be hit than others. 5 processes , when one process hits a bug, that does not mean the other 4 will hit the same bug. Obscure bugs kill software all the time, the trick is to minimize the impact. Software is not perfect, assuming it can be is a mistake. ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/users-lounge/docs/faq.html From pgsql-hackers-owner+M16320=candle.pha.pa.us=pgman@postgresql.org Thu Dec 6 10:16:20 2001 Return-path: Received: from west.navpoint.com (west.navpoint.com [207.106.42.13]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id fB6FGJZ29347 for ; Thu, 6 Dec 2001 10:16:19 -0500 (EST) Received: from rs.postgresql.org (server1.pgsql.org [64.39.15.238] (may be forged)) by west.navpoint.com (8.11.6/8.10.1) with ESMTP id fB6FGIE25797 for ; Thu, 6 Dec 2001 10:16:18 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by rs.postgresql.org (8.11.6/8.11.6) with ESMTP id fB6F8MR55154 for ; Thu, 6 Dec 2001 09:12:12 -0600 (CST) (envelope-from pgsql-hackers-owner+M16320=candle.pha.pa.us=pgman@postgresql.org) Received: from dendrite.sacadia.com (adsl-64-168-22-137.dsl.snfc21.pacbell.net [64.168.22.137]) by postgresql.org (8.11.3/8.11.4) with ESMTP id fB3NNfm32380 for ; Mon, 3 Dec 2001 18:23:42 -0500 (EST) (envelope-from mkscott@sacadia.com) Received: from sacadia.com (localhost [127.0.0.1]) by dendrite.sacadia.com (8.10.2+Sun/8.10.2) with ESMTP id fB3NKiK16816 for ; Mon, 3 Dec 2001 15:20:44 -0800 (PST) Message-ID: <3C0C08CC.917CC04B@sacadia.com> Date: Mon, 03 Dec 2001 15:20:44 -0800 From: mkscott@sacadia.com X-Mailer: Mozilla 4.78 [en] (X11; U; SunOS 5.8 sun4u) X-Accept-Language: en MIME-Version: 1.0 To: PostgreSQL Hackers Mailing List Subject: [HACKERS] Using Threads (again) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org Status: OR Hi All, Since I last posted to this list I have done some work on a multi-threaded port of Postgres 7.0.2 that I have been kicking around for a while. There has been some mild interest in this in the past so I thought I might try and start a sourceforge project with what I have so far. >From past discussions, it is clear to me that a direct port of postgres which uses threads instead of processes is not a good idea, how about an embedded version that uses threads. A multi-threaded postgres might be good for that. The version I am working on is slower in terms of transaction throughput than the current postgres but it uses less system resources and does not require shared memory. I know it is possible to embed the current postgres but I believe that is a single user system. Comments? Myron Scott mkscott@sacadia.com ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@postgresql.org so that your message can get through to the mailing list cleanly From pgsql-hackers-owner+M33671@postgresql.org Fri Jan 3 10:27:00 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h03FQwl07124 for ; Fri, 3 Jan 2003 10:26:58 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id EDEBC4764DE; Fri, 3 Jan 2003 10:26:53 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 31554476422; Fri, 3 Jan 2003 10:25:46 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 69252476286 for ; Fri, 3 Jan 2003 10:25:29 -0500 (EST) Received: from www.pspl.co.in (www.pspl.co.in [202.54.11.65]) by postgresql.org (Postfix) with ESMTP id 98F754764C3 for ; Fri, 3 Jan 2003 10:23:52 -0500 (EST) Received: (from root@localhost) by www.pspl.co.in (8.11.6/8.11.6) id h03FNtK17518 for ; Fri, 3 Jan 2003 20:53:55 +0530 Received: from daithan.itnranet.pspl.co.in (daithan.intranet.pspl.co.in [192.168.7.161]) by www.pspl.co.in (8.11.6/8.11.0) with ESMTP id h03FNsf17512 for ; Fri, 3 Jan 2003 20:53:54 +0530 From: Shridhar Daithankar To: PGHackers Subject: [HACKERS] Threads Date: Fri, 3 Jan 2003 20:54:11 +0530 User-Agent: KMail/1.4.3 MIME-Version: 1.0 Content-Type: Multipart/Mixed; boundary="------------Boundary-00=_BG9530ZI94UNRKSGBVL5" Message-ID: <200301032054.11125.shridhar_daithankar@persistent.co.in> X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR --------------Boundary-00=_BG9530ZI94UNRKSGBVL5 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Hi all, I am sure, many of you would like to delete this message before reading, ho= ld=20 on. :-) There is much talk about threading on this list and the idea is always=20 deferred for want of robust thread models across all supported platforms an= d=20 feasibility of gains v/s efforts required. I think threads are useful in difference situations namely parallelising=20 blocking conditions and using multiple CPUs. Attached is a framework that I ported to C from a C++ server I have written= .=20 It has threadpool and threads implementation based on pthreads. This code expects minimum pthreads implementation and does not assume anyth= ing=20 on threads part (e.g kernel threads or not etc.) I request hackers on this list to take a look at it. It should be easily=20 pluggable in any source code and is released without any strings for any us= e. This framework allows to plug-in the worker function and argument on the fl= y.=20 The threads created are sleeping by default and can be woken up s and when= =20 required. I propose to use it incrementally in postgresql. Let's start with I/O. When= a=20 block of data is being read, rather than blocking for read, we can set up= =20 creator-consumer link between two threads That we way can utilize that I/O= =20 time in a overlapped fashion. Further threads can be useful when the server has more CPUs. It can spread = CPU=20 intensive work to different threads such as index creation or sorting. This= =20 way we can utilise idle CPU which we can not as of now. There are many advantages that I can see. 1)Threads can be optionally turned on/off depending upon the configuration.= So=20 we can entirely keep existing functionality and convert them one-by-one to= =20 threaded application. 2)For each functionality we can have two code branches, one that do not use= =20 threads i.e. current code base and one that can use threads. Agreed the=20 binary will be bit bloated but that would give enormous flexibility. If we= =20 find a thread implementation buggy, we simply switch it off either in=20 compilation or inconfiguration. 3) Not much efforts should be required to plug code into this model. The id= ea=20 of using threads is to assign exclusive work to each thread. So that should= =20 not require much of a locking. In case of using multiple CPUs, separate functions need be written that can= =20 handle the things in a thread-safe fashion. Also a merger function would be= =20 required which would merge results of worker threads. That would be totally= =20 additional. I would say two threads per CPU per back-end should be a reasonable default= as=20 that would cover I/O blocking well. Of course unless threading is turned of= f=20 in build or in configuration. Please note that I have tested the code in C++ and my C is rusty. Quite lik= ely=20 there are bugs in the code. I will stress test the code on monday but I wou= ld=20 like to seek an opinion on this as soon as possible. ( Hey but it compiles= =20 clean..) If required I can post example usage of this code, but I don't think that= =20 should be necessary.:-) Bye Shridhar --------------Boundary-00=_BG9530ZI94UNRKSGBVL5 Content-Type: text/x-chdr; charset="us-ascii"; name="thread.h" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="thread.h" #define _REENTRANT #include #include #include #include //typedefs typedef void* (*function)(void *); typedef void* argtype; typedef struct { pthread_mutex_t lock; pthread_cond_t cond; unsigned short freeCount,n,count; void *pool; } threadPool; typedef struct { pthread_t t; pthread_attr_t tattr; pthread_mutex_t lock; pthread_cond_t cond; argtype arg; function f; unsigned short quit; threadPool *p; } thread; /*Thread functions*/ void initThread(thread **t,threadPool *pool); void deleteThread(thread **t); void stop(thread *thr); void wakeForWork(thread *thr,function func,argtype a); argtype runner(void *ptr); /*thread pool functions*/ void initPool(threadPool **pool,unsigned short numthreads); void deletePool(threadPool **p); void putThread(threadPool *p,thread *t); thread *getThread(threadPool *p); --------------Boundary-00=_BG9530ZI94UNRKSGBVL5 Content-Type: text/x-csrc; charset="us-ascii"; name="thread.c" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="thread.c" #include "thread.h" void initThread(thread **t,threadPool *pool) { thread *thr=(thread *)malloc(sizeof(thread)); if(!thr) { fprintf(stderr,"\nCan not allocate memory for thread. Quitting...\n"); exit(1); } *t=thr; pthread_attr_init(&(thr->tattr)); pthread_mutex_init(&(thr->lock), NULL); pthread_cond_init(&(thr->cond), NULL); pthread_attr_setdetachstate(&(thr->tattr),PTHREAD_CREATE_DETACHED); thr->quit=0; thr->p=pool; //Create the thread int ret=pthread_create(&(thr->t),&(thr->tattr),runner,(void *)thr); if(ret!=0) { fprintf(stderr,"\nCan not create thread. Quitting...