Provide multi-block smgrprefetch().

Previously smgrprefetch() could issue POSIX_FADV_WILLNEED advice for a
single block at a time.  Add an nblocks argument so that we can do the
same for a range of blocks.  This usually produces a single system call,
but might need to loop if it crosses a segment boundary.  Initially it
is only called with nblocks == 1, but proposed patches will make wider
calls.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi> (earlier version)
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -569,7 +569,7 @@ PrefetchSharedBuffer(SMgrRelation smgr_reln,
 		 * recovery if the relation file doesn't exist.
 		 */
 		if ((io_direct_flags & IO_DIRECT_DATA) == 0 &&
-			smgrprefetch(smgr_reln, forkNum, blockNum))
+			smgrprefetch(smgr_reln, forkNum, blockNum, 1))
 		{
 			result.initiated_io = true;
 		}

src/backend/storage/buffer/localbuf.c

@@ -94,7 +94,7 @@ PrefetchLocalBuffer(SMgrRelation smgr, ForkNumber forkNum,
 #ifdef USE_PREFETCH
 		/* Not in buffers, so initiate prefetch */
 		if ((io_direct_flags & IO_DIRECT_DATA) == 0 &&
-			smgrprefetch(smgr, forkNum, blockNum))
+			smgrprefetch(smgr, forkNum, blockNum, 1))
 		{
 			result.initiated_io = true;
 		}

src/backend/storage/smgr/md.c

@@ -710,17 +710,25 @@ mdclose(SMgrRelation reln, ForkNumber forknum)
 }
 
 /*
- * mdprefetch() -- Initiate asynchronous read of the specified block of a relation
+ * mdprefetch() -- Initiate asynchronous read of the specified blocks of a relation
  */
 bool
-mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum)
+mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
+		   int nblocks)
 {
 #ifdef USE_PREFETCH
-	off_t		seekpos;
-	MdfdVec    *v;
 
 	Assert((io_direct_flags & IO_DIRECT_DATA) == 0);
 
+	if ((uint64) blocknum + nblocks > (uint64) MaxBlockNumber + 1)
+		return false;
+
+	while (nblocks > 0)
+	{
+		off_t		seekpos;
+		MdfdVec    *v;
+		int			nblocks_this_segment;
+
 		v = _mdfd_getseg(reln, forknum, blocknum, false,
 						 InRecovery ? EXTENSION_RETURN_NULL : EXTENSION_FAIL);
 		if (v == NULL)
@@ -730,7 +738,16 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum)
 
 		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
 
-	(void) FilePrefetch(v->mdfd_vfd, seekpos, BLCKSZ, WAIT_EVENT_DATA_FILE_PREFETCH);
+		nblocks_this_segment =
+			Min(nblocks,
+				RELSEG_SIZE - (blocknum % ((BlockNumber) RELSEG_SIZE)));
+
+		(void) FilePrefetch(v->mdfd_vfd, seekpos, BLCKSZ * nblocks_this_segment,
+							WAIT_EVENT_DATA_FILE_PREFETCH);
+
+		blocknum += nblocks_this_segment;
+		nblocks -= nblocks_this_segment;
+	}
 #endif							/* USE_PREFETCH */
 
 	return true;

src/backend/storage/smgr/smgr.c

@@ -54,7 +54,7 @@ typedef struct f_smgr
 	void		(*smgr_zeroextend) (SMgrRelation reln, ForkNumber forknum,
 									BlockNumber blocknum, int nblocks, bool skipFsync);
 	bool		(*smgr_prefetch) (SMgrRelation reln, ForkNumber forknum,
-								  BlockNumber blocknum);
+								  BlockNumber blocknum, int nblocks);
 	void		(*smgr_read) (SMgrRelation reln, ForkNumber forknum,
 							  BlockNumber blocknum, void *buffer);
 	void		(*smgr_write) (SMgrRelation reln, ForkNumber forknum,
@@ -547,9 +547,10 @@ smgrzeroextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
  * record).
  */
 bool
-smgrprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum)
+smgrprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
+			 int nblocks)
 {
-	return smgrsw[reln->smgr_which].smgr_prefetch(reln, forknum, blocknum);
+	return smgrsw[reln->smgr_which].smgr_prefetch(reln, forknum, blocknum, nblocks);
 }
 
 /*

src/include/storage/md.h

@@ -31,7 +31,7 @@ extern void mdextend(SMgrRelation reln, ForkNumber forknum,
 extern void mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 						 BlockNumber blocknum, int nblocks, bool skipFsync);
 extern bool mdprefetch(SMgrRelation reln, ForkNumber forknum,
-					   BlockNumber blocknum);
+					   BlockNumber blocknum, int nblocks);
 extern void mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 				   void *buffer);
 extern void mdwrite(SMgrRelation reln, ForkNumber forknum,

src/include/storage/smgr.h

@@ -95,7 +95,7 @@ extern void smgrextend(SMgrRelation reln, ForkNumber forknum,
 extern void smgrzeroextend(SMgrRelation reln, ForkNumber forknum,
 						   BlockNumber blocknum, int nblocks, bool skipFsync);
 extern bool smgrprefetch(SMgrRelation reln, ForkNumber forknum,
-						 BlockNumber blocknum);
+						 BlockNumber blocknum, int nblocks);
 extern void smgrread(SMgrRelation reln, ForkNumber forknum,
 					 BlockNumber blocknum, void *buffer);
 extern void smgrwrite(SMgrRelation reln, ForkNumber forknum,