Tweak buffer manager so that 'internal' accesses to a buffer do not

advance its usage_count.  This includes writes of dirty buffers triggered
by bgwriter, checkpoint, or FlushRelationBuffers, as well as various
corner cases that really ought not count as accesses to the page.
Should make for some marginal improvement in the quality of our decisions
about when to recycle buffers.  Per suggestion from ITAGAKI Takahiro.

src/backend/storage/buffer/bufmgr.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.197 2005/10/15 02:49:24 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.198 2005/10/27 17:07:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -85,7 +85,8 @@ static volatile BufferDesc *PinCountWaitBuf = NULL;
 
 static bool PinBuffer(volatile BufferDesc *buf);
 static void PinBuffer_Locked(volatile BufferDesc *buf);
-static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner, bool trashOK);
+static void UnpinBuffer(volatile BufferDesc *buf,
+						bool fixOwner, bool normalAccess);
 static bool SyncOneBuffer(int buf_id, bool skip_pinned);
 static void WaitIO(volatile BufferDesc *buf);
 static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
@@ -780,12 +781,18 @@ PinBuffer_Locked(volatile BufferDesc *buf)
  * This should be applied only to shared buffers, never local ones.
  *
  * Most but not all callers want CurrentResourceOwner to be adjusted.
+ * Those that don't should pass fixOwner = FALSE.
+ *
+ * normalAccess indicates that we are finishing a "normal" page access,
+ * that is, one requested by something outside the buffer subsystem.
+ * Passing FALSE means it's an internal access that should not update the
+ * buffer's usage count nor cause a change in the freelist.
  *
  * If we are releasing a buffer during VACUUM, and it's not been otherwise
- * used recently, and trashOK is true, send the buffer to the freelist.
+ * used recently, and normalAccess is true, we send the buffer to the freelist.
  */
 static void
-UnpinBuffer(volatile BufferDesc *buf, bool fixOwner, bool trashOK)
+UnpinBuffer(volatile BufferDesc *buf, bool fixOwner, bool normalAccess)
 {
 	int			b = buf->buf_id;
 
@@ -797,7 +804,7 @@ UnpinBuffer(volatile BufferDesc *buf, bool fixOwner, bool trashOK)
 	PrivateRefCount[b]--;
 	if (PrivateRefCount[b] == 0)
 	{
-		bool		trash_buffer = false;
+		bool		immed_free_buffer = false;
 
 		/* I'd better not still hold any locks on the buffer */
 		Assert(!LWLockHeldByMe(buf->content_lock));
@@ -810,16 +817,21 @@ UnpinBuffer(volatile BufferDesc *buf, bool fixOwner, bool trashOK)
 		Assert(buf->refcount > 0);
 		buf->refcount--;
 
-		/* Mark the buffer recently used, unless we are in VACUUM */
-		if (!strategy_hint_vacuum)
+		/* Update buffer usage info, unless this is an internal access */
+		if (normalAccess)
 		{
-			if (buf->usage_count < BM_MAX_USAGE_COUNT)
-				buf->usage_count++;
+			if (!strategy_hint_vacuum)
+			{
+				if (buf->usage_count < BM_MAX_USAGE_COUNT)
+					buf->usage_count++;
+			}
+			else
+			{
+				/* VACUUM accesses don't bump usage count, instead... */
+				if (buf->refcount == 0 && buf->usage_count == 0)
+					immed_free_buffer = true;
+			}
 		}
-		else if (trashOK &&
-				 buf->refcount == 0 &&
-				 buf->usage_count == 0)
-			trash_buffer = true;
 
 		if ((buf->flags & BM_PIN_COUNT_WAITER) &&
 			buf->refcount == 1)
@@ -839,7 +851,7 @@ UnpinBuffer(volatile BufferDesc *buf, bool fixOwner, bool trashOK)
 		 * freelist for near-term reuse.  We put it at the tail so that it
 		 * won't be used before any invalid buffers that may exist.
 		 */
-		if (trash_buffer)
+		if (immed_free_buffer)
 			StrategyFreeBuffer(buf, false);
 	}
 }