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Commit 11e178d

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kgrittnkgrittn
committed
Inline initial comparisons in TestForOldSnapshot()
Even with old_snapshot_threshold = -1 (which disables the "snapshot too old" feature), performance regressions were seen at moderate to high concurrency. For example, a one-socket, four-core system running 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 the TestForOldSnapshot() function, the i7 case dropped to a 0.2% regression, which could easily just be noise, and is clearly an improvement. Further testing will show whether more is needed.
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‎src/backend/storage/buffer/bufmgr.c

Copy file name to clipboardExpand all lines: src/backend/storage/buffer/bufmgr.c
+5-23Lines changed: 5 additions & 23 deletions
Original file line numberDiff line numberDiff line change
@@ -4282,33 