Track shared buffer hits in pg_stat_io

Among other things, this should make it easier to calculate a useful cache hit
ratio by excluding buffer reads via buffer access strategies. As buffer access
strategies reuse buffers (and thus evict the prior buffer contents), it is
normal to see reads on repeated scans of the same data.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAAKRu_beMa9Hzih40%3DXPYqhDVz6tsgUGTrhZXRo%3Dunp%2Bszb%3DUA%40mail.gmail.com

2 files changed, 16 insertions, 33 deletions

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index fe029d2ea6..b3adbbe7d2 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -472,7 +472,7 @@ static BufferDesc *BufferAlloc(SMgrRelation smgr,
 							   ForkNumber forkNum,
 							   BlockNumber blockNum,
 							   BufferAccessStrategy strategy,
-							   bool *foundPtr, IOContext *io_context);
+							   bool *foundPtr, IOContext io_context);
 static void FlushBuffer(BufferDesc *buf, SMgrRelation reln,
 						IOObject io_object, IOContext io_context);
 static void FindAndDropRelationBuffers(RelFileLocator rlocator,
@@ -850,13 +850,14 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	if (isLocalBuf)
 	{
 		/*
-		 * LocalBufferAlloc() will set the io_context to IOCONTEXT_NORMAL. We
-		 * do not use a BufferAccessStrategy for I/O of temporary tables.
+		 * We do not use a BufferAccessStrategy for I/O of temporary tables.
 		 * However, in some cases, the "strategy" may not be NULL, so we can't
 		 * rely on IOContextForStrategy() to set the right IOContext for us.
 		 * This may happen in cases like CREATE TEMPORARY TABLE AS...
 		 */
-		bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found, &io_context);
+		io_context = IOCONTEXT_NORMAL;
+		io_object = IOOBJECT_TEMP_RELATION;
+		bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
 		if (found)
 			pgBufferUsage.local_blks_hit++;
 		else if (isExtend)
@@ -871,8 +872,10 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
 		 * not currently in memory.
 		 */
+		io_context = IOContextForStrategy(strategy);
+		io_object = IOOBJECT_RELATION;
 		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
-							 strategy, &found, &io_context);
+							 strategy, &found, io_context);
 		if (found)
 			pgBufferUsage.shared_blks_hit++;
 		else if (isExtend)
@@ -892,6 +895,7 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			/* Just need to update stats before we exit */
 			*hit = true;
 			VacuumPageHit++;
+			pgstat_count_io_op(io_object, io_context, IOOP_HIT);
 
 			if (VacuumCostActive)
 				VacuumCostBalance += VacuumCostPageHit;
@@ -987,16 +991,7 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	 */
 	Assert(!(pg_atomic_read_u32(&bufHdr->state) & BM_VALID));	/* spinlock not needed */
 
-	if (isLocalBuf)
-	{
-		bufBlock = LocalBufHdrGetBlock(bufHdr);
-		io_object = IOOBJECT_TEMP_RELATION;
-	}
-	else
-	{
-		bufBlock = BufHdrGetBlock(bufHdr);
-		io_object = IOOBJECT_RELATION;
-	}
+	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
 
 	if (isExtend)
 	{
@@ -1139,7 +1134,7 @@ static BufferDesc *
 BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr, IOContext *io_context)
+			bool *foundPtr, IOContext io_context)
 {
 	bool		from_ring;
 	BufferTag	newTag;			/* identity of requested block */
@@ -1193,11 +1188,8 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			{
 				/*
 				 * If we get here, previous attempts to read the buffer must
-				 * have failed ... but we shall bravely try again. Set
-				 * io_context since we will in fact need to count an IO
-				 * Operation.
+				 * have failed ... but we shall bravely try again.
 				 */
-				*io_context = IOContextForStrategy(strategy);
 				*foundPtr = false;
 			}
 		}
@@ -1211,8 +1203,6 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	 */
 	LWLockRelease(newPartitionLock);
 
-	*io_context = IOContextForStrategy(strategy);
-
 	/* Loop here in case we have to try another victim buffer */
 	for (;;)
 	{
@@ -1295,7 +1285,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 														  smgr->smgr_rlocator.locator.dbOid,
 														  smgr->smgr_rlocator.locator.relNumber);
 
-				FlushBuffer(buf, NULL, IOOBJECT_RELATION, *io_context);
+				FlushBuffer(buf, NULL, IOOBJECT_RELATION, io_context);
 				LWLockRelease(BufferDescriptorGetContentLock(buf));
 
 				ScheduleBufferTagForWriteback(&BackendWritebackContext,
@@ -1494,7 +1484,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		 * we may have been forced to release the buffer due to concurrent
 		 * pinners or erroring out.
 		 */
-		pgstat_count_io_op(IOOBJECT_RELATION, *io_context,
+		pgstat_count_io_op(IOOBJECT_RELATION, io_context,
 						   from_ring ? IOOP_REUSE : IOOP_EVICT);
 	}
 
diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c
index 68b4817c67..6f9e7eda57 100644
--- a/src/backend/storage/buffer/localbuf.c
+++ b/src/backend/storage/buffer/localbuf.c
@@ -108,7 +108,7 @@ PrefetchLocalBuffer(SMgrRelation smgr, ForkNumber forkNum,
  */
 BufferDesc *
 LocalBufferAlloc(SMgrRelation smgr, ForkNumber forkNum, BlockNumber blockNum,
-				 bool *foundPtr, IOContext *io_context)
+				 bool *foundPtr)
 {
 	BufferTag	newTag;			/* identity of requested block */
 	LocalBufferLookupEnt *hresult;
@@ -128,14 +128,6 @@ LocalBufferAlloc(SMgrRelation smgr, ForkNumber forkNum, BlockNumber blockNum,
 	hresult = (LocalBufferLookupEnt *)
 		hash_search(LocalBufHash, &newTag, HASH_FIND, NULL);
 
-	/*
-	 * IO Operations on local buffers are only done in IOCONTEXT_NORMAL. Set
-	 * io_context here (instead of after a buffer hit would have returned) for
-	 * convenience since we don't have to worry about the overhead of calling
-	 * IOContextForStrategy().
-	 */
-	*io_context = IOCONTEXT_NORMAL;
-
 	if (hresult)
 	{
 		b = hresult->id;
@@ -239,6 +231,7 @@ LocalBufferAlloc(SMgrRelation smgr, ForkNumber forkNum, BlockNumber blockNum,
 		buf_state &= ~BM_DIRTY;
 		pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
 
+		/* Temporary table I/O does not use Buffer Access Strategies */
 		pgstat_count_io_op(IOOBJECT_TEMP_RELATION, IOCONTEXT_NORMAL, IOOP_WRITE);
 		pgBufferUsage.local_blks_written++;
 	}