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Make large sequential scans and VACUUMs work in a limited-size "ring" of

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buffers, rather than blowing out the whole shared-buffer arena.  Aside from
avoiding cache spoliation, this fixes the problem that VACUUM formerly tended
to cause a WAL flush for every page it modified, because we had it hacked to
use only a single buffer.  Those flushes will now occur only once per
ring-ful.  The exact ring size, and the threshold for seqscans to switch into
the ring usage pattern, remain under debate; but the infrastructure seems
done.  The key bit of infrastructure is a new optional BufferAccessStrategy
object that can be passed to ReadBuffer operations; this replaces the former
StrategyHintVacuum API.

This patch also changes the buffer usage-count methodology a bit: we now
advance usage_count when first pinning a buffer, rather than when last
unpinning it.  To preserve the behavior that a buffer's lifetime starts to
decrease when it's released, the clock sweep code is modified to not decrement
usage_count of pinned buffers.

Work not done in this commit: teach GiST and GIN indexes to use the vacuum
BufferAccessStrategy for vacuum-driven fetches.

Original patch by Simon, reworked by Heikki and again by Tom.
This commit is contained in:
Tom Lane
2007-05-30 20:12:03 +00:00
parent 0a6f2ee84d
commit d526575f89
24 changed files with 722 additions and 262 deletions
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15
src/include/storage/bufmgr.h
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@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/storage/bufmgr.h,v 1.103 2007/05/02 23:18:03 tgl Exp $
* $PostgreSQL: pgsql/src/include/storage/bufmgr.h,v 1.104 2007/05/30 20:12:03 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -19,6 +19,14 @@
typedef void *Block;
/* Possible arguments for GetAccessStrategy() */
typedef enum BufferAccessStrategyType
{
BAS_NORMAL, /* Normal random access */
BAS_BULKREAD, /* Large read-only scan (hint bit updates are ok) */
BAS_VACUUM /* VACUUM */
} BufferAccessStrategyType;
/* in globals.c ... this duplicates miscadmin.h */
extern DLLIMPORT int NBuffers;
@@ -111,6 +119,8 @@ extern DLLIMPORT int32 *LocalRefCount;
* prototypes for functions in bufmgr.c
*/
extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
extern Buffer ReadBufferWithStrategy(Relation reln, BlockNumber blockNum,
BufferAccessStrategy strategy);
extern Buffer ReadOrZeroBuffer(Relation reln, BlockNumber blockNum);
extern void ReleaseBuffer(Buffer buffer);
extern void UnlockReleaseBuffer(Buffer buffer);
@@ -157,6 +167,7 @@ extern void BgBufferSync(void);
extern void AtProcExit_LocalBuffers(void);
/* in freelist.c */
extern void StrategyHintVacuum(bool vacuum_active);
extern BufferAccessStrategy GetAccessStrategy(BufferAccessStrategyType btype);
extern void FreeAccessStrategy(BufferAccessStrategy strategy);
#endif