\n"); exit(1); } } void deleteThread(thread **t) { thread *thr=*t; if(!t) return; stop(thr); pthread_attr_destroy(&(thr->tattr)); pthread_cond_destroy(&(thr->cond)); pthread_mutex_destroy(&(thr->lock)); free(thr); } void stop(thread *thr) { unsigned short i; thr->quit=1; pthread_cond_signal(&(thr->cond)); for(i=0;thr->quit && i<10;i++) { if(i>=10) { pthread_kill(thr->t,9); break; } usleep(400); } } void wakeForWork(thread *thr,function func,argtype a) { thr->f=func; thr->arg=a; pthread_cond_signal(&(thr->cond)); } argtype runner(void* arg) { thread *ptr=(thread *)arg; while(1) { pthread_mutex_lock(&(ptr->lock)); if(ptr->p) putThread(ptr->p,ptr); pthread_cond_wait(&(ptr->cond),&(ptr->lock)); if(ptr->quit) break; ptr->f((void *)ptr->arg); pthread_mutex_unlock(&(ptr->lock)); } ptr->quit=0; return NULL; } void initPool(threadPool **pool,unsigned short numthreads) { thread **thr; threadPool *p=(threadPool *)malloc(sizeof(threadPool)); if(!p) { fprintf(stderr,"Can not get memory to create threadpool. Quitting\n"); exit(1); } if(!pool) { free(p); return; } *pool=p; pthread_mutex_init(&(p->lock), NULL); pthread_cond_init(&(p->cond), NULL); p->n=numthreads; p->freeCount=0; p->n=numthreads; thr=(thread **)malloc(numthreads*sizeof(thread *)); if(!thr) { fprintf(stderr,"Can not get memory to create pool of threads. Quitting\n"); exit(1); } p->pool=(void *)thr; } void deletePool(threadPool **pool) { threadPool *p=(threadPool *)pool; if(!pool) return; thread **thr=(thread **)p->pool; unsigned short i; for(i=0;in;i++) stop(thr[i]); free(p->pool); pthread_cond_destroy(&(p->cond)); pthread_mutex_destroy(&(p->lock)); free(p); } void putThread(threadPool *p,thread *t) { unsigned short i; thread **pool; if(!p || !t) return; pool=(thread **)p->pool; pthread_mutex_lock(&(p->lock)); i=p->freeCount; pool[(p->freeCount)++]=t; if(i<=0)pthread_cond_signal(&(p->cond)); pthread_mutex_unlock(&(p->lock)); } thread *getThread(threadPool *p) { thread *t,**t1; if(!p) return NULL; t1=(thread **)p->pool; pthread_mutex_lock(&(p->lock)); if((p->freeCount)<=0)pthread_cond_wait(&(p->cond),&(p->lock)); t=t1[--(p->freeCount)]; pthread_mutex_unlock(&(p->lock)); return t; } --------------Boundary-00=_BG9530ZI94UNRKSGBVL5 Content-Type: text/plain Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org --------------Boundary-00=_BG9530ZI94UNRKSGBVL5-- From pgsql-hackers-owner+M33682@postgresql.org Fri Jan 3 15:43:54 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h03Khhl06938 for ; Fri, 3 Jan 2003 15:43:45 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id DF70F476EA6; Fri, 3 Jan 2003 15:43:34 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 95BA8476514; Fri, 3 Jan 2003 15:43:26 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 71F4E475DBC for ; Fri, 3 Jan 2003 15:43:14 -0500 (EST) Received: from snoopy.mohawksoft.com (h0030f1382639.ne.client2.attbi.com [24.60.194.163]) by postgresql.org (Postfix) with ESMTP id ACE5B475DAD for ; Fri, 3 Jan 2003 15:43:13 -0500 (EST) Received: from mohawksoft.com (snoopy.mohawksoft.com [127.0.0.1]) by snoopy.mohawksoft.com (8.11.6/8.11.6) with ESMTP id h03KlMs24421; Fri, 3 Jan 2003 15:47:27 -0500 Message-ID: <3E15F6DA.8000209@mohawksoft.com> Date: Fri, 03 Jan 2003 15:47:22 -0500 From: mlw User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Shridhar Daithankar cc: PGHackers Subject: Re: [HACKERS] Threads References: <200301032054.11125.shridhar_daithankar@persistent.co.in> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR Please no threading threads!!! Has anyone calculated the interval and period of "PostgreSQL needs threads" posts? The *ONLY* advantage threading has over multiple processes is the time and resources used in creating new processes. That being said, I admit that creating a threaded program is easier than one with multiple processes, but PostgreSQL is already there and working. Drawbacks to a threaded model: (1) One thread screws up, the whole process dies. In a multiple process application this is not too much of an issue. (2) Heap fragmentation. In a long uptime application, such as a database, heap fragmentation is an important consideration. With multiple processes, each process manages its own heap and what ever fragmentation that exists goes away when the connection is closed. A threaded server is far more vulnerable because the heap has to manage many threads and the heap has to stay active and unfragmented in perpetuity. This is why Windows applications usually end up using 2G of memory after 3 months of use. (Well, this AND memory leaks) (3) Stack space. In a threaded application they are more limits to stack usage. I'm not sure, but I bet PostgreSQL would have a problem with a fixed size stack, I know the old ODBC driver did. (4) Lock Contention. The various single points of access in a process have to be serialized for multiple threads. heap allocation, deallocation, etc all have to be managed. In a multple process model, these resources would be separated by process contexts. (5) Lastly, why bother? Seriously? Process creation time is an issue true, but its an issue with threads as well, just not as bad. Anyone who is looking for performance should be using a connection pooling mechanism as is done in things like PHP. I have done both threaded and process servers. The threaded servers are easier to write. The process based severs are more robust. From an operational point of view, a "select foo from bar where x > y" will take he same amount of time. ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org From pgsql-hackers-owner+M33684@postgresql.org Fri Jan 3 15:56:48 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h03Kufl08003 for ; Fri, 3 Jan 2003 15:56:43 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id D0392477118; Fri, 3 Jan 2003 15:56:31 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 31FDC475461; Fri, 3 Jan 2003 15:55:26 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id BD892477147 for ; Fri, 3 Jan 2003 15:55:09 -0500 (EST) Received: from voyager.corporate.connx.com (unknown [209.20.248.131]) by postgresql.org (Postfix) with ESMTP id 7EE644771A0 for ; Fri, 3 Jan 2003 15:52:47 -0500 (EST) content-class: urn:content-classes:message Subject: Re: [HACKERS] Threads MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Date: Fri, 3 Jan 2003 12:52:48 -0800 X-MimeOLE: Produced By Microsoft Exchange V6.0.6249.0 Message-ID: Thread-Topic: [HACKERS] Threads Thread-Index: AcKzaMsucwBFaOikSjKML8BqvR/gCAAACDPA From: "Dann Corbit" To: "PGHackers" X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by candle.pha.pa.us id h03Kufl08003 Status: OR > -----Original Message----- > From: mlw [mailto:pgsql@mohawksoft.com] > Sent: Friday, January 03, 2003 12:47 PM > To: Shridhar Daithankar > Cc: PGHackers > Subject: Re: [HACKERS] Threads > > > Please no threading threads!!! > > Has anyone calculated the interval and period of "PostgreSQL needs > threads" posts? > > The *ONLY* advantage threading has over multiple processes is > the time > and resources used in creating new processes. Threading is absurdly easier to do portably than fork(). Will you fork() successfully on MVS, VMS, OS/2, Win32? On some operating systems, thread creation is absurdly faster than process creation (many orders of magnitude). > That being said, I admit that creating a threaded program is > easier than > one with multiple processes, but PostgreSQL is already there > and working. > > Drawbacks to a threaded model: > > (1) One thread screws up, the whole process dies. In a > multiple process > application this is not too much of an issue. If you use C++ you can try/catch and nothing bad happens to anything but the naughty thread. > (2) Heap fragmentation. In a long uptime application, such as a > database, heap fragmentation is an important consideration. With > multiple processes, each process manages its own heap and what ever > fragmentation that exists goes away when the connection is closed. A > threaded server is far more vulnerable because the heap has to manage > many threads and the heap has to stay active and unfragmented in > perpetuity. This is why Windows applications usually end up > using 2G of > memory after 3 months of use. (Well, this AND memory leaks) Poorly written applications leak memory. Fragmentation is a legitimate concern. > (3) Stack space. In a threaded application they are more > limits to stack > usage. I'm not sure, but I bet PostgreSQL would have a problem with a > fixed size stack, I know the old ODBC driver did. A single server with 20 threads will consume less total free store memory and automatic memory than 20 servers. You have to decide how much stack to give a thread, that's true. > (4) Lock Contention. The various single points of access in a process > have to be serialized for multiple threads. heap allocation, > deallocation, etc all have to be managed. In a multple process model, > these resources would be separated by process contexts. Semaphores are more complicated than critical sections. If anything, a shared memory approach is more problematic and fragile, especially when porting to multiple operating systems. > (5) Lastly, why bother? Seriously? Process creation time is an issue > true, but its an issue with threads as well, just not as bad. > Anyone who > is looking for performance should be using a connection pooling > mechanism as is done in things like PHP. > > I have done both threaded and process servers. The threaded > servers are > easier to write. The process based severs are more robust. From an > operational point of view, a "select foo from bar where x > > y" will take > he same amount of time. Probably true. I think a better solution is a server that can start threads or processes or both. But that's neither here nor there and I'm certainly not volunteering to write it. Here is a solution to the dilemma. Make the one who suggests the feature be the first volunteer on the team that writes it. Is it a FAQ? If not, it ought to be. ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@postgresql.org so that your message can get through to the mailing list cleanly From pgsql-hackers-owner+M33685@postgresql.org Fri Jan 3 16:35:02 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h03LYsl11402 for ; Fri, 3 Jan 2003 16:34:56 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 0F09B477168; Fri, 3 Jan 2003 16:34:48 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id C1A9C477132; Fri, 3 Jan 2003 16:34:39 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id D830847630B for ; Fri, 3 Jan 2003 16:34:25 -0500 (EST) Received: from CopelandConsulting.Net (dsl-24293-ld.customer.centurytel.net [209.142.135.135]) by postgresql.org (Postfix) with ESMTP id 025DD476417 for ; Fri, 3 Jan 2003 16:34:24 -0500 (EST) Received: from [192.168.1.2] (mouse.copelandconsulting.net [192.168.1.2]) by CopelandConsulting.Net (8.10.1/8.10.1) with ESMTP id h03LY2700731; Fri, 3 Jan 2003 15:34:03 -0600 (CST) X-Trade-Id: To: mlw cc: Shridhar Daithankar , PGHackers In-Reply-To: <3E15F6DA.8000209@mohawksoft.com> References: <200301032054.11125.shridhar_daithankar@persistent.co.in> <3E15F6DA.8000209@mohawksoft.com> Content-Type: text/plain Organization: Copeland Computer Consulting Message-ID: <1041629649.15933.135.camel@mouse.copelandconsulting.net> MIME-Version: 1.0 X-Mailer: Ximian Evolution 1.2.0 Date: 03 Jan 2003 15:34:10 -0600 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On Fri, 2003-01-03 at 14:47, mlw wrote: > Please no threading threads!!! > Ya, I'm very pro threads but I've long since been sold on no threads for PostgreSQL. AIO on the other hand... ;) Your summary so accurately addresses the issue it should be a whole FAQ entry on threads and PostgreSQL. :) > Drawbacks to a threaded model: > > (1) One thread screws up, the whole process dies. In a multiple process > application this is not too much of an issue. > > (2) Heap fragmentation. In a long uptime application, such as a > database, heap fragmentation is an important consideration. With > multiple processes, each process manages its own heap and what ever > fragmentation that exists goes away when the connection is closed. A > threaded server is far more vulnerable because the heap has to manage > many threads and the heap has to stay active and unfragmented in > perpetuity. This is why Windows applications usually end up using 2G of > memory after 3 months of use. (Well, this AND memory leaks) These are things that can't be stressed enough. IMO, these are some of the many reasons why applications running on MS platforms tend to have much lower application and system up times (that and resources leaks which are inherent to the platform). BTW, if you do much in the way of threaded coding, there is libHorde which is a heap library for heavily threaded, memory hungry applications. It excels in performance, reduces heap lock contention (maintains multiple heaps in a very thread smart manner), and goes a long way toward reducing heap fragmentation which is common for heavily memory based, threaded applications. > (3) Stack space. In a threaded application they are more limits to stack > usage. I'm not sure, but I bet PostgreSQL would have a problem with a > fixed size stack, I know the old ODBC driver did. > Most modern thread implementations use a page guard on the stack to determine if it needs to grow or not. Generally speaking, for most modern platforms which support threading, stack considerations rarely become an issue. > (5) Lastly, why bother? Seriously? Process creation time is an issue > true, but its an issue with threads as well, just not as bad. Anyone who > is looking for performance should be using a connection pooling > mechanism as is done in things like PHP. > > I have done both threaded and process servers. The threaded servers are > easier to write. The process based severs are more robust. From an > operational point of view, a "select foo from bar where x > y" will take > he same amount of time. > I agree with this, however, using threads does open the door for things like splitting queries and sorts across multiple CPUs. Something the current process model, which was previously agreed on, would not be able to address because of cost. Example: "select foo from bar where x > y order by foo ;", could be run on multiple CPUs if the sort were large enough to justify. After it's all said and done, I do agree that threading just doesn't seem like a good fit for PostgreSQL. -- Greg Copeland Copeland Computer Consulting ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster From pgsql-hackers-owner+M33686@postgresql.org Fri Jan 3 16:47:20 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h03LlBl12502 for ; Fri, 3 Jan 2003 16:47:12 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 6873147621D; Fri, 3 Jan 2003 16:47:06 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 97466477133; Fri, 3 Jan 2003 16:46:41 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id E25BB477152 for ; Fri, 3 Jan 2003 16:46:24 -0500 (EST) Received: from CopelandConsulting.Net (dsl-24293-ld.customer.centurytel.net [209.142.135.135]) by postgresql.org (Postfix) with ESMTP id 84A87477157 for ; Fri, 3 Jan 2003 16:45:21 -0500 (EST) Received: from [192.168.1.2] (mouse.copelandconsulting.net [192.168.1.2]) by CopelandConsulting.Net (8.10.1/8.10.1) with ESMTP id h03LjC712426; Fri, 3 Jan 2003 15:45:13 -0600 (CST) X-Trade-Id: To: Dann Corbit cc: PGHackers In-Reply-To: References: Content-Type: text/plain Organization: Copeland Computer Consulting Message-ID: <1041630319.15927.146.camel@mouse.copelandconsulting.net> MIME-Version: 1.0 X-Mailer: Ximian Evolution 1.2.0 Date: 03 Jan 2003 15:45:20 -0600 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On Fri, 2003-01-03 at 14:52, Dann Corbit wrote: > > -----Original Message----- > > (1) One thread screws up, the whole process dies. In a > > multiple process > > application this is not too much of an issue. > > If you use C++ you can try/catch and nothing bad happens to anything but > the naughty thread. That doesn't protect against the type of issues he's talking about. Invalid pointer reference is a very common snafu which really hoses threaded applications. Not to mention resource leaks AND LOCKED resources which are inherently an issue on Win32. Besides, it's doubtful that PostgreSQL is going to be rewritten in C++ so bringing up try/catch is pretty much an invalid argument. > > > (2) Heap fragmentation. In a long uptime application, such as a > > database, heap fragmentation is an important consideration. With > > multiple processes, each process manages its own heap and what ever > > fragmentation that exists goes away when the connection is closed. A > > threaded server is far more vulnerable because the heap has to manage > > many threads and the heap has to stay active and unfragmented in > > perpetuity. This is why Windows applications usually end up > > using 2G of > > memory after 3 months of use. (Well, this AND memory leaks) > > Poorly written applications leak memory. Fragmentation is a legitimate > concern. And well written applications which attempt to safely handle segfaults, etc., often leak memory and lock resources like crazy. On Win32, depending on the nature of the resources, once this happens, even process termination will not free/unlock the resources. > > (4) Lock Contention. The various single points of access in a process > > have to be serialized for multiple threads. heap allocation, > > deallocation, etc all have to be managed. In a multple process model, > > these resources would be separated by process contexts. > > Semaphores are more complicated than critical sections. If anything, a > shared memory approach is more problematic and fragile, especially when > porting to multiple operating systems. And critical sections lead to low performance on SMP systems for Win32 platforms. No task can switch on ANY CPU for the duration of the critical section. It's highly recommend by MS as the majority of Win32 applications expect uniprocessor systems and they are VERY fast. As soon as multiple processors come into the mix, critical sections become a HORRIBLE idea if any soft of scalability is desired. > Is it a FAQ? If not, it ought to be. I agree. I think mlw's list of reasons should be added to a faq. It terse yet says it all! -- Greg Copeland Copeland Computer Consulting ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to majordomo@postgresql.org) From pgsql-hackers-owner+M33703@postgresql.org Fri Jan 3 20:41:10 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h041f9l05824 for ; Fri, 3 Jan 2003 20:41:09 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 7F5764764C8; Fri, 3 Jan 2003 20:41:04 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id BE24547606D; Fri, 3 Jan 2003 20:38:53 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 4D50D476165 for ; Fri, 3 Jan 2003 20:38:39 -0500 (EST) Received: from sss.pgh.pa.us (unknown [192.204.191.242]) by postgresql.org (Postfix) with ESMTP id 20C8547659F for ; Fri, 3 Jan 2003 20:34:10 -0500 (EST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss.pgh.pa.us (8.12.6/8.12.6) with ESMTP id h041Y20U023764; Fri, 3 Jan 2003 20:34:03 -0500 (EST) To: "Serguei Mokhov" cc: "Greg Copeland" , "Dann Corbit" , "PGHackers" Subject: Re: [HACKERS] Threads In-Reply-To: <004101c2b37b$0f261ae0$0301a8c0@gunnymede.lan> References: <1041630319.15927.146.camel@mouse.copelandconsulting.net> <004101c2b37b$0f261ae0$0301a8c0@gunnymede.lan> Comments: In-reply-to "Serguei Mokhov" message dated "Fri, 03 Jan 2003 17:54:20 -0500" Date: Fri, 03 Jan 2003 20:34:02 -0500 Message-ID: <23763.1041644042@sss.pgh.pa.us> From: Tom Lane X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR "Serguei Mokhov" writes: >>> (1) One thread screws up, the whole process dies. In a >>> multiple process application this is not too much of an issue. > (1) is an issue only for user-level threads. Uh, what other kind of thread have you got in mind here? I suppose the lack-of-cross-thread-protection issue would go away if our objective was only to use threads for internal parallelism in each backend instance (ie, you still have one process per connection, but