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Commit37a6e5d

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Optimize xid/subxid searches in XidInMVCCSnapshot().
As reported by Yura Sokolov, scanning the snapshot->xip array hasnoticeable impact on scalability when there are a large number ofconcurrent writers. Use the optimized (on x86-64) routine fromb6ef167to speed up searches through the [sub]xip arrays. One benchmark showeda 5% increase in transaction throughput with 128 concurrent writers,and a 50% increase in a pathological case of 1024 writers. While a hashtable would have scaled even better, it was ultimately rejected becauseof concerns around code complexity and memory allocation. Credit to AndresFreund for the idea to optimize linear search using SIMD instructions.Nathan BossartReviewed by: Andres Freund, John Naylor, Bharath Rupireddy, Masahiko SawadaDiscussion:https://postgr.es/m/20220713170950.GA3116318%40nathanxps13
1 parenta8c0128 commit37a6e5d

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+7
-21
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+7
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‎src/backend/utils/time/snapmgr.c

Lines changed: 7 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -56,6 +56,7 @@
5656
#include"datatype/timestamp.h"
5757
#include"lib/pairingheap.h"
5858
#include"miscadmin.h"
59+
#include"port/pg_lfind.h"
5960
#include"storage/predicate.h"
6061
#include"storage/proc.h"
6162
#include"storage/procarray.h"
@@ -2284,8 +2285,6 @@ RestoreTransactionSnapshot(Snapshot snapshot, void *source_pgproc)
22842285
bool
22852286
XidInMVCCSnapshot(TransactionIdxid,Snapshotsnapshot)
22862287
{
2287-
uint32i;
2288-
22892288
/*
22902289
* Make a quick range check to eliminate most XIDs without looking at the
22912290
* xip arrays. Note that this is OK even if we convert a subxact XID to
@@ -2317,13 +2316,8 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
23172316
if (!snapshot->suboverflowed)
23182317
{
23192318
/* we have full data, so search subxip */
2320-
int32j;
2321-
2322-
for (j=0;j<snapshot->subxcnt;j++)
2323-
{
2324-
if (TransactionIdEquals(xid,snapshot->subxip[j]))
2325-
return true;
2326-
}
2319+
if (pg_lfind32(xid,snapshot->subxip,snapshot->subxcnt))
2320+
return true;
23272321

23282322
/* not there, fall through to search xip[] */
23292323
}
@@ -2344,16 +2338,11 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
23442338
return false;
23452339
}
23462340

2347-
for (i=0;i<snapshot->xcnt;i++)
2348-
{
2349-
if (TransactionIdEquals(xid,snapshot->xip[i]))
2350-
return true;
2351-
}
2341+
if (pg_lfind32(xid,snapshot->xip,snapshot->xcnt))
2342+
return true;
23522343
}
23532344
else
23542345
{
2355-
int32j;
2356-
23572346
/*
23582347
* In recovery we store all xids in the subxact array because it is by
23592348
* far the bigger array, and we mostly don't know which xids are
@@ -2383,11 +2372,8 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
23832372
* indeterminate xid. We don't know whether it's top level or subxact
23842373
* but it doesn't matter. If it's present, the xid is visible.
23852374
*/
2386-
for (j=0;j<snapshot->subxcnt;j++)
2387-
{
2388-
if (TransactionIdEquals(xid,snapshot->subxip[j]))
2389-
return true;
2390-
}
2375+
if (pg_lfind32(xid,snapshot->subxip,snapshot->subxcnt))
2376+
return true;
23912377
}
23922378

23932379
return false;

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