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Commit11e178d

Browse files
committed
Inline initial comparisons in TestForOldSnapshot()
Even with old_snapshot_threshold = -1 (which disables the "snapshottoo old" feature), performance regressions were seen at moderate tohigh concurrency. For example, a one-socket, four-core systemrunning 200 connections at saturation could see up to a 2.3%regression, with larger regressions possible on NUMA machines.By inlining the early (smaller, faster) tests in theTestForOldSnapshot() function, the i7 case dropped to a 0.2%regression, which could easily just be noise, and is clearly animprovement. Further testing will show whether more is needed.
1 parent5b1f9ce commit11e178d

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2 files changed

+36
-24
lines changed

2 files changed

+36
-24
lines changed

‎src/backend/storage/buffer/bufmgr.c

Lines changed: 5 additions & 23 deletions
Original file line numberDiff line numberDiff line change
@@ -4282,33 +4282,15 @@ IssuePendingWritebacks(WritebackContext *context)
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42844284
/*
4285-
* Check whether the given snapshot is too old to have safely read the given
4286-
* page from the given table. If so, throw a "snapshot too old" error.
4285+
* Implement slower/larger portions of TestForOldSnapshot
42874286
*
4288-
* This test generally needs to be performed after every BufferGetPage() call
4289-
* that is executed as part of a scan. It is not needed for calls made for
4290-
* modifying the page (for example, to position to the right place to insert a
4291-
* new index tuple or for vacuuming). It may also be omitted where calls to
4292-
* lower-level functions will have already performed the test.
4293-
*
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* Note that a NULL snapshot argument is allowed and causes a fast return
4295-
* without error; this is to support call sites which can be called from
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* either scans or index modification areas.
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*
4298-
* For best performance, keep the tests that are fastest and/or most likely to
4299-
* exclude a page from old snapshot testing near the front.
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* Smaller/faster portions are put inline, but the entire set of logic is too
4288+
* big for that.
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*/
43014290
void
4302-
TestForOldSnapshot(Snapshotsnapshot,Relationrelation,Pagepage)
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TestForOldSnapshot_impl(Snapshotsnapshot,Relationrelation)
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{
4304-
Assert(relation!=NULL);
4305-
4306-
if (old_snapshot_threshold >=0
4307-
&& (snapshot)!=NULL
4308-
&& (snapshot)->satisfies==HeapTupleSatisfiesMVCC
4309-
&& !XLogRecPtrIsInvalid((snapshot)->lsn)
4310-
&&PageGetLSN(page)> (snapshot)->lsn
4311-
&& !IsCatalogRelation(relation)
4293+
if (!IsCatalogRelation(relation)
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&& !RelationIsAccessibleInLogicalDecoding(relation)
43134295
&& (snapshot)->whenTaken<GetOldSnapshotThresholdTimestamp())
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ereport(ERROR,

‎src/include/storage/bufmgr.h

Lines changed: 31 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -239,7 +239,7 @@ extern bool BgBufferSync(struct WritebackContext *wb_context);
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externvoidAtProcExit_LocalBuffers(void);
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242-
externvoidTestForOldSnapshot(Snapshotsnapshot,Relationrelation,Pagepage);
242+
externvoidTestForOldSnapshot_impl(Snapshotsnapshot,Relationrelation);
243243

244244
/* in freelist.c */
245245
externBufferAccessStrategyGetAccessStrategy(BufferAccessStrategyTypebtype);
@@ -257,6 +257,36 @@ extern void FreeAccessStrategy(BufferAccessStrategy strategy);
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258258
#ifndefFRONTEND
259259

260+
/*
261+
* Check whether the given snapshot is too old to have safely read the given
262+
* page from the given table. If so, throw a "snapshot too old" error.
263+
*
264+
* This test generally needs to be performed after every BufferGetPage() call
265+
* that is executed as part of a scan. It is not needed for calls made for
266+
* modifying the page (for example, to position to the right place to insert a
267+
* new index tuple or for vacuuming). It may also be omitted where calls to
268+
* lower-level functions will have already performed the test.
269+
*
270+
* Note that a NULL snapshot argument is allowed and causes a fast return
271+
* without error; this is to support call sites which can be called from
272+
* either scans or index modification areas.
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*
274+
* For best performance, keep the tests that are fastest and/or most likely to
275+
* exclude a page from old snapshot testing near the front.
276+
*/
277+
staticinlinevoid
278+
TestForOldSnapshot(Snapshotsnapshot,Relationrelation,Pagepage)
279+
{
280+
Assert(relation!=NULL);
281+
282+
if (old_snapshot_threshold >=0
283+
&& (snapshot)!=NULL
284+
&& (snapshot)->satisfies==HeapTupleSatisfiesMVCC
285+
&& !XLogRecPtrIsInvalid((snapshot)->lsn)
286+
&&PageGetLSN(page)> (snapshot)->lsn)
287+
TestForOldSnapshot_impl(snapshot,relation);
288+
}
289+
260290
#endif/* FRONTEND */
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262292
#endif/* BUFMGR_H */

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