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.

4 files changed, 14 insertions, 6 deletions

diff --git a/src/include/access/genam.h b/src/include/access/genam.h
index 1f31baf0e4..98296e62be 100644
--- a/src/include/access/genam.h
+++ b/src/include/access/genam.h
@@ -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/access/genam.h,v 1.66 2007/01/05 22:19:50 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/access/genam.h,v 1.67 2007/05/30 20:12:02 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -40,6 +40,7 @@ typedef struct IndexVacuumInfo
 	bool		vacuum_full;	/* VACUUM FULL (we have exclusive lock) */
 	int			message_level;	/* ereport level for progress messages */
 	double		num_heap_tuples;	/* tuples remaining in heap */
+	BufferAccessStrategy strategy;	/* access strategy for reads */
 } IndexVacuumInfo;
 
 /*
diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index d382ee6ee9..2bd314a8aa 100644
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -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/access/hash.h,v 1.80 2007/05/03 16:45:58 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/hash.h,v 1.81 2007/05/30 20:12:02 tgl Exp $
  *
  * NOTES
  *		modeled after Margo Seltzer's hash implementation for unix.
@@ -273,11 +273,13 @@ extern void _hash_doinsert(Relation rel, IndexTuple itup);
 
 /* hashovfl.c */
 extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf);
-extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf);
+extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf,
+									  BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
 				 BlockNumber blkno);
 extern void _hash_squeezebucket(Relation rel,
-					Bucket bucket, BlockNumber bucket_blkno);
+								Bucket bucket, BlockNumber bucket_blkno,
+								BufferAccessStrategy bstrategy);
 
 /* hashpage.c */
 extern void _hash_getlock(Relation rel, BlockNumber whichlock, int access);
@@ -287,6 +289,9 @@ extern Buffer _hash_getbuf(Relation rel, BlockNumber blkno,
 						   int access, int flags);
 extern Buffer _hash_getinitbuf(Relation rel, BlockNumber blkno);
 extern Buffer _hash_getnewbuf(Relation rel, BlockNumber blkno);
+extern Buffer _hash_getbuf_with_strategy(Relation rel, BlockNumber blkno,
+										 int access, int flags,
+										 BufferAccessStrategy bstrategy);
 extern void _hash_relbuf(Relation rel, Buffer buf);
 extern void _hash_dropbuf(Relation rel, Buffer buf);
 extern void _hash_wrtbuf(Relation rel, Buffer buf);
diff --git a/src/include/access/relscan.h b/src/include/access/relscan.h
index 7a1ea39352..200b45713e 100644
--- a/src/include/access/relscan.h
+++ b/src/include/access/relscan.h
@@ -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/access/relscan.h,v 1.53 2007/05/27 03:50:39 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/relscan.h,v 1.54 2007/05/30 20:12:02 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -27,6 +27,7 @@ typedef struct HeapScanDescData
 	int			rs_nkeys;		/* number of scan keys */
 	ScanKey		rs_key;			/* array of scan key descriptors */
 	BlockNumber rs_nblocks;		/* number of blocks to scan */
+	BufferAccessStrategy rs_strategy;	/* access strategy for reads */
 	bool		rs_pageatatime; /* verify visibility page-at-a-time? */
 
 	/* scan current state */
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 87ff6aba50..1c741f38fd 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.77 2007/05/20 21:08:19 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.78 2007/05/30 20:12:02 tgl Exp $
  */
 #ifndef XLOG_H
 #define XLOG_H
@@ -159,6 +159,7 @@ extern bool XLOG_DEBUG;
 
 extern XLogRecPtr XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata);
 extern void XLogFlush(XLogRecPtr RecPtr);
+extern bool XLogNeedsFlush(XLogRecPtr RecPtr);
 
 extern void xlog_redo(XLogRecPtr lsn, XLogRecord *record);
 extern void xlog_desc(StringInfo buf, uint8 xl_info, char *rec);