From 13453eedd3f692f8dcf8e334396eee84f00fdde2 Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Sun, 7 Apr 2024 09:13:17 +1200 Subject: [PATCH] Add pg_buffercache_evict() function for testing. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit When testing buffer pool logic, it is useful to be able to evict arbitrary blocks. This function can be used in SQL queries over the pg_buffercache view to set up a wide range of buffer pool states. Of course, buffer mappings might change concurrently so you might evict a block other than the one you had in mind, and another session might bring it back in at any time. That's OK for the intended purpose of setting up developer testing scenarios, and more complicated interlocking schemes to give stronger guararantees about that would likely be less flexible for actual testing work anyway. Superuser-only. Author: Palak Chaturvedi Author: Thomas Munro (docs, small tweaks) Reviewed-by: Nitin Jadhav Reviewed-by: Andres Freund Reviewed-by: Cary Huang Reviewed-by: Cédric Villemain Reviewed-by: Jim Nasby Reviewed-by: Maxim Orlov Reviewed-by: Thomas Munro Reviewed-by: Melanie Plageman Discussion: https://postgr.es/m/CALfch19pW48ZwWzUoRSpsaV9hqt0UPyaBPC4bOZ4W+c7FF566A@mail.gmail.com --- contrib/pg_buffercache/Makefile | 2 +- contrib/pg_buffercache/meson.build | 1 + .../pg_buffercache--1.4--1.5.sql | 6 ++ contrib/pg_buffercache/pg_buffercache.control | 2 +- contrib/pg_buffercache/pg_buffercache_pages.c | 20 ++++++ doc/src/sgml/pgbuffercache.sgml | 39 ++++++++++-- src/backend/storage/buffer/bufmgr.c | 63 +++++++++++++++++++ src/include/storage/bufmgr.h | 2 + 8 files changed, 127 insertions(+), 8 deletions(-) create mode 100644 contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql diff --git a/contrib/pg_buffercache/Makefile b/contrib/pg_buffercache/Makefile index d6b58d4da9..eae65ead9e 100644 --- a/contrib/pg_buffercache/Makefile +++ b/contrib/pg_buffercache/Makefile @@ -8,7 +8,7 @@ OBJS = \ EXTENSION = pg_buffercache DATA = pg_buffercache--1.2.sql pg_buffercache--1.2--1.3.sql \ pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql \ - pg_buffercache--1.3--1.4.sql + pg_buffercache--1.3--1.4.sql pg_buffercache--1.4--1.5.sql PGFILEDESC = "pg_buffercache - monitoring of shared buffer cache in real-time" REGRESS = pg_buffercache diff --git a/contrib/pg_buffercache/meson.build b/contrib/pg_buffercache/meson.build index c86e33cc95..1ca3452918 100644 --- a/contrib/pg_buffercache/meson.build +++ b/contrib/pg_buffercache/meson.build @@ -22,6 +22,7 @@ install_data( 'pg_buffercache--1.2--1.3.sql', 'pg_buffercache--1.2.sql', 'pg_buffercache--1.3--1.4.sql', + 'pg_buffercache--1.4--1.5.sql', 'pg_buffercache.control', kwargs: contrib_data_args, ) diff --git a/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql b/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql new file mode 100644 index 0000000000..0fb18ff786 --- /dev/null +++ b/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql @@ -0,0 +1,6 @@ +\echo Use "ALTER EXTENSION pg_buffercache UPDATE TO '1.5'" to load this file. \quit + +CREATE FUNCTION pg_buffercache_evict(IN int) +RETURNS bool +AS 'MODULE_PATHNAME', 'pg_buffercache_evict' +LANGUAGE C PARALLEL SAFE VOLATILE STRICT; diff --git a/contrib/pg_buffercache/pg_buffercache.control b/contrib/pg_buffercache/pg_buffercache.control index a82ae5f9bb..5ee875f77d 100644 --- a/contrib/pg_buffercache/pg_buffercache.control +++ b/contrib/pg_buffercache/pg_buffercache.control @@ -1,5 +1,5 @@ # pg_buffercache extension comment = 'examine the shared buffer cache' -default_version = '1.4' +default_version = '1.5' module_pathname = '$libdir/pg_buffercache' relocatable = true diff --git a/contrib/pg_buffercache/pg_buffercache_pages.c b/contrib/pg_buffercache/pg_buffercache_pages.c index 3316732365..3ae0a018e1 100644 --- a/contrib/pg_buffercache/pg_buffercache_pages.c +++ b/contrib/pg_buffercache/pg_buffercache_pages.c @@ -63,6 +63,7 @@ typedef struct PG_FUNCTION_INFO_V1(pg_buffercache_pages); PG_FUNCTION_INFO_V1(pg_buffercache_summary); PG_FUNCTION_INFO_V1(pg_buffercache_usage_counts); +PG_FUNCTION_INFO_V1(pg_buffercache_evict); Datum pg_buffercache_pages(PG_FUNCTION_ARGS) @@ -347,3 +348,22 @@ pg_buffercache_usage_counts(PG_FUNCTION_ARGS) return (Datum) 0; } + +/* + * Try to evict a shared buffer. + */ +Datum +pg_buffercache_evict(PG_FUNCTION_ARGS) +{ + Buffer buf = PG_GETARG_INT32(0); + + if (!superuser()) + ereport(ERROR, + (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE), + errmsg("must be superuser to use pg_buffercache_evict function"))); + + if (buf < 1 || buf > NBuffers) + elog(ERROR, "bad buffer ID: %d", buf); + + PG_RETURN_BOOL(EvictUnpinnedBuffer(buf)); +} diff --git a/doc/src/sgml/pgbuffercache.sgml b/doc/src/sgml/pgbuffercache.sgml index afe2d97834..4b90eefc0b 100644 --- a/doc/src/sgml/pgbuffercache.sgml +++ b/doc/src/sgml/pgbuffercache.sgml @@ -11,6 +11,8 @@ The pg_buffercache module provides a means for examining what's happening in the shared buffer cache in real time. + It also offers a low-level way to evict data from it, for testing + purposes. @@ -21,11 +23,16 @@ pg_buffercache_summary + + pg_buffercache_evict + + This module provides the pg_buffercache_pages() function (wrapped in the pg_buffercache view), - the pg_buffercache_summary() function, and the - pg_buffercache_usage_counts() function. + the pg_buffercache_summary() function, the + pg_buffercache_usage_counts() function and + the pg_buffercache_evict() function. @@ -47,9 +54,15 @@ - By default, use is restricted to superusers and roles with privileges of the - pg_monitor role. Access may be granted to others - using GRANT. + By default, use of the above functions is restricted to superusers and roles + with privileges of the pg_monitor role. Access may be + granted to others using GRANT. + + + + The pg_buffercache_evict() function allows a block to + be evicted from the buffer pool given a buffer identifier. Use of this + function is restricted to superusers only. @@ -351,7 +364,21 @@ - + + The <structname>pg_buffercache_evict</structname> Function + + The pg_buffercache_evict() function takes a buffer + identifier, as shown in the bufferid column of + the pg_buffercache view. It returns true on success, + and false if the buffer wasn't valid, if it couldn't be evicted because it + was pinned, or if it became dirty again after an attempt to write it out. + The result is immediately out of date upon return, as the buffer might + become valid again at any time due to concurrent activity. The function is + intended for developer testing only. + + + + Sample Output diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index 06e9ffd2b0..44836751b7 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -6003,3 +6003,66 @@ ResOwnerPrintBufferPin(Datum res) { return DebugPrintBufferRefcount(DatumGetInt32(res)); } + +/* + * Try to evict the current block in a shared buffer. + * + * This function is intended for testing/development use only! + * + * To succeed, the buffer must not be pinned on entry, so if the caller had a + * particular block in mind, it might already have been replaced by some other + * block by the time this function runs. It's also unpinned on return, so the + * buffer might be occupied again by the time control is returned, potentially + * even by the same block. This inherent raciness without other interlocking + * makes the function unsuitable for non-testing usage. + * + * Returns true if the buffer was valid and it has now been made invalid. + * Returns false if it wasn't valid, if it couldn't be evicted due to a pin, + * or if the buffer becomes dirty again while we're trying to write it out. + */ +bool +EvictUnpinnedBuffer(Buffer buf) +{ + BufferDesc *desc; + uint32 buf_state; + bool result; + + /* Make sure we can pin the buffer. */ + ResourceOwnerEnlarge(CurrentResourceOwner); + ReservePrivateRefCountEntry(); + + Assert(!BufferIsLocal(buf)); + desc = GetBufferDescriptor(buf - 1); + + /* Lock the header and check if it's valid. */ + buf_state = LockBufHdr(desc); + if ((buf_state & BM_VALID) == 0) + { + UnlockBufHdr(desc, buf_state); + return false; + } + + /* Check that it's not pinned already. */ + if (BUF_STATE_GET_REFCOUNT(buf_state) > 0) + { + UnlockBufHdr(desc, buf_state); + return false; + } + + PinBuffer_Locked(desc); /* releases spinlock */ + + /* If it was dirty, try to clean it once. */ + if (buf_state & BM_DIRTY) + { + LWLockAcquire(BufferDescriptorGetContentLock(desc), LW_SHARED); + FlushBuffer(desc, NULL, IOOBJECT_RELATION, IOCONTEXT_NORMAL); + LWLockRelease(BufferDescriptorGetContentLock(desc)); + } + + /* This will return false if it becomes dirty or someone else pins it. */ + result = InvalidateVictimBuffer(desc); + + UnpinBuffer(desc); + + return result; +} diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h index 07ba1a6050..42211bfec4 100644 --- a/src/include/storage/bufmgr.h +++ b/src/include/storage/bufmgr.h @@ -305,6 +305,8 @@ extern bool BgBufferSync(struct WritebackContext *wb_context); extern void LimitAdditionalPins(uint32 *additional_pins); extern void LimitAdditionalLocalPins(uint32 *additional_pins); +extern bool EvictUnpinnedBuffer(Buffer buf); + /* in buf_init.c */ extern void InitBufferPool(void); extern Size BufferShmemSize(void);