Optimize prefetch patterns in both heap seqscan and vacuum scans.

Previously, we called PrefetchBuffer [NBlkScanned * seqscan_prefetch_buffers] times in each of those situations, but now only NBlkScanned. In addition, the prefetch mechanism for the vacuum scans is now based on blocks instead of tuples - improving the efficiency.
neondatabase · Oct 24, 2022 · 0a675c1 · 0a675c1
1 parent b1c21a4
commit 0a675c1
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Showing 2 changed files with 60 additions and 21 deletions.
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
@@ -400,19 +400,32 @@ heapgetpage(TableScanDesc sscan, BlockNumber page)
 	CHECK_FOR_INTERRUPTS();
 
 	/* Prefetch next block */
-	if (enable_seqscan_prefetch)
+	if (enable_seqscan_prefetch && seqscan_prefetch_buffers > 0)
 	{
-		int prefetch_limit = seqscan_prefetch_buffers;
+		uint32 prefetch_limit = seqscan_prefetch_buffers;
+		BlockNumber	prefetch_start = page;
 		ParallelBlockTableScanWorker pbscanwork = scan->rs_parallelworkerdata;
+
 		if (pbscanwork != NULL && pbscanwork->phsw_chunk_remaining < prefetch_limit)
 			prefetch_limit = pbscanwork->phsw_chunk_remaining;
-		if (page + prefetch_limit >= scan->rs_nblocks)
-			prefetch_limit = scan->rs_nblocks - page - 1;
+
+		/*
+		 * If we've started prefetching buffers, we don't have to prefetch
+		 * all pages in page..page + n, but only page + n; saving N-1 prefetch
+		 * calls.
+		 */
+		if (scan->rs_startblock != page)
+		{
+			prefetch_start = (page + prefetch_limit - 1) % scan->rs_nblocks;
+			prefetch_limit = 1;
+		}
+		else
+			prefetch_start = page;
 
 		RelationOpenSmgr(scan->rs_base.rs_rd);
-		smgr_reset_prefetch(scan->rs_base.rs_rd->rd_smgr);
 		for (int i = 1; i <= prefetch_limit; i++)
-			PrefetchBuffer(scan->rs_base.rs_rd, MAIN_FORKNUM, page+i);
+			PrefetchBuffer(scan->rs_base.rs_rd, MAIN_FORKNUM,
+						   (prefetch_start+i) % scan->rs_nblocks);
 	}
 
 	/* read page using selected strategy */

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
@@ -1218,14 +1218,6 @@ lazy_scan_heap(LVRelState *vacrel, VacuumParams *params, bool aggressive)
 		 */
 		visibilitymap_pin(vacrel->rel, blkno, &vmbuffer);
 
-		if (enable_seqscan_prefetch)
-		{
-			int prefetch_limit = Min(nblocks - blkno - 1, seqscan_prefetch_buffers);
-			RelationOpenSmgr(vacrel->rel);
-			smgr_reset_prefetch(vacrel->rel->rd_smgr);
-			for (int i = 1; i <= prefetch_limit; i++)
-				PrefetchBuffer(vacrel->rel, MAIN_FORKNUM, blkno+i);
-		}
 		buf = ReadBufferExtended(vacrel->rel, MAIN_FORKNUM, blkno,
 								 RBM_NORMAL, vacrel->bstrategy);
 
@@ -2323,7 +2315,8 @@ lazy_vacuum_all_indexes(LVRelState *vacrel)
 static void
 lazy_vacuum_heap_rel(LVRelState *vacrel)
 {
-	int			tupindex;
+	int			tupindex,
+				ptupindex;
 	BlockNumber vacuumed_pages;
 	PGRUsage	ru0;
 	Buffer		vmbuffer = InvalidBuffer;
@@ -2346,6 +2339,7 @@ lazy_vacuum_heap_rel(LVRelState *vacrel)
 	vacuumed_pages = 0;
 
 	tupindex = 0;
+	ptupindex = 0;
 	while (tupindex < vacrel->dead_tuples->num_tuples)
 	{
 		BlockNumber tblk;
@@ -2356,14 +2350,46 @@ lazy_vacuum_heap_rel(LVRelState *vacrel)
 		vacuum_delay_point();
 
 		tblk = ItemPointerGetBlockNumber(&vacrel->dead_tuples->itemptrs[tupindex]);
-		if (enable_seqscan_prefetch)
+
+		if (enable_seqscan_prefetch && seqscan_prefetch_buffers > 0)
 		{
-			int prefetch_limit = Min(vacrel->dead_tuples->num_tuples - tupindex - 1, seqscan_prefetch_buffers);
-			RelationOpenSmgr(vacrel->rel);
-			smgr_reset_prefetch(vacrel->rel->rd_smgr);
-			for (int i = 1; i <= prefetch_limit; i++)
-				PrefetchBuffer(vacrel->rel, MAIN_FORKNUM, ItemPointerGetBlockNumber(&vacrel->dead_tuples->itemptrs[tupindex + i]));
+			/*
+			 * If we're just starting out, prefetch N consecutive blocks.
+			 * If not, only the next 1 block
+			 */
+			if (tupindex == 0)
+			{
+				int prefetch_limit = Min(vacrel->dead_tuples->num_tuples - tupindex - 1,
+										 Min(vacrel->rel_pages,
+											 seqscan_prefetch_buffers));
+				BlockNumber prev_prefetch = 0;
+
+				RelationOpenSmgr(vacrel->rel);
+
+				while (++ptupindex < vacrel->dead_tuples->num_tuples &&
+					   prefetch_limit > 0)
+				{
+					ItemPointer ptr = &vacrel->dead_tuples->itemptrs[ptupindex];
+					if (ItemPointerGetBlockNumber(ptr) != prev_prefetch)
+					{
+						prev_prefetch = ItemPointerGetBlockNumber(ptr);
+						prefetch_limit -= 1;
+						PrefetchBuffer(vacrel->rel, MAIN_FORKNUM, prev_prefetch);
+					}
+				}
+			}
+			else
+			{
+				BlockNumber toPrefetch = ItemPointerGetBlockNumber(&vacrel->dead_tuples->itemptrs[ptupindex]);
+				while (ptupindex < vacrel->dead_tuples->num_tuples)
+				{
+					if (toPrefetch != ItemPointerGetBlockNumber(&vacrel->dead_tuples->itemptrs[ptupindex]))
+						break;
+				}
+				PrefetchBuffer(vacrel->rel, MAIN_FORKNUM, toPrefetch);
+			}
 		}
+
 		vacrel->blkno = tblk;
 		buf = ReadBufferExtended(vacrel->rel, MAIN_FORKNUM, tblk, RBM_NORMAL,
 								 vacrel->bstrategy);