From 7e26e02eec90370dd222f35f00042f8188488ac4 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Mon, 23 Jan 2017 12:55:18 -0300
Subject: [PATCH] Prefetch blocks during lazy vacuum's truncation scan
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Vacuum truncation scan can be sped up on rotating media by prefetching
blocks in forward direction. That makes the blocks already present in
memory by the time they are needed, while also letting OS read-ahead
kick in.
The truncate scan has been measured to be five times faster than without
this patch (that was on a slow disk, but it shouldn't hurt on fast
disks.)
Author: Ãlvaro Herrera, loosely based on a submission by Claudio Freire
Discussion: https://postgr.es/m/CAGTBQpa6NFGO_6g_y_7zQx8L9GcHDSQKYdo1tGuh791z6PYgEg@mail.gmail.com
---
src/backend/commands/vacuumlazy.c | 32 ++++++++++++++++++++++++++++++-
1 file changed, 31 insertions(+), 1 deletion(-)
diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index a2999b3bf77..005440eb392 100644
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -98,6 +98,12 @@
*/
#define SKIP_PAGES_THRESHOLD ((BlockNumber) 32)
+/*
+ * Size of the prefetch window for lazy vacuum backwards truncation scan.
+ * Needs to be a power of 2.
+ */
+#define PREFETCH_SIZE ((BlockNumber) 32)
+
typedef struct LVRelStats
{
/* hasindex = true means two-pass strategy; false means one-pass */
@@ -1826,13 +1832,22 @@ static BlockNumber
count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
{
BlockNumber blkno;
+ BlockNumber prefetchedUntil;
instr_time starttime;
/* Initialize the starttime if we check for conflicting lock requests */
INSTR_TIME_SET_CURRENT(starttime);
- /* Strange coding of loop control is needed because blkno is unsigned */
+ /*
+ * Start checking blocks at what we believe relation end to be and move
+ * backwards. (Strange coding of loop control is needed because blkno is
+ * unsigned.) To make the scan faster, we prefetch a few blocks at a time
+ * in forward direction, so that OS-level readahead can kick in.
+ */
blkno = vacrelstats->rel_pages;
+ StaticAssertStmt((PREFETCH_SIZE & (PREFETCH_SIZE - 1)) == 0,
+ "prefetch size must be power of 2");
+ prefetchedUntil = InvalidBlockNumber;
while (blkno > vacrelstats->nonempty_pages)
{
Buffer buf;
@@ -1882,6 +1897,21 @@ count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
blkno--;
+ /* If we haven't prefetched this lot yet, do so now. */
+ if (prefetchedUntil > blkno)
+ {
+ BlockNumber prefetchStart;
+ BlockNumber pblkno;
+
+ prefetchStart = blkno & ~(PREFETCH_SIZE - 1);
+ for (pblkno = prefetchStart; pblkno <= blkno; pblkno++)
+ {
+ PrefetchBuffer(onerel, MAIN_FORKNUM, pblkno);
+ CHECK_FOR_INTERRUPTS();
+ }
+ prefetchedUntil = prefetchStart;
+ }
+
buf = ReadBufferExtended(onerel, MAIN_FORKNUM, blkno,
RBM_NORMAL, vac_strategy);
--
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