Tweak indexscan and seqscan code to arrange that steps from one page to

the next are handled by ReleaseAndReadBuffer rather than separate
ReleaseBuffer and ReadBuffer calls.  This cuts the number of acquisitions
of the BufMgrLock by a factor of 2 (possibly more, if an indexscan happens
to pull successive rows from the same heap page).  Unfortunately this
doesn't seem enough to get us out of the recently discussed context-switch
storm problem, but it's surely worth doing anyway.

Author: tglsfdc

src/backend/access/heap/heapam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.164 2004/04/01 21:28:43 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.165 2004/04/21 18:24:23 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -888,6 +888,28 @@ heap_fetch(Relation relation,
 		   Buffer *userbuf,
 		   bool keep_buf,
 		   PgStat_Info *pgstat_info)
+{
+	/* Assume *userbuf is undefined on entry */
+	*userbuf = InvalidBuffer;
+	return heap_release_fetch(relation, snapshot, tuple,
+							  userbuf, keep_buf, pgstat_info);
+}
+
+/*
+ *	heap_release_fetch		- retrieve tuple with given tid
+ *
+ * This has the same API as heap_fetch except that if *userbuf is not
+ * InvalidBuffer on entry, that buffer will be released before reading
+ * the new page.  This saves a separate ReleaseBuffer step and hence
+ * one entry into the bufmgr when looping through multiple fetches.
+ */
+bool
+heap_release_fetch(Relation relation,
+				   Snapshot snapshot,
+				   HeapTuple tuple,
+				   Buffer *userbuf,
+				   bool keep_buf,
+				   PgStat_Info *pgstat_info)
 {
 	ItemPointer tid = &(tuple->t_self);
 	ItemId		lp;
@@ -898,9 +920,10 @@ heap_fetch(Relation relation,
 
 	/*
 	 * get the buffer from the relation descriptor. Note that this does a
-	 * buffer pin.
+	 * buffer pin, and releases the old *userbuf if not InvalidBuffer.
 	 */
-	buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));
+	buffer = ReleaseAndReadBuffer(*userbuf, relation,
+								  ItemPointerGetBlockNumber(tid));
 
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "ReadBuffer(\"%s\", %lu) failed",

src/backend/access/index/indexam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.72 2003/11/29 19:51:40 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.73 2004/04/21 18:24:24 tgl Exp $
  *
  * INTERFACE ROUTINES
  *		index_open		- open an index relation by relation OID
@@ -316,6 +316,13 @@ index_rescan(IndexScanDesc scan, ScanKey key)
 	SCAN_CHECKS;
 	GET_SCAN_PROCEDURE(rescan, amrescan);
 
+	/* Release any held pin on a heap page */
+	if (BufferIsValid(scan->xs_cbuf))
+	{
+		ReleaseBuffer(scan->xs_cbuf);
+		scan->xs_cbuf = InvalidBuffer;
+	}
+
 	scan->kill_prior_tuple = false;		/* for safety */
 	scan->keys_are_unique = false;		/* may be set by index AM */
 	scan->got_tuple = false;
@@ -419,13 +426,6 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 
 	SCAN_CHECKS;
 
-	/* Release any previously held pin */
-	if (BufferIsValid(scan->xs_cbuf))
-	{
-		ReleaseBuffer(scan->xs_cbuf);
-		scan->xs_cbuf = InvalidBuffer;
-	}
-
 	/*
 	 * If we already got a tuple and it must be unique, there's no need to
 	 * make the index AM look through any additional tuples.  (This can
@@ -508,14 +508,22 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 		scan->kill_prior_tuple = false;
 
 		if (!found)
+		{
+			/* Release any held pin on a heap page */
+			if (BufferIsValid(scan->xs_cbuf))
+			{
+				ReleaseBuffer(scan->xs_cbuf);
+				scan->xs_cbuf = InvalidBuffer;
+			}
 			return NULL;		/* failure exit */
+		}
 
 		/*
 		 * Fetch the heap tuple and see if it matches the snapshot.
 		 */
-		if (heap_fetch(scan->heapRelation, scan->xs_snapshot,
-					   heapTuple, &scan->xs_cbuf, true,
-					   &scan->xs_pgstat_info))
+		if (heap_release_fetch(scan->heapRelation, scan->xs_snapshot,
+							   heapTuple, &scan->xs_cbuf, true,
+							   &scan->xs_pgstat_info))
 			break;
 
 		/* Skip if no tuple at this location */
@@ -527,7 +535,7 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 		 * if the tuple is dead to all transactions.  If so, signal the
 		 * index AM to not return it on future indexscans.
 		 *
-		 * We told heap_fetch to keep a pin on the buffer, so we can
+		 * We told heap_release_fetch to keep a pin on the buffer, so we can
 		 * re-access the tuple here.  But we must re-lock the buffer
 		 * first. Also, it's just barely possible for an update of hint
 		 * bits to occur here.
@@ -542,8 +550,6 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 		if (sv_infomask != heapTuple->t_data->t_infomask)
 			SetBufferCommitInfoNeedsSave(scan->xs_cbuf);
 		LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);
-		ReleaseBuffer(scan->xs_cbuf);
-		scan->xs_cbuf = InvalidBuffer;
 	}
 
