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

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.

1 files changed, 24 insertions, 15 deletions

diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c
index 306ffe45769..ad2bcf8dac6 100644
--- a/src/backend/storage/buffer/localbuf.c
+++ b/src/backend/storage/buffer/localbuf.c
@@ -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)
-		{
-			LocalRefCount[b]++;
-			ResourceOwnerRememberBuffer(CurrentResourceOwner,
-										BufferDescriptorGetBuffer(bufHdr));
-			break;
-		}
-
-		if (bufHdr->usage_count > 0)
+		if (LocalRefCount[b] == 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;