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Optimize xid/subxid searches in XidInMVCCSnapshot(). 

As reported by Yura Sokolov, scanning the snapshot->xip array has
noticeable impact on scalability when there are a large number of
concurrent writers. Use the optimized (on x86-64) routine from b6ef16756
to speed up searches through the [sub]xip arrays. One benchmark showed
a 5% increase in transaction throughput with 128 concurrent writers,
and a 50% increase in a pathological case of 1024 writers. While a hash
table would have scaled even better, it was ultimately rejected because
of concerns around code complexity and memory allocation. Credit to Andres
Freund for the idea to optimize linear search using SIMD instructions.

Nathan Bossart

Reviewed by: Andres Freund, John Naylor, Bharath Rupireddy, Masahiko Sawada
Discussion: https://postgr.es/m/20220713170950.GA3116318%40nathanxps13
This commit is contained in:
John Naylor 2022-08-03 09:59:28 -07:00
parent a8c0128697
commit 37a6e5df37
1 changed files with 7 additions and 21 deletions
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28
src/backend/utils/time/snapmgr.c
View File

@ -56,6 +56,7 @@
#include "datatype/timestamp.h"
#include "lib/pairingheap.h"
#include "miscadmin.h"
#include "port/pg_lfind.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
@ -2284,8 +2285,6 @@ RestoreTransactionSnapshot(Snapshot snapshot, void *source_pgproc)
bool
XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
{
uint32 i;
/*
* Make a quick range check to eliminate most XIDs without looking at the
* xip arrays. Note that this is OK even if we convert a subxact XID to
@ -2317,13 +2316,8 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
if (!snapshot->suboverflowed)
{
/* we have full data, so search subxip */
int32 j;
for (j = 0; j < snapshot->subxcnt; j++)
{
if (TransactionIdEquals(xid, snapshot->subxip[j]))
return true;
}
if (pg_lfind32(xid, snapshot->subxip, snapshot->subxcnt))
return true;
/* not there, fall through to search xip[] */
}
@ -2344,16 +2338,11 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
return false;
}
for (i = 0; i < snapshot->xcnt; i++)
{
if (TransactionIdEquals(xid, snapshot->xip[i]))
return true;
}
if (pg_lfind32(xid, snapshot->xip, snapshot->xcnt))
return true;
}
else
{
int32 j;
/*
* In recovery we store all xids in the subxact array because it is by
* far the bigger array, and we mostly don't know which xids are
@ -2383,11 +2372,8 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
* indeterminate xid. We don't know whether it's top level or subxact
* but it doesn't matter. If it's present, the xid is visible.
*/
for (j = 0; j < snapshot->subxcnt; j++)
{
if (TransactionIdEquals(xid, snapshot->subxip[j]))
return true;
}
if (pg_lfind32(xid, snapshot->subxip, snapshot->subxcnt))
return true;
}
return false;