 	/* Success exit */

src/backend/access/nbtree/nbtinsert.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.111 2004/01/07 18:56:24 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.112 2004/04/21 18:24:25 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -297,9 +297,7 @@ _bt_check_unique(Relation rel, BTItem btitem, Relation heapRel,
 			for (;;)
 			{
 				nblkno = opaque->btpo_next;
-				if (nbuf != InvalidBuffer)
-					_bt_relbuf(rel, nbuf);
-				nbuf = _bt_getbuf(rel, nblkno, BT_READ);
+				nbuf = _bt_relandgetbuf(rel, nbuf, nblkno, BT_READ);
 				page = BufferGetPage(nbuf);
 				opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 				if (!P_IGNORE(opaque))
@@ -454,9 +452,7 @@ _bt_insertonpg(Relation rel,
 			{
 				BlockNumber rblkno = lpageop->btpo_next;
 
-				if (rbuf != InvalidBuffer)
-					_bt_relbuf(rel, rbuf);
-				rbuf = _bt_getbuf(rel, rblkno, BT_WRITE);
+				rbuf = _bt_relandgetbuf(rel, rbuf, rblkno, BT_WRITE);
 				page = BufferGetPage(rbuf);
 				lpageop = (BTPageOpaque) PageGetSpecialPointer(page);
 				if (!P_IGNORE(lpageop))
@@ -1357,7 +1353,7 @@ _bt_getstackbuf(Relation rel, BTStack stack, int access)
 		if (P_RIGHTMOST(opaque))
 		{
 			_bt_relbuf(rel, buf);
-			return (InvalidBuffer);
+			return InvalidBuffer;
 		}
 		blkno = opaque->btpo_next;
 		start = InvalidOffsetNumber;

src/backend/access/nbtree/nbtpage.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtpage.c,v 1.74 2003/12/21 01:23:06 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtpage.c,v 1.75 2004/04/21 18:24:25 tgl Exp $
  *
  *	NOTES
  *	   Postgres btree pages look like ordinary relation pages.	The opaque
@@ -262,11 +262,15 @@ _bt_getroot(Relation rel, int access)
 		Assert(rootblkno != P_NONE);
 		rootlevel = metad->btm_fastlevel;
 
-		_bt_relbuf(rel, metabuf);		/* done with the meta page */
+		/*
+		 * We are done with the metapage; arrange to release it via
+		 * first _bt_relandgetbuf call
+		 */
+		rootbuf = metabuf;
 
 		for (;;)
 		{
-			rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
+			rootbuf = _bt_relandgetbuf(rel, rootbuf, rootblkno, BT_READ);
 			rootpage = BufferGetPage(rootbuf);
 			rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 
@@ -278,8 +282,6 @@ _bt_getroot(Relation rel, int access)
 				elog(ERROR, "no live root page found in \"%s\"",
 					 RelationGetRelationName(rel));
 			rootblkno = rootopaque->btpo_next;
-
-			_bt_relbuf(rel, rootbuf);
 		}
 
 		/* Note: can't check btpo.level on deleted pages */
@@ -352,11 +354,15 @@ _bt_gettrueroot(Relation rel)
 	rootblkno = metad->btm_root;
 	rootlevel = metad->btm_level;
 
-	_bt_relbuf(rel, metabuf);	/* done with the meta page */
+	/*
+	 * We are done with the metapage; arrange to release it via
+	 * first _bt_relandgetbuf call
+	 */
+	rootbuf = metabuf;
 
 	for (;;)
 	{
-		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
+		rootbuf = _bt_relandgetbuf(rel, rootbuf, rootblkno, BT_READ);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 
@@ -368,8 +374,6 @@ _bt_gettrueroot(Relation rel)
 			elog(ERROR, "no live root page found in \"%s\"",
 				 RelationGetRelationName(rel));
 		rootblkno = rootopaque->btpo_next;
-
-		_bt_relbuf(rel, rootbuf);
 	}
 
 	/* Note: can't check btpo.level on deleted pages */
@@ -492,6 +496,28 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)
 	return buf;
 }
 
