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Commit7e26e02

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Prefetch blocks during lazy vacuum's truncation scan
Vacuum truncation scan can be sped up on rotating media by prefetchingblocks in forward direction. That makes the blocks already present inmemory by the time they are needed, while also letting OS read-aheadkick in.The truncate scan has been measured to be five times faster than withoutthis patch (that was on a slow disk, but it shouldn't hurt on fastdisks.)Author: Álvaro Herrera, loosely based on a submission by Claudio FreireDiscussion:https://postgr.es/m/CAGTBQpa6NFGO_6g_y_7zQx8L9GcHDSQKYdo1tGuh791z6PYgEg@mail.gmail.com
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‎src/backend/commands/vacuumlazy.c

Lines changed: 31 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -98,6 +98,12 @@
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*/
9999
#defineSKIP_PAGES_THRESHOLD((BlockNumber) 32)
100100

101+
/*
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* Size of the prefetch window for lazy vacuum backwards truncation scan.
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* Needs to be a power of 2.
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*/
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#definePREFETCH_SIZE((BlockNumber) 32)
106+
101107
typedefstructLVRelStats
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{
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/* hasindex = true means two-pass strategy; false means one-pass */
@@ -1826,13 +1832,22 @@ static BlockNumber
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count_nondeletable_pages(Relationonerel,LVRelStats*vacrelstats)
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{
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BlockNumberblkno;
1835+
BlockNumberprefetchedUntil;
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instr_timestarttime;
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/* Initialize the starttime if we check for conflicting lock requests */
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INSTR_TIME_SET_CURRENT(starttime);
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1834-
/* Strange coding of loop control is needed because blkno is unsigned */
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/*
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* Start checking blocks at what we believe relation end to be and move
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* backwards. (Strange coding of loop control is needed because blkno is
1844+
* unsigned.) To make the scan faster, we prefetch a few blocks at a time
1845+
* in forward direction, so that OS-level readahead can kick in.
1846+
*/
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blkno=vacrelstats->rel_pages;
1848+
StaticAssertStmt((PREFETCH_SIZE& (PREFETCH_SIZE-1))==0,
1849+
"prefetch size must be power of 2");
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prefetchedUntil=InvalidBlockNumber;
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while (blkno>vacrelstats->nonempty_pages)
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{
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Bufferbuf;
@@ -1882,6 +1897,21 @@ count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
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blkno--;
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1900+
/* If we haven't prefetched this lot yet, do so now. */
1901+
if (prefetchedUntil>blkno)
1902+
{
1903+
BlockNumberprefetchStart;
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BlockNumberpblkno;
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1906+
prefetchStart=blkno& ~(PREFETCH_SIZE-1);
1907+
for (pblkno=prefetchStart;pblkno <=blkno;pblkno++)
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{
1909+
PrefetchBuffer(onerel,MAIN_FORKNUM,pblkno);
1910+
CHECK_FOR_INTERRUPTS();
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}
1912+
prefetchedUntil=prefetchStart;
1913+
}
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buf=ReadBufferExtended(onerel,MAIN_FORKNUM,blkno,
18861916
RBM_NORMAL,vac_strategy);
18871917

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