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Only skip pages marked as clean in the visibility map, if the last 32

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pages were marked as clean as well. The idea is to avoid defeating OS
readahead by skipping a page here and there, and also makes it less likely
that we miss an opportunity to advance relfrozenxid, for the sake of only
a few skipped pages.
This commit is contained in:
Heikki Linnakangas 2009-01-22 19:25:00 +00:00
parent c079090bbc
commit bf136cf6e3
1 changed files with 25 additions and 4 deletions
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29
src/backend/commands/vacuumlazy.c
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@ -29,7 +29,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.117 2009/01/16 13:27:23 heikki Exp $
* $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.118 2009/01/22 19:25:00 heikki Exp $
*
*-------------------------------------------------------------------------
*/
@ -74,6 +74,12 @@
*/
#define LAZY_ALLOC_TUPLES MaxHeapTuplesPerPage
/*
* Before we consider skipping a page that's marked as clean in
* visibility map, we must've seen at least this many clean pages.
*/
#define SKIP_PAGES_THRESHOLD 32
typedef struct LVRelStats
{
/* hasindex = true means two-pass strategy; false means one-pass */
@ -271,6 +277,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
int i;
PGRUsage ru0;
Buffer vmbuffer = InvalidBuffer;
BlockNumber all_visible_streak;
pg_rusage_init(&ru0);
@ -292,6 +299,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
lazy_space_alloc(vacrelstats, nblocks);
all_visible_streak = 0;
for (blkno = 0; blkno < nblocks; blkno++)
{
Buffer buf;
@ -309,7 +317,14 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
/*
* Skip pages that don't require vacuuming according to the
* visibility map.
* visibility map. But only if we've seen a streak of at least
* SKIP_PAGES_THRESHOLD pages marked as clean. Since we're reading
* sequentially, the OS should be doing readahead for us and there's
* no gain in skipping a page now and then. You need a longer run of
* consecutive skipped pages before it's worthwhile. Also, skipping
* even a single page means that we can't update relfrozenxid or
* reltuples, so we only want to do it if there's a good chance to
* skip a goodly number of pages.
*/
if (!scan_all)
{
@ -317,9 +332,15 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
visibilitymap_test(onerel, blkno, &vmbuffer);
if (all_visible_according_to_vm)
{
vacrelstats->scanned_all = false;
continue;
all_visible_streak++;
if (all_visible_streak >= SKIP_PAGES_THRESHOLD)
{
vacrelstats->scanned_all = false;
continue;
}
}
else
all_visible_streak = 0;
}
vacuum_delay_point();