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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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39
src/backend/storage/buffer/localbuf.c
View File

@@ -9,7 +9,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/buffer/localbuf.c,v 1.76 2007/01/05 22:19:37 momjian Exp $
* $PostgreSQL: pgsql/src/backend/storage/buffer/localbuf.c,v 1.77 2007/05/30 20:11:59 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -57,7 +57,8 @@ static Block GetLocalBufferStorage(void);
*
* API is similar to bufmgr.c's BufferAlloc, except that we do not need
* to do any locking since this is all local. Also, IO_IN_PROGRESS
* does not get set.
* does not get set. Lastly, we support only default access strategy
* (hence, usage_count is always advanced).
*/
BufferDesc *
LocalBufferAlloc(Relation reln, BlockNumber blockNum, bool *foundPtr)
@@ -88,7 +89,12 @@ LocalBufferAlloc(Relation reln, BlockNumber blockNum, bool *foundPtr)
fprintf(stderr, "LB ALLOC (%u,%d) %d\n",
RelationGetRelid(reln), blockNum, -b - 1);
#endif
/* this part is equivalent to PinBuffer for a shared buffer */
if (LocalRefCount[b] == 0)
{
if (bufHdr->usage_count < BM_MAX_USAGE_COUNT)
bufHdr->usage_count++;
}
LocalRefCount[b]++;
ResourceOwnerRememberBuffer(CurrentResourceOwner,
BufferDescriptorGetBuffer(bufHdr));
@@ -121,18 +127,21 @@ LocalBufferAlloc(Relation reln, BlockNumber blockNum, bool *foundPtr)
bufHdr = &LocalBufferDescriptors[b];
if (LocalRefCount[b] == 0 && bufHdr->usage_count == 0)
if (LocalRefCount[b] == 0)
{
LocalRefCount[b]++;
ResourceOwnerRememberBuffer(CurrentResourceOwner,
BufferDescriptorGetBuffer(bufHdr));
break;
}
if (bufHdr->usage_count > 0)
{
bufHdr->usage_count--;
trycounter = NLocBuffer;
if (bufHdr->usage_count > 0)
{
bufHdr->usage_count--;
trycounter = NLocBuffer;
}
else
{
/* Found a usable buffer */
LocalRefCount[b]++;
ResourceOwnerRememberBuffer(CurrentResourceOwner,
BufferDescriptorGetBuffer(bufHdr));
break;
}
}
else if (--trycounter == 0)
ereport(ERROR,
@@ -199,7 +208,7 @@ LocalBufferAlloc(Relation reln, BlockNumber blockNum, bool *foundPtr)
bufHdr->tag = newTag;
bufHdr->flags &= ~(BM_VALID | BM_DIRTY | BM_JUST_DIRTIED | BM_IO_ERROR);
bufHdr->flags |= BM_TAG_VALID;
bufHdr->usage_count = 0;
bufHdr->usage_count = 1;
*foundPtr = FALSE;
return bufHdr;