internally it would use multiple threads to process subqueries in parallel). Of course that gives up the hope of faster connection startup that has always been touted as a major reason to want Postgres to be threaded... regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/users-lounge/docs/faq.html From pgsql-hackers-owner+M33706@postgresql.org Fri Jan 3 21:16:55 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h042Gsl08584 for ; Fri, 3 Jan 2003 21:16:54 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 8F2EB475E22; Fri, 3 Jan 2003 21:16:49 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 72017475FDA; Fri, 3 Jan 2003 21:15:21 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id EA790476242 for ; Fri, 3 Jan 2003 21:15:00 -0500 (EST) Received: from CopelandConsulting.Net (dsl-24293-ld.customer.centurytel.net [209.142.135.135]) by postgresql.org (Postfix) with ESMTP id BB7A0475D0D for ; Fri, 3 Jan 2003 21:11:20 -0500 (EST) Received: from [192.168.1.2] (mouse.copelandconsulting.net [192.168.1.2]) by CopelandConsulting.Net (8.10.1/8.10.1) with ESMTP id h042B8729407; Fri, 3 Jan 2003 20:11:08 -0600 (CST) X-Trade-Id: To: Tom Lane cc: Serguei Mokhov , Dann Corbit , PGHackers In-Reply-To: <23763.1041644042@sss.pgh.pa.us> References: <1041630319.15927.146.camel@mouse.copelandconsulting.net> <004101c2b37b$0f261ae0$0301a8c0@gunnymede.lan> <23763.1041644042@sss.pgh.pa.us> Content-Type: text/plain Organization: Copeland Computer Consulting Message-ID: <1041646276.15927.202.camel@mouse.copelandconsulting.net> MIME-Version: 1.0 X-Mailer: Ximian Evolution 1.2.0 Date: 03 Jan 2003 20:11:17 -0600 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On Fri, 2003-01-03 at 19:34, Tom Lane wrote: > "Serguei Mokhov" writes: > >>> (1) One thread screws up, the whole process dies. In a > >>> multiple process application this is not too much of an issue. > > > (1) is an issue only for user-level threads. > Umm. No. User or system level threads, the statement is true. If a thread kills over, the process goes with it. Furthermore, on Win32 platforms, it opens a whole can of worms no matter how you care to address it. > Uh, what other kind of thread have you got in mind here? > > I suppose the lack-of-cross-thread-protection issue would go away if > our objective was only to use threads for internal parallelism in each > backend instance (ie, you still have one process per connection, but > internally it would use multiple threads to process subqueries in > parallel). > Several have previously spoken about a hybrid approach (ala Apache). IIRC, it was never ruled out but it was simply stated that no one had the energy to put into such a concept. > Of course that gives up the hope of faster connection startup that has > always been touted as a major reason to want Postgres to be threaded... > > regards, tom lane Faster startup, should never be the primary reason as there are many ways to address that issue already. Connection pooling and caching are by far, the most common way to address this issue. Not only that, but by definition, it's almost an oxymoron. If you really need high performance, you shouldn't be using transient connections, no matter how fast they are. This, in turn, brings you back to persistent connections or connection pools/caches. -- Greg Copeland Copeland Computer Consulting ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster From pgsql-hackers-owner+M33709@postgresql.org Fri Jan 3 22:39:26 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h043dOl13614 for ; Fri, 3 Jan 2003 22:39:25 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id CA13B47621C; Fri, 3 Jan 2003 22:39:20 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 8DE1D475DFF; Fri, 3 Jan 2003 22:39:04 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 15AA1475AFF for ; Fri, 3 Jan 2003 22:39:00 -0500 (EST) Received: from CopelandConsulting.Net (dsl-24293-ld.customer.centurytel.net [209.142.135.135]) by postgresql.org (Postfix) with ESMTP id 19D8F475ADD for ; Fri, 3 Jan 2003 22:38:59 -0500 (EST) Received: from [192.168.1.2] (mouse.copelandconsulting.net [192.168.1.2]) by CopelandConsulting.Net (8.10.1/8.10.1) with ESMTP id h043ca714568; Fri, 3 Jan 2003 21:38:36 -0600 (CST) X-Trade-Id: To: mlw cc: Tom Lane , Serguei Mokhov , Dann Corbit , PGHackers In-Reply-To: <3E16575C.1030805@mohawksoft.com> References: <1041630319.15927.146.camel@mouse.copelandconsulting.net> <004101c2b37b$0f261ae0$0301a8c0@gunnymede.lan> <23763.1041644042@sss.pgh.pa.us> <1041646276.15927.202.camel@mouse.copelandconsulting.net> <3E16575C.1030805@mohawksoft.com> Content-Type: text/plain Organization: Copeland Computer Consulting Message-ID: <1041651525.15927.207.camel@mouse.copelandconsulting.net> MIME-Version: 1.0 X-Mailer: Ximian Evolution 1.2.0 Date: 03 Jan 2003 21:38:46 -0600 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On Fri, 2003-01-03 at 21:39, mlw wrote: > Connection time should *never* be in the critical path. There, I've > said it!! People who complain about connection time are barking up the > wrong tree. Regardless of the methodology, EVERY OS has issues with > thread creation, process creation, the memory allocation, and system > manipulation required to manage it. Under load this is ALWAYS slower. > > I think that if there is ever a choice, "do I make startup time > faster?" or "Do I make PostgreSQL not need a dump/restore for upgrade" > the upgrade problem has a much higher impact to real PostgreSQL sites. Exactly. Trying to speed up something that shouldn't be in the critical path is exactly what I'm talking about. I completely agree with you! -- Greg Copeland Copeland Computer Consulting ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster From pgsql-hackers-owner+M33708@postgresql.org Fri Jan 3 22:35:26 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h043ZOl13418 for ; Fri, 3 Jan 2003 22:35:25 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 2277B475FDA; Fri, 3 Jan 2003 22:35:21 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id DA681475E18; Fri, 3 Jan 2003 22:35:12 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 8254047595A for ; Fri, 3 Jan 2003 22:34:58 -0500 (EST) Received: from snoopy.mohawksoft.com (h0030f1382639.ne.client2.attbi.com [24.60.194.163]) by postgresql.org (Postfix) with ESMTP id A4D60475921 for ; Fri, 3 Jan 2003 22:34:57 -0500 (EST) Received: from mohawksoft.com (snoopy.mohawksoft.com [127.0.0.1]) by snoopy.mohawksoft.com (8.11.6/8.11.6) with ESMTP id h043d8s26180; Fri, 3 Jan 2003 22:39:09 -0500 Message-ID: <3E16575C.1030805@mohawksoft.com> Date: Fri, 03 Jan 2003 22:39:08 -0500 From: mlw User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Greg Copeland cc: Tom Lane , Serguei Mokhov , Dann Corbit , PGHackers Subject: Re: [HACKERS] Threads References: <1041630319.15927.146.camel@mouse.copelandconsulting.net> <004101c2b37b$0f261ae0$0301a8c0@gunnymede.lan> <23763.1041644042@sss.pgh.pa.us> <1041646276.15927.202.camel@mouse.copelandconsulting.net> Content-Type: multipart/alternative; boundary="------------030005060103020905060907" X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR --------------030005060103020905060907 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Greg Copeland wrote: > > >>Of course that gives up the hope of faster connection startup that has >>always been touted as a major reason to want Postgres to be threaded... >> >> regards, tom lane >> >> > >Faster startup, should never be the primary reason as there are many >ways to address that issue already. Connection pooling and caching are >by far, the most common way to address this issue. Not only that, but >by definition, it's almost an oxymoron. If you really need high >performance, you shouldn't be using transient connections, no matter how >fast they are. This, in turn, brings you back to persistent connections >or connection pools/caches. > Connection time should *never* be in the critical path. There, I've said it!! People who complain about connection time are barking up the wrong tree. Regardless of the methodology, EVERY OS has issues with thread creation, process creation, the memory allocation, and system manipulation required to manage it. Under load this is ALWAYS slower. I think that if there is ever a choice, "do I make startup time faster?" or "Do I make PostgreSQL not need a dump/restore for upgrade" the upgrade problem has a much higher impact to real PostgreSQL sites. --------------030005060103020905060907 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit

Greg Copeland wrote:
  
Of course that gives up the hope of faster connection startup that has
always been touted as a major reason to want Postgres to be threaded...

			regards, tom lane
    

Faster startup, should never be the primary reason as there are many
ways to address that issue already.  Connection pooling and caching are
by far, the most common way to address this issue.  Not only that, but
by definition, it's almost an oxymoron.  If you really need high
performance, you shouldn't be using transient connections, no matter how
fast they are.  This, in turn, brings you back to persistent connections
or connection pools/caches.
Connection time should *never* be in the critical path. There, I've said it!! People who complain about connection time are barking up the wrong tree. Regardless of the methodology, EVERY OS has issues with thread creation, process creation, the memory allocation, and system manipulation  required to manage it. Under load this is ALWAYS slower.

I think that if there is ever a choice, "do I make startup time faster?" or "Do I make PostgreSQL not need a dump/restore for upgrade" the upgrade problem has a much higher impact to real PostgreSQL sites.
--------------030005060103020905060907-- From pgsql-hackers-owner+M33713@postgresql.org Sat Jan 4 00:34:04 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h045Y2l23520 for ; Sat, 4 Jan 2003 00:34:02 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id BCA39476226; Sat, 4 Jan 2003 00:33:56 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 1B030475F09; Sat, 4 Jan 2003 00:33:47 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id A42D847595A for ; Sat, 4 Jan 2003 00:33:37 -0500 (EST) Received: from houston.familyhealth.com.au (unknown [203.59.48.253]) by postgresql.org (Postfix) with ESMTP id C14B4475921 for ; Sat, 4 Jan 2003 00:33:35 -0500 (EST) Received: from localhost (chriskl@localhost) by houston.familyhealth.com.au (8.11.6/8.11.6) with ESMTP id h045XKt36362; Sat, 4 Jan 2003 13:33:23 +0800 (WST) (envelope-from chriskl@familyhealth.com.au) Date: Sat, 4 Jan 2003 13:33:20 +0800 (WST) From: Christopher Kings-Lynne To: mlw cc: Shridhar Daithankar , PGHackers Subject: Re: [HACKERS] Threads In-Reply-To: <3E15F6DA.8000209@mohawksoft.com> Message-ID: <20030104133226.N36192-100000@houston.familyhealth.com.au> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR Also remember that in even well developed OS's like FreeBSD, all a process's threads will execute only on one CPU. This might change in FreeBSD 5.0, but still a threaded app (such as MySQL) cannot use mutliple CPUs on a FreeBSD system. Chris On Fri, 3 Jan 2003, mlw wrote: > Please no threading threads!!! > > Has anyone calculated the interval and period of "PostgreSQL needs > threads" posts? > > The *ONLY* advantage threading has over multiple processes is the time > and resources used in creating new processes. > > That being said, I admit that creating a threaded program is easier than > one with multiple processes, but PostgreSQL is already there and working. > > Drawbacks to a threaded model: > > (1) One thread screws up, the whole process dies. In a multiple process > application this is not too much of an issue. > > (2) Heap fragmentation. In a long uptime application, such as a > database, heap fragmentation is an important consideration. With > multiple processes, each process manages its own heap and what ever > fragmentation that exists goes away when the connection is closed. A > threaded server is far more vulnerable because the heap has to manage > many threads and the heap has to stay active and unfragmented in > perpetuity. This is why Windows applications usually end up using 2G of > memory after 3 months of use. (Well, this AND memory leaks) > > (3) Stack space. In a threaded application they are more limits to stack > usage. I'm not sure, but I bet PostgreSQL would have a problem with a > fixed size stack, I know the old ODBC driver did. > > (4) Lock Contention. The various single points of access in a process > have to be serialized for multiple threads. heap allocation, > deallocation, etc all have to be managed. In a multple process model, > these resources would be separated by process contexts. > > (5) Lastly, why bother? Seriously? Process creation time is an issue > true, but its an issue with threads as well, just not as bad. Anyone who > is looking for performance should be using a connection pooling > mechanism as is done in things like PHP. > > I have done both threaded and process servers. The threaded servers are > easier to write. The process based severs are more robust. From an > operational point of view, a "select foo from bar where x > y" will take > he same amount of time. > > > > > ---------------------------(end of broadcast)--------------------------- > TIP 6: Have you searched our list archives? > > http://archives.postgresql.org > ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@postgresql.org so that your message can get through to the mailing list cleanly From pgsql-hackers-owner+M33723@postgresql.org Sat Jan 4 13:21:52 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h04ILpl25640 for ; Sat, 4 Jan 2003 13:21:51 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id A5E5D4764F0; Sat, 4 Jan 2003 13:21:50 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id B8D94476021; Sat, 4 Jan 2003 13:21:37 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 7FDFE475CE7 for ; Sat, 4 Jan 2003 13:21:28 -0500 (EST) Received: from CopelandConsulting.Net (dsl-24293-ld.customer.centurytel.net [209.142.135.135]) by postgresql.org (Postfix) with ESMTP id 71C47474E42 for ; Sat, 4 Jan 2003 13:21:27 -0500 (EST) Received: from [192.168.1.2] (mouse.copelandconsulting.net [192.168.1.2]) by CopelandConsulting.Net (8.10.1/8.10.1) with ESMTP id h04ILF721061; Sat, 4 Jan 2003 12:21:15 -0600 (CST) X-Trade-Id: To: kar@kakidata.dk cc: PGHackers In-Reply-To: <200301041359.35715.kar@kakidata.dk> References: <23763.1041644042@sss.pgh.pa.us> <1041646276.15927.202.camel@mouse.copelandconsulting.net> <200301041359.35715.kar@kakidata.dk> Content-Type: text/plain Organization: Copeland Computer Consulting Message-ID: <1041704480.15927.224.camel@mouse.copelandconsulting.net> MIME-Version: 1.0 X-Mailer: Ximian Evolution 1.2.0 Date: 04 Jan 2003 12:21:20 -0600 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On Sat, 2003-01-04 at 06:59, Kaare Rasmussen wrote: > > Umm. No. User or system level threads, the statement is true. If a > > thread kills over, the process goes with it. Furthermore, on Win32 > > Hm. This is a database system. If one of the backend processes dies > unexpectedly, I'm not sure I would trust the consistency and state of the > others. > > Or maybe I'm just being chicken. I'd call that being wise. That's the problem with using threads. Should a thread do something naughty, the state of the entire process is in question. This is true regardless if it is a user mode, kernel mode, or hybrid thread implementation. That's the power of using the process model that is currently in use. Should it do something naughty, we bitch and complain politely, throw our hands in the air and exit. We no longer have to worry about the state and validity of that backend. This creates a huge systemic reliability surplus. This is also why the concept of a hybrid thread/process implementation keeps coming to the surface on the list. If you maintain the process model and only use threads for things that ONLY relate to the single process (single session/connection), should a thread cause a problem, you can still throw you hands in the air and exit just as is done now without causing problems for, or questioning the validity of, other backends. The cool thing about such a concept is that it still opens the door for things like parallel sorts and queries as it relates to a single backend. -- Greg Copeland Copeland Computer Consulting ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org From pgsql-hackers-owner+M33819@postgresql.org Mon Jan 6 02:41:01 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h067exi23864 for ; Mon, 6 Jan 2003 02:40:59 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id CCD564763B7; Mon, 6 Jan 2003 02:40:56 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 4A6574762E0; Mon, 6 Jan 2003 02:40:54 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 2C31947606A for ; Mon, 6 Jan 2003 02:40:50 -0500 (EST) Received: from datafix.CS.Berkeley.EDU (datafix.CS.Berkeley.EDU [128.32.37.185]) by postgresql.org (Postfix) with ESMTP id 8D7AF47603D for ; Mon, 6 Jan 2003 02:40:49 -0500 (EST) Received: (from sailesh@localhost) by datafix.CS.Berkeley.EDU (8.11.6/8.11.6) id h067ac532006; Sun, 5 Jan 2003 23:36:38 -0800 X-Authentication-Warning: datafix.CS.Berkeley.EDU: sailesh set sender to sailesh@cs.berkeley.edu using -f Reply-To: sailesh@cs.berkeley.edu X-URL: http://www.cs.berkeley.edu/~sailesh X-Attribution: Sailesh To: Shridhar Daithankar cc: PGHackers Subject: Re: [HACKERS] Threads References: <200301032054.11125.shridhar_daithankar@persistent.co.in> <3E1605B8.5060403@priefert.com> <200301061202.43247.shridhar_daithankar@persistent.co.in> From: Sailesh Krishnamurthy Date: 05 Jan 2003 23:36:38 -0800 In-Reply-To: <200301061202.43247.shridhar_daithankar@persistent.co.in> Message-ID: Lines: 50 User-Agent: Gnus/5.0808 (Gnus v5.8.8) XEmacs/21.1 (Cuyahoga Valley) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR >>>>> "Shridhar" == Shridhar Daithankar writes: Shridhar> On Saturday 04 January 2003 03:20 am, you wrote: >> >I am sure, many of you would like to delete this message >> before reading, > hold on. :-) >> >> I'm afraid most posters did not read the message. Those who >> replied >> >> "Why bother?" did not address your challenge: Shridhar> Our challenges may be..;-) Not having threading does reduce some of the freedom we've been having in our work. But then we have ripped the process model a fair bit and we have the freedom of an entirely new process to deal with data streams entering the system and we're experimenting with threading for asynchronous I/O there. However, in general I agree with the spirit of the previous messages in this thread that threading isn't the main issue for PG. One thing that I missed so far in the threading thread. Context switches are (IMHO) far cheaper between threads, because you save TLB flushes. Whether this makes a real difference in a data intensive application, I don't know. I wonder how easy it is to measure the x86 counters to see TLB flushes/misses. In a database system, even if one process dies, I'd be very chary of trusting it. So I am not too swayed by the fact that a process-per-connection gets you better isolation. BTW, many commercial database systems also use per-process models on Unix. However they are very aggressive with connection sharing and reuse - even to the point of reusing the same process for multiple active connections .. maybe at transaction boundaries. Good when a connection is maintained for a long duaration with short-lived transactions separated by fair amouns of time. Moreover, in db2 for instance, the same code base is used for both per-thread and per-process models - in other words, the entire code is MT-safe, and the scheduling mechanism is treated as a policy (Win32 is MT, and some Unices MP). AFAICT though, postgres code, such as perhaps the memory contexts is not MT-safe (of course the bufferpool/shmem accesses are safe). -- Pip-pip Sailesh http://www.cs.berkeley.edu/~sailesh ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org From pgsql-hackers-owner+M33822@postgresql.org Mon Jan 6 06:23:29 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h06BNSi17987 for ; Mon, 6 Jan 2003 06:23:28 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id A1204476260; Mon, 6 Jan 2003 06:23:21 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 0B78D476060; Mon, 6 Jan 2003 06:23:19 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 50277475BA0 for ; Mon, 6 Jan 2003 06:23:14 -0500 (EST) Received: from mail.gne.de (mail.gne.de [213.83.0.2]) by postgresql.org (Postfix) with ESMTP id 27B244758E6 for ; Mon, 6 Jan 2003 06:23:13 -0500 (EST) Received: from DO5GNE-MTA by mail.gne.de with Novell_GroupWise; Mon, 06 Jan 2003 12:23:02 +0100 Message-ID: X-Mailer: Novell GroupWise Internet Agent 6.0.2 Date: Mon, 06 Jan 2003 12:22:57 +0100 From: "Ulrich Neumann" To: " Subject: Re: [HACKERS] Threads MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Content-Disposition: inline X-Guinevere: 1.1.14 ; GNE Grebe Neumann Gl X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR Hello all, it's very interesting to see the discussion of "threads" again. I've portet PostgreSQL to a "thread-per-connection" model based on pthreads and it is functional. Most of the work was finding all the static globals in the sourcefiles and swapping them between threads and freeing memory if a thread terminates. (PostgreSQL isn't written very clean in the aspects of memory handling). My version of the thread-based PostgreSQL is not very efficient at the moment because I haven't done any optimisation of the code to better support threads and I'm using just a simple semaphore to control switching of data but this could be a starting point for others who want to see this code. If this direction will be taken seriously I'm very willing to help. If someone is interested in the code I can send a zip file to everyone who wants. Ulrich ---------------------------------- This e-mail is virus scanned Diese e-mail ist virusgeprueft ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/users-lounge/docs/faq.html From pgsql-hackers-owner+M33824@postgresql.org Mon Jan 6 07:49:46 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h06Cnii03541 for ; Mon, 6 Jan 2003 07:49:44 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id C409E476778; Mon, 6 Jan 2003 07:49:36 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 617C04768C8; Mon, 6 Jan 2003 07:49:01 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id EA9284768AA for ; Mon, 6 Jan 2003 07:48:56 -0500 (EST) Received: from CopelandConsulting.Net (dsl-24293-ld.customer.centurytel.net [209.142.135.135]) by postgresql.org (Postfix) with ESMTP id 2DB74476191 for ; Mon, 6 Jan 2003 07:48:41 -0500 (EST) Received: from [192.168.1.2] (mouse.copelandconsulting.net [192.168.1.2]) by CopelandConsulting.Net (8.10.1/8.10.1) with ESMTP id h06CmL702059; Mon, 6 Jan 2003 06:48:21 -0600 (CST) X-Trade-Id: To: shridhar_daithankar@persistent.co.in cc: " In-Reply-To: <3E19B78B.25689.15BFFE@localhost> References: <3E19B78B.25689.15BFFE@localhost> Content-Type: text/plain Organization: Copeland Computer Consulting Message-ID: <1041857302.17321.49.camel@mouse.copelandconsulting.net> MIME-Version: 1.0 X-Mailer: Ximian Evolution 1.2.0 Date: 06 Jan 2003 06:48:23 -0600 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On Mon, 2003-01-06 at 05:36, Shridhar Daithankar wrote: > On 6 Jan 2003 at 12:22, Ulrich Neumann wrote: > > > Hello all, > > If someone is interested in the code I can send a zip file to everyone > > who wants. > > I suggest you preserver your work. The reason I suggested thread are mainly two > folds. > > 1) Get I/O time used fuitfully AIO may address this without the need for integrated threading. Arguably, from the long thread that last appeared on the topic of AIO, some hold that AIO doesn't even offer anything beyond the current implementation. As such, it's highly doubtful that integrated threading is going to offer anything beyond what a sound AIO implementation can achieve. > 2) Use multiple CPU better. > Multiple processes tend to universally support multiple CPUs better than does threading. On some platforms, the level of threading support is currently only user mode implementations which means no additional CPU use. Furthermore, some platforms where user-mode threads are defacto, they don't even allow for scheduling bias resulting is less work being accomplished within the same time interval (work slice must be divided between n-threads within the process, all of which run on a single CPU). > It will not require as much code cleaning as your efforts might had. However > your work will be very useful if somebody decides to use thread in any fashion > in core postgresql. > > I was hoping for bit more optimistic response given that what I suggested was > totally optional at any point of time but very important from performance > point. Besides the change would have been gradual as required.. > Speaking for my self, I probably would of been more excited if the offered framework had addressed several issues. The short list is: o Code needs to be more robust. It shouldn't be calling exit directly as, I believe, it should be allowing for PostgreSQL to clean up some. Correct me as needed. I would of also expected the code of adopted PostgreSQL's semantics and mechanisms as needed (error reporting, etc). I do understand it was an initial attempt to simply get something in front of some eyes and have something to talk about. Just the same, I was expecting something that we could actually pull the trigger with. o Code isn't very portable. Looked fairly okay for pthread platforms, however, there is new emphasis on the Win32 platform. I think it would be a mistake to introduce something as significant as threading without addressing Win32 from the get-go. o I would desire a more highly abstracted/portable interface which allows for different threading and synchronization primitives to be used. Current implementation is tightly coupled to pthreads. Furthermore, on platforms such as Solaris, I would hope it would easily allow for plugging in its native threading primitives which are touted to be much more efficient than pthreads on said platform. o Code is not commented. I would hope that adding new code for something as important as threading would be commented. o Code is fairly trivial and does not address other primitives (semaphores, mutexs, conditions, TSS, etc) portably which would be required for anything but the most trivial of threaded work. This is especially true in such an application where data IS the application. As such, you must reasonably assume that threads need some form of portable serialization primitives, not to mention mechanisms for non-trivial communication. o Does not address issues such as thread signaling or status reporting. o Pool interface is rather simplistic. Does not currently support concepts such as wake pool, stop pool, pool status, assigning a pool to work, etc. In fact, it's not altogether obvious what the capabilities intent is of the current pool implementation. o Doesn't seem to address any form of thread communication facilities (mailboxes, queues, etc). There are probably other things that I can find if I spend more than just a couple of minutes looking at the code. Honestly, I love threads but I can see that the current code offering is not much more than a token in its current form. No offense meant. After it's all said and done, I'd have to see a lot more meat before I'd be convinced that threading is ready for PostgreSQL; from both a social and technological perspective. Regards, -- Greg Copeland Copeland Computer Consulting ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to majordomo@postgresql.org) From pgsql-hackers-owner+M33899@postgresql.org Tue Jan 7 03:00:25 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h0780Mi00624 for ; Tue, 7 Jan 2003 03:00:23 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 46FA747687C; Tue, 7 Jan 2003 03:00:21 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 69717475F25; Tue, 7 Jan 2003 03:00:13 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 5323E475F39 for ; Tue, 7 Jan 2003 03:00:01 -0500 (EST) Received: from www.pspl.co.in (www.pspl.co.in [202.54.11.65]) by postgresql.org (Postfix) with ESMTP id 351DD475EE1 for ; Tue, 7 Jan 2003 02:59:58 -0500 (EST) Received: (from root@localhost) by www.pspl.co.in (8.11.6/8.11.6) id h077xvs03265 for ; Tue, 7 Jan 2003 13:29:57 +0530 Received: from daithan (daithan.intranet.pspl.co.in [192.168.7.161]) by www.pspl.co.in (8.11.6/8.11.0) with ESMTP id h077xvr03260 for ; Tue, 7 Jan 2003 13:29:57 +0530 From: "Shridhar Daithankar" To: " Date: Tue, 07 Jan 2003 13:30:05 +0530 MIME-Version: 1.0 Subject: Re: [HACKERS] Threads Reply-To: shridhar_daithankar@persistent.co.in Message-ID: <3E1AD65D.10112.192793@localhost> References: <3E19B78B.25689.15BFFE@localhost> In-Reply-To: <1041857302.17321.49.camel@mouse.copelandconsulting.net> X-Mailer: Pegasus Mail for Windows (v4.02) Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT Content-Description: Mail message body X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On 6 Jan 2003 at 6:48, Greg Copeland wrote: > > 1) Get I/O time used fuitfully > AIO may address this without the need for integrated threading. > Arguably, from the long thread that last appeared on the topic of AIO, > some hold that AIO doesn't even offer anything beyond the current > implementation. As such, it's highly doubtful that integrated threading > is going to offer anything beyond what a sound AIO implementation can > achieve. Either way, a complete aio or threading implementation is not available on major platforms that postgresql runs. Linux definitely does not have one, last I checked. If postgresql is not using aio or threading, we should start using one of them, is what I feel. What do you say? > > 2) Use multiple CPU better. > Multiple processes tend to universally support multiple CPUs better than > does threading. On some platforms, the level of threading support is > currently only user mode implementations which means no additional CPU > use. Furthermore, some platforms where user-mode threads are defacto, > they don't even allow for scheduling bias resulting is less work being > accomplished within the same time interval (work slice must be divided > between n-threads within the process, all of which run on a single CPU). The frame-work I have posted, threading is optional at build and should be a configuration option if it gets integrated. So for the platforms that can not spread threads across multiple CPUs, it can simply be turned off.. > Speaking for my self, I probably would of been more excited if the > offered framework had addressed several issues. The short list is: > > o Code needs to be more robust. It shouldn't be calling exit directly > as, I believe, it should be allowing for PostgreSQL to clean up some. > Correct me as needed. I would of also expected the code of adopted > PostgreSQL's semantics and mechanisms as needed (error reporting, etc). > I do understand it was an initial attempt to simply get something in > front of some eyes and have something to talk about. Just the same, I > was expecting something that we could actually pull the trigger with. That could be done. > > o Code isn't very portable. Looked fairly okay for pthread platforms, > however, there is new emphasis on the Win32 platform. I think it would > be a mistake to introduce something as significant as threading without > addressing Win32 from the get-go. If you search for "pthread" in thread.c, there are not many instances. Same goes for thread.h. From what I understand windows threading, it would be less than 10 minutes job to #ifdef the pthread related part on either file. It is just that I have not played with windows threading and nor I am inclined to...;-) > > o I would desire a more highly abstracted/portable interface which > allows for different threading and synchronization primitives to be > used. Current implementation is tightly coupled to pthreads. > Furthermore, on platforms such as Solaris, I would hope it would easily > allow for plugging in its native threading primitives which are touted > to be much more efficient than pthreads on said platform. Same as above. If there can be two cases separated with #ifdef, there can be more.. But what is important is to have a thread that can be woken up as and when required with any function desired. That is the basic idea. > o Code is not commented. I would hope that adding new code for > something as important as threading would be commented. Agreed. > o Code is fairly trivial and does not address other primitives > (semaphores, mutexs, conditions, TSS, etc) portably which would be > required for anything but the most trivial of threaded work. This is > especially true in such an application where data IS the application. > As such, you must reasonably assume that threads need some form of > portable serialization primitives, not to mention mechanisms for > non-trivial communication. I don't get this. Probably I should post a working example. It is not threads responsibility to make a function thread safe which is changed on the fly. The function has to make sure that it is thread safe. That is altogether different effort.. > o Does not address issues such as thread signaling or status reporting. >From what I learnt from pthreads on linux, I would not mix threads and signals. One can easily add code in runner function that disables any signals for thread while the thread starts running. This would leave original signal handling mechanism in place. As far as status reporting is concerned, the thread sould be initiated while back-end starts and terminated with backend termination. What is about status reporting? > o Pool interface is rather simplistic. Does not currently support > concepts such as wake pool, stop pool, pool status, assigning a pool to > work, etc. In fact, it's not altogether obvious what the capabilities > intent is of the current pool implementation. Could you please elaborate? I am using same interface in c++ for a server application and never faced a problem like that..;-) > o Doesn't seem to address any form of thread communication facilities > (mailboxes, queues, etc). Not part of this abstraction of threading mechanism. Intentionally left out to keep things clean. > There are probably other things that I can find if I spend more than > just a couple of minutes looking at the code. Honestly, I love threads > but I can see that the current code offering is not much more than a > token in its current form. No offense meant. None taken. Point is it is useful and that is enough for me. If you could elaborate examples for any problems you see, I can probably modify it. (Code documentation is what I will do now) > After it's all said and done, I'd have to see a lot more meat before I'd > be convinced that threading is ready for PostgreSQL; from both a social > and technological perspective. Tell me about it.. Bye Shridhar -- What's this script do? unzip ; touch ; finger ; mount ; gasp ; yes ; umount ; sleepHint for the answer: not everything is computer-oriented. Sometimes you'rein a sleeping bag, camping out.(Contributed by Frans van der Zande.) ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster From pgsql-hackers-owner+M33921@postgresql.org Tue Jan 7 11:10:53 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h07GApX13277 for ; Tue, 7 Jan 2003 11:10:51 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 5BDC0477200; Tue, 7 Jan 2003 11:06:58 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 9EE41477268; Tue, 7 Jan 2003 11:06:40 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id ACEA5477260 for ; Tue, 7 Jan 2003 11:06:35 -0500 (EST) Received: from CopelandConsulting.Net (dsl-24293-ld.customer.centurytel.net [209.142.135.135]) by postgresql.org (Postfix) with ESMTP id 78B51477165 for ; Tue, 7 Jan 2003 11:06:28 -0500 (EST) Received: from [192.168.1.2] (mouse.copelandconsulting.net [192.168.1.2]) by CopelandConsulting.Net (8.10.1/8.10.1) with ESMTP id h07G68711510; Tue, 7 Jan 2003 10:06:09 -0600 (CST) X-Trade-Id: To: shridhar_daithankar@persistent.co.in cc: " In-Reply-To: <3E1AD65D.10112.192793@localhost> References: <3E19B78B.25689.15BFFE@localhost> <3E1AD65D.10112.192793@localhost> Content-Type: text/plain Organization: Copeland Computer Consulting Message-ID: <1041955572.17639.148.camel@mouse.copelandconsulting.net> MIME-Version: 1.0 X-Mailer: Ximian Evolution 1.2.0 Date: 07 Jan 2003 10:06:12 -0600 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On Tue, 2003-01-07 at 02:00, Shridhar Daithankar wrote: > On 6 Jan 2003 at 6:48, Greg Copeland wrote: > > > 1) Get I/O time used fuitfully > > AIO may address this without the need for integrated threading. > > Arguably, from the long thread that last appeared on the topic of AIO, > > some hold that AIO doesn't even offer anything beyond the current > > implementation. As such, it's highly doubtful that integrated threading > > is going to offer anything beyond what a sound AIO implementation can > > achieve. > > Either way, a complete aio or threading implementation is not available on > major platforms that postgresql runs. Linux definitely does not have one, last > I checked. > There are two or three significant AIO implementation efforts currently underway for Linux. One such implementation is available from the Red Hat Server Edition (IIRC) and has been available for some time now. I believe Oracle is using it. SGI also has an effort and I forget where the other one comes from. Nonetheless, I believe it's going to be a hard fought battle to get AIO implemented simply because I don't think anyone, yet, can truly argue a case on the gain vs effort. > If postgresql is not using aio or threading, we should start using one of them, > is what I feel. What do you say? > I did originally say that I'd like to see an AIO implementation. Then again, I don't current have a position to stand other than simply saying it *might* perform better. ;) Not exactly a position that's going to win the masses over. > > was expecting something that we could actually pull the trigger with. > > That could be done. > I'm sure it can, but that's probably the easiest item to address. > > > > o Code isn't very portable. Looked fairly okay for pthread platforms, > > however, there is new emphasis on the Win32 platform. I think it would > > be a mistake to introduce something as significant as threading without > > addressing Win32 from the get-go. > > If you search for "pthread" in thread.c, there are not many instances. Same > goes for thread.h. From what I understand windows threading, it would be less > than 10 minutes job to #ifdef the pthread related part on either file. > > It is just that I have not played with windows threading and nor I am inclined > to...;-) > Well, the method above is going to create a semi-ugly mess. I've written thread abstraction layers which cover OS/2, NT, and pthreads. Each have subtle distinction. What really needs to be done is the creation of another abstraction layer which your current code would sit on top of. That way, everything contained within is clear and easy to read. The big bonus is that as additional threading implementations need to be added, only the "low-level" abstraction stuff needs to modified. Done properly, each thread implementation would be it's own module requiring little #if clutter. As you can see, that's a fair amount of work and far from where the code currently is. > > > > o I would desire a more highly abstracted/portable interface which > > allows for different threading and synchronization primitives to be > > used. Current implementation is tightly coupled to pthreads. > > Furthermore, on platforms such as Solaris, I would hope it would easily > > allow for plugging in its native threading primitives which are touted > > to be much more efficient than pthreads on said platform. > > Same as above. If there can be two cases separated with #ifdef, there can be > more.. But what is important is to have a thread that can be woken up as and > when required with any function desired. That is the basic idea. > Again, there's a lot of work in creating a well formed abstraction layer for all of the mechanics that are required. Furthermore, different thread implementations have slightly different semantics which further complicates things. Worse, some types of primitives are simply not available with some thread implementations. That means those platforms require it to be written from the primitives that are available on the platform. Yet more work. > > o Code is fairly trivial and does not address other primitives > > (semaphores, mutexs, conditions, TSS, etc) portably which would be > > required for anything but the most trivial of threaded work. This is > > especially true in such an application where data IS the application. > > As such, you must reasonably assume that threads need some form of > > portable serialization primitives, not to mention mechanisms for > > non-trivial communication. > > I don't get this. Probably I should post a working example. It is not threads > responsibility to make a function thread safe which is changed on the fly. The > function has to make sure that it is thread safe. That is altogether different > effort.. You're right, it's not the thread's responsibility, however, it is the threading toolkit's. In this case, you're offering to be the toolkit which functions across two platforms, just for starters. Reasonably, you should expect a third to quickly follow. > > > o Does not address issues such as thread signaling or status reporting. > > >From what I learnt from pthreads on linux, I would not mix threads and signals. > One can easily add code in runner function that disables any signals for thread > while the thread starts running. This would leave original signal handling > mechanism in place. > > As far as status reporting is concerned, the thread sould be initiated while > back-end starts and terminated with backend termination. What is about status > reporting? > > > o Pool interface is rather simplistic. Does not currently support > > concepts such as wake pool, stop pool, pool status, assigning a pool to > > work, etc. In fact, it's not altogether obvious what the capabilities > > intent is of the current pool implementation. > > Could you please elaborate? I am using same interface in c++ for a server > application and never faced a problem like that..;-) > > > > o Doesn't seem to address any form of thread communication facilities > > (mailboxes, queues, etc). > > Not part of this abstraction of threading mechanism. Intentionally left out to > keep things clean. > > > There are probably other things that I can find if I spend more than > > just a couple of minutes looking at the code. Honestly, I love threads > > but I can see that the current code offering is not much more than a > > token in its current form. No offense meant. > > None taken. Point is it is useful and that is enough for me. If you could > elaborate examples for any problems you see, I can probably modify it. (Code > documentation is what I will do now) > > > After it's all said and done, I'd have to see a lot more meat before I'd > > be convinced that threading is ready for PostgreSQL; from both a social > > and technological perspective. > > Tell me about it.. > Long story short, if PostgreSQL is to use threads, it shouldn't be handicapped by having a very limited subset of functionality. With the code that has been currently submitted, I don't believe you could even effectively implement a parallel sort. To get an idea of the types of things that would be needed, check out the ACE Toolkit. There are a couple of other fairly popular toolkits as well. Nonetheless, it's a significant effort and the current code is a long ways off from being usable. -- Greg Copeland Copeland Computer Consulting ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/users-lounge/docs/faq.html From pgsql-hackers-owner+M33944@postgresql.org Tue Jan 7 13:22:04 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h07IM2X05350 for ; Tue, 7 Jan 2003 13:22:02 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 544EF476AC1; Tue, 7 Jan 2003 13:22:05 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 341134761E8; Tue, 7 Jan 2003 13:21:55 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 48974475ADE for ; Tue, 7 Jan 2003 13:21:50 -0500 (EST) Received: from sabre.velocet.net (sabre.velocet.net [216.138.209.205]) by postgresql.org (Postfix) with ESMTP id B8D40475AD7 for ; Tue, 7 Jan 2003 13:21:49 -0500 (EST) Received: from stark.dyndns.tv (H162.C233.tor.velocet.net [216.138.233.162]) by sabre.velocet.net (Postfix) with ESMTP id 887681382B9; Tue, 7 Jan 2003 13:21:48 -0500 (EST) Received: from localhost ([127.0.0.1] helo=stark.dyndns.tv ident=foobar) by stark.dyndns.tv with smtp (Exim 3.36 #1 (Debian)) id 18VyMF-0002zN-00; Tue, 07 Jan 2003 13:21:47 -0500 To: Greg Copeland cc: kar@kakidata.dk, PGHackers Subject: Re: [HACKERS] Threads References: <23763.1041644042@sss.pgh.pa.us> <1041646276.15927.202.camel@mouse.copelandconsulting.net> <200301041359.35715.kar@kakidata.dk> <1041704480.15927.224.camel@mouse.copelandconsulting.net> In-Reply-To: <1041704480.15927.224.camel@mouse.copelandconsulting.net> From: Greg Stark Organization: The Emacs Conspiracy; member since 1992 Date: 07 Jan 2003 13:21:47 -0500 Message-ID: <87isx0izwk.fsf@stark.dyndns.tv> Lines: 43 User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.2 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR Greg Copeland writes: > That's the power of using the process model that is currently in use. Should > it do something naughty, we bitch and complain politely, throw our hands in > the air and exit. We no longer have to worry about the state and validity of > that backend. You missed the point of his post. If one process in your database does something nasty you damn well should worry about the state of and validity of the entire database, not just that one backend. Are you really sure you caught the problem before it screwed up the data in shared memory? On disk? This whole topic is in need of some serious FUD-dispelling and careful analysis. Here's a more calm explanation of the situation on this particular point. Perhaps I'll follow up with something on IO concurrency later. The point in consideration here is really memory isolation. Threads by default have zero isolation between threads. They can all access each other's memory even including their stack. Most of that memory is in fact only needed by a single thread. Processes by default have complete memory isolation. However postgres actually weakens that by doing a lot of work in a shared memory pool. That memory gets exactly the same protection as it would get in a threaded model, which is to say none. So the reality is that if you have a bug most likely you've only corrupted the local data which can be easily cleaned up either way. In the thread model there's also the unlikely but scary risk that you've damaged other threads' memory. And in either case there's the possibility that you've damaged the shared pool which is unrecoverable. In theory minimising the one case of corrupting other threads' local data shouldn't make a big difference to the risk in the case of an assertion failure. I'm not sure in practice if that's true though. Processes probably reduce the temptation to do work in the shared area too. -- greg ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to majordomo@postgresql.org) From pgsql-hackers-owner+M33945@postgresql.org Tue Jan 7 13:48:12 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h07Im8X15155 for ; Tue, 7 Jan 2003 13:48:08 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id BF0454773D3; Tue, 7 Jan 2003 13:43:10 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 781634773A6; Tue, 7 Jan 2003 13:43:03 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 28074477390 for ; Tue, 7 Jan 2003 13:42:59 -0500 (EST) Received: from CopelandConsulting.Net (dsl-24293-ld.customer.centurytel.net [209.142.135.135]) by postgresql.org (Postfix) with ESMTP id 1392B476682 for ; Tue, 7 Jan 2003 13:42:42 -0500 (EST) Received: from [192.168.1.2] (mouse.copelandconsulting.net [192.168.1.2]) by CopelandConsulting.Net (8.10.1/8.10.1) with ESMTP id h07IgS715128; Tue, 7 Jan 2003 12:42:28 -0600 (CST) X-Trade-Id: To: Greg Stark cc: kar@kakidata.dk, PGHackers In-Reply-To: <87isx0izwk.fsf@stark.dyndns.tv> References: <23763.1041644042@sss.pgh.pa.us> <1041646276.15927.202.camel@mouse.copelandconsulting.net> <200301041359.35715.kar@kakidata.dk> <1041704480.15927.224.camel@mouse.copelandconsulting.net> <87isx0izwk.fsf@stark.dyndns.tv> Content-Type: text/plain Organization: Copeland Computer Consulting Message-ID: <1041964952.29180.10.camel@mouse.copelandconsulting.net> MIME-Version: 1.0 X-Mailer: Ximian Evolution 1.2.0 Date: 07 Jan 2003 12:42:33 -0600 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR On Tue, 2003-01-07 at 12:21, Greg Stark wrote: > Greg Copeland writes: > > > That's the power of using the process model that is currently in use. Should > > it do something naughty, we bitch and complain politely, throw our hands in > > the air and exit. We no longer have to worry about the state and validity of > > that backend. > > You missed the point of his post. If one process in your database does > something nasty you damn well should worry about the state of and validity of > the entire database, not just that one backend. > I can assure you I did not miss the point. No idea why you're continuing to spell it out. In this case, it appears the quotation is being taken out of context or it was originally stated in an improper context. > Are you really sure you caught the problem before it screwed up the data in > shared memory? On disk? > > > This whole topic is in need of some serious FUD-dispelling and careful > analysis. Here's a more calm explanation of the situation on this particular > point. Perhaps I'll follow up with something on IO concurrency later. > Hmmm. Not sure what needs to be dispelled since I've not seen any FUD. > The point in consideration here is really memory isolation. Threads by default > have zero isolation between threads. They can all access each other's memory > even including their stack. Most of that memory is in fact only needed by a > single thread. > Again, this has been covered already. > Processes by default have complete memory isolation. However postgres actually > weakens that by doing a lot of work in a shared memory pool. That memory gets > exactly the same protection as it would get in a threaded model, which is to > say none. > Again, this has all been covered, more or less. You're comments seem to imply that you did not fully read what has been said on the topic thus far or that you misunderstood something that was said. Of course, it's also possible that I may of said something out of it's proper context which may be confusing you. I think it's safe to say I don't have any further comment unless something new is being brought to the table. Should there be something new to cover, I'm happy to talk about it. At this point, however, it appears that it's been beat to death already. -- Greg Copeland Copeland Computer Consulting ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@postgresql.org so that your message can get through to the mailing list cleanly From pgsql-hackers-owner+M33946@postgresql.org Tue Jan 7 14:02:33 2003 Return-path: Received: from postgresql.org (postgresql.org [64.49.215.8]) by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h07J2TX22478 for ; Tue, 7 Jan 2003 14:02:30 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 6A905477204; Tue, 7 Jan 2003 14:02:32 -0500 (EST) Received: from postgresql.org (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with SMTP id 3546E476688; Tue, 7 Jan 2003 14:02:21 -0500 (EST) Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id E3CC44760BD for ; Tue, 7 Jan 2003 14:02:14 -0500 (EST) Received: from sss.pgh.pa.us (unknown [192.204.191.242]) by postgresql.org (Postfix) with ESMTP id 3D8FA475AD7 for ; Tue, 7 Jan 2003 14:02:14 -0500 (EST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss.pgh.pa.us (8.12.6/8.12.6) with ESMTP id h07J1s0U019750; Tue, 7 Jan 2003 14:01:54 -0500 (EST) To: Greg Stark cc: Greg Copeland , kar@kakidata.dk, PGHackers Subject: Re: [HACKERS] Threads In-Reply-To: <87isx0izwk.fsf@stark.dyndns.tv> References: <23763.1041644042@sss.pgh.pa.us> <1041646276.15927.202.camel@mouse.copelandconsulting.net> <200301041359.35715.kar@kakidata.dk> <1041704480.15927.224.camel@mouse.copelandconsulting.net> <87isx0izwk.fsf@stark.dyndns.tv> Comments: In-reply-to Greg Stark message dated "07 Jan 2003 13:21:47 -0500" Date: Tue, 07 Jan 2003 14:01:53 -0500 Message-ID: <19749.1041966113@sss.pgh.pa.us> From: Tom Lane X-Virus-Scanned: by AMaViS new-20020517 Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org X-Virus-Scanned: by AMaViS new-20020517 Status: OR Greg Stark writes: > You missed the point of his post. If one process in your database does > something nasty you damn well should worry about the state of and validity of > the entire database, not just that one backend. Right. And in fact we do blow away all the processes when any one of them crashes or panics. Nonetheless, memory isolation between processes is a Good Thing, because it reduces the chances that a process gone wrong will cause damage via other processes before they can be shut down. Here is a simple example of a scenario where that isolation buys us something: suppose that we have a bug that tromps on memory starting at some point X until it falls off the sbrk boundary and dumps core. (There are plenty of ways to make that happen, such as miscalculating the length of a memcpy or memset operation as -1.) Such a bug causes no serious damage in isolation, because the process suffering the failure will be in a tight data-copying or data-zeroing loop until it gets the SIGSEGV exception. It won't do anything bad based on all the data structures it has clobbered during its march to the end of memory. However, put that same bug in a multithreading context, and it becomes entirely possible that some other thread will be dispatched and will try to make use of already-clobbered data structures before the ultimate SIGSEGV exception happens. Now you have the potential for unlimited trouble. In general, isolation buys you some safety anytime there is a delay between the occurrence of a failure and its detection. > Processes by default have complete memory isolation. However postgres > actually weakens that by doing a lot of work in a shared memory > pool. That memory gets exactly the same protection as it would get in > a threaded model, which is to say none. Yes. We try to minimize the risk by keeping the shared memory pool relatively small and not doing more than we have to in it. (For example, this was one of the arguments against creating a shared plan cache.) It's also very helpful that in most platforms, shared memory is not address-wise contiguous to normal memory; thus for example a process caught in a memset death march will hit a SIGSEGV before it gets to the shared memory block. It's interesting to note that this can be made into an argument for not making shared_buffers very large: the larger the fraction of your address space that the shared buffers occupy, the larger the chance that a wild store will overwrite something you'd wish it didn't. I can't recall anyone having made that point during our many discussions of appropriate shared_buffer sizing. > So the reality is that if you have a bug most likely you've only corrupted the > local data which can be easily cleaned up either way. In the thread model > there's also the unlikely but scary risk that you've damaged other threads' > memory. And in either case there's the possibility that you've damaged the > shared pool which is unrecoverable. In a thread model, *most* of the accessible memory space would be shared with other threads, at least potentially. So I think you're wrong to categorize the second case as unlikely. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org