+/*
+ *	_bt_relandgetbuf() -- release a locked buffer and get another one.
+ *
+ * This is equivalent to _bt_relbuf followed by _bt_getbuf, with the
+ * exception that blkno may not be P_NEW.  Also, if obuf is InvalidBuffer
+ * then it reduces to just _bt_getbuf; allowing this case simplifies some
+ * callers. The motivation for using this is to avoid two entries to the
+ * bufmgr when one will do.
+ */
+Buffer
+_bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
+{
+	Buffer		buf;
+
+	Assert(blkno != P_NEW);
+	if (BufferIsValid(obuf))
+		LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
+	buf = ReleaseAndReadBuffer(obuf, rel, blkno);
+	LockBuffer(buf, access);
+	return buf;
+}
+
 /*
  *	_bt_relbuf() -- release a locked buffer.
  *

src/backend/access/nbtree/nbtree.c

@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.113 2004/02/10 03:42:43 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.114 2004/04/21 18:24:26 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -919,9 +919,7 @@ _bt_restscan(IndexScanDesc scan)
 		for (;;)
 		{
 			blkno = opaque->btpo_next;
-			if (nextbuf != InvalidBuffer)
-				_bt_relbuf(rel, nextbuf);
-			nextbuf = _bt_getbuf(rel, blkno, BT_READ);
+			nextbuf = _bt_relandgetbuf(rel, nextbuf, blkno, BT_READ);
 			page = BufferGetPage(nextbuf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 			if (!P_IGNORE(opaque))

src/backend/access/nbtree/nbtsearch.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.86 2003/12/21 17:52:34 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.87 2004/04/21 18:24:26 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -108,8 +108,7 @@ _bt_search(Relation rel, int keysz, ScanKey scankey, bool nextkey,
 		new_stack->bts_parent = stack_in;
 
 		/* drop the read lock on the parent page, acquire one on the child */
-		_bt_relbuf(rel, *bufP);
-		*bufP = _bt_getbuf(rel, blkno, BT_READ);
+		*bufP = _bt_relandgetbuf(rel, *bufP, blkno, BT_READ);
 
 		/* okay, all set to move down a level */
 		stack_in = new_stack;
@@ -178,8 +177,7 @@ _bt_moveright(Relation rel,
 		/* step right one page */
 		BlockNumber rblkno = opaque->btpo_next;
 
-		_bt_relbuf(rel, buf);
-		buf = _bt_getbuf(rel, rblkno, access);
+		buf = _bt_relandgetbuf(rel, buf, rblkno, access);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	}
@@ -933,8 +931,7 @@ _bt_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 				}
 				/* step right one page */
 				blkno = opaque->btpo_next;
-				_bt_relbuf(rel, *bufP);
-				*bufP = _bt_getbuf(rel, blkno, BT_READ);
+				*bufP = _bt_relandgetbuf(rel, *bufP, blkno, BT_READ);
 				page = BufferGetPage(*bufP);
 				opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 				if (!P_IGNORE(opaque))
@@ -1041,8 +1038,7 @@ _bt_walk_left(Relation rel, Buffer buf)
 		obknum = BufferGetBlockNumber(buf);
 		/* step left */
 		blkno = lblkno = opaque->btpo_prev;
-		_bt_relbuf(rel, buf);
-		buf = _bt_getbuf(rel, blkno, BT_READ);
+		buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1069,15 +1065,13 @@ _bt_walk_left(Relation rel, Buffer buf)
 			if (P_RIGHTMOST(opaque) || ++tries > 4)
 				break;
 			blkno = opaque->btpo_next;
-			_bt_relbuf(rel, buf);
-			buf = _bt_getbuf(rel, blkno, BT_READ);
+			buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
 			page = BufferGetPage(buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		}
 
 		/* Return to the original page to see what's up */
-		_bt_relbuf(rel, buf);
-		buf = _bt_getbuf(rel, obknum, BT_READ);
+		buf = _bt_relandgetbuf(rel, buf, obknum, BT_READ);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		if (P_ISDELETED(opaque))
@@ -1094,8 +1088,7 @@ _bt_walk_left(Relation rel, Buffer buf)
 					elog(ERROR, "fell off the end of \"%s\"",
 						 RelationGetRelationName(rel));
 				blkno = opaque->btpo_next;
-				_bt_relbuf(rel, buf);
-				buf = _bt_getbuf(rel, blkno, BT_READ);
+				buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
 				page = BufferGetPage(buf);
 				opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 				if (!P_ISDELETED(opaque))
@@ -1177,8 +1170,7 @@ _bt_get_endpoint(Relation rel, uint32 level, bool rightmost)
 			if (blkno == P_NONE)
 				elog(ERROR, "fell off the end of \"%s\"",
 					 RelationGetRelationName(rel));
-			_bt_relbuf(rel, buf);
-			buf = _bt_getbuf(rel, blkno, BT_READ);
+			buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
 			page = BufferGetPage(buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		}
@@ -1199,8 +1191,7 @@ _bt_get_endpoint(Relation rel, uint32 level, bool rightmost)
 		itup = &(btitem->bti_itup);
 		blkno = ItemPointerGetBlockNumber(&(itup->t_tid));
 
-		_bt_relbuf(rel, buf);
-		buf = _bt_getbuf(rel, blkno, BT_READ);
+		buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	}