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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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38
src/backend/access/heap/heapam.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.233 2007/05/27 03:50:38 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.234 2007/05/30 20:11:53 tgl Exp $
*
*
* INTERFACE ROUTINES
@ -83,6 +83,24 @@ initscan(HeapScanDesc scan, ScanKey key)
*/
scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_rd);
/*
* If the table is large relative to NBuffers, use a bulk-read access
* strategy, else use the default random-access strategy. During a
* rescan, don't make a new strategy object if we don't have to.
*/
if (scan->rs_nblocks > NBuffers / 4 &&
!scan->rs_rd->rd_istemp)
{
if (scan->rs_strategy == NULL)
scan->rs_strategy = GetAccessStrategy(BAS_BULKREAD);
}
else
{
if (scan->rs_strategy != NULL)
FreeAccessStrategy(scan->rs_strategy);
scan->rs_strategy = NULL;
}
scan->rs_inited = false;
scan->rs_ctup.t_data = NULL;
ItemPointerSetInvalid(&scan->rs_ctup.t_self);
@ -123,9 +141,17 @@ heapgetpage(HeapScanDesc scan, BlockNumber page)
Assert(page < scan->rs_nblocks);
scan->rs_cbuf = ReleaseAndReadBuffer(scan->rs_cbuf,
scan->rs_rd,
page);
/* release previous scan buffer, if any */
if (BufferIsValid(scan->rs_cbuf))
{
ReleaseBuffer(scan->rs_cbuf);
scan->rs_cbuf = InvalidBuffer;
}
/* read page using selected strategy */
scan->rs_cbuf = ReadBufferWithStrategy(scan->rs_rd,
page,
scan->rs_strategy);
scan->rs_cblock = page;
if (!scan->rs_pageatatime)
@ -938,6 +964,7 @@ heap_beginscan(Relation relation, Snapshot snapshot,
scan->rs_rd = relation;
scan->rs_snapshot = snapshot;
scan->rs_nkeys = nkeys;
scan->rs_strategy = NULL; /* set in initscan */
/*
* we can use page-at-a-time mode if it's an MVCC-safe snapshot
@ -1007,6 +1034,9 @@ heap_endscan(HeapScanDesc scan)
if (scan->rs_key)
pfree(scan->rs_key);
if (scan->rs_strategy != NULL)
FreeAccessStrategy(scan->rs_strategy);
pfree(scan);
}