No longer need TODO.detail/lock.
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Bruce Momjian
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From owner-pgsql-hackers@hub.org Sat Dec 18 17:22:09 1999
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Date: Sat, 18 Dec 1999 16:14:50 -0700 (MST)
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From: Brook Milligan <brook@biology.nmsu.edu>
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To: pgman@candle.pha.pa.us
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CC: peter_e@gmx.net, pgsql-hackers@postgreSQL.org
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In-reply-to: <199912182026.PAA05926@candle.pha.pa.us> (message from Bruce
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Momjian on Sat, 18 Dec 1999 15:26:15 -0500 (EST))
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Subject: Re: [HACKERS] Re: [PATCHES] Lock
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References: <199912182026.PAA05926@candle.pha.pa.us>
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Sender: owner-pgsql-hackers@postgreSQL.org
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Status: OR
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> > * Allow LOCK TABLE tab1, tab2, tab3 so all tables locked in unison
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Let me add to this. One problem is that my description would sometimes
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lock the tables in different orders, and that is a recipe for deadlock.
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If you have to release earlier locks to wait on a later lock, once you
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get the later lock, you must release it and then start from the
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beginning, locking them in order again. If you don't, the system could
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report a deadlock at random times, which would be very bad.
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I'll add something, too. :) I think this derived from a suggestion I
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made long ago. My idea was that when multiple tables need locking, a
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deadlock can occur in the process of doing them one at a time. My
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suggested solution was based on an analogy with the way ethernet
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packets work.
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- go through the list locking tables along the way.
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- if a lock cannot be obtained within some time, release some (all?) locks,
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and try again after some random time.
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- keep trying (and releasing as needed) until some other timeout
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passes, and then punt.
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My thought was that if colliding locks are occuring, some sequence of
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relinquishing locks (not necessarily all of them with each trial),
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waiting, and reasserting them should work around the collisions.
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Introducing random components to this might reduce the overall waiting
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time, but I suppose a careful analysis of this needs to be done.
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Perhaps just releasing all of the locks, waiting a random time, and
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trying again is enough.
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Somehow there has to be a mechanism for atomically asserting locks on
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more than one table.
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Cheers,
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Brook
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************
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From owner-pgsql-patches@hub.org Sat Dec 18 22:51:06 1999
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Sat, 18 Dec 1999 21:12:09 -0800 (PST)
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Date: Sat, 18 Dec 1999 21:12:09 -0800 (PST)
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From: Alfred Perlstein <bright@wintelcom.net>
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To: Bruce Momjian <pgman@candle.pha.pa.us>
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cc: Peter Eisentraut <peter_e@gmx.net>, patches@postgreSQL.org
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Subject: Re: [PATCHES] Lock
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In-Reply-To: <199912181828.NAA01486@candle.pha.pa.us>
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Status: OR
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On Sat, 18 Dec 1999, Bruce Momjian wrote:
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> [Charset ISO-8859-1 unsupported, filtering to ASCII...]
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> > I was looking at this
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> >
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> > * Allow LOCK TABLE tab1, tab2, tab3 so all tables locked in unison
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> >
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> > but I'm not sure if my solution is really what was wanted, because it
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> > doesn't actually guarantee an all-or-nothing lock, it just locks each
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> > table in order. Thus it's more like a syntax simplification and reduces
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> > overhead.
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> >
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>
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> It took a few minutes, but I remember the use for this. If you are
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> going to hang waiting to lock tab3, you don't want to lock tab1 and tab2
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> while you are waiting for tab3 lock. The user wanted all tables to lock
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> in one operation without holding locks while waiting to complete all
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> locking.
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>
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> Can you do the locks, and if one fails, not hang, but unlock the
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> previous tables, go lock/hang on the failure, and go back and lock the
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> others? Seems it would have to be some kind of lock/fail/unlock/wait
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> loop.
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>
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> Does this make sense? It did to me.
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Guys, have a look at:
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http://www.freebsd.org/~terry/iml.txt
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http://jazz.external.hp.com/training/sqltables/c5s17.html
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It's a way to do locking with deadlock detection, and without loosing
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your place in line for locks, very nifty imo.
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-Alfred
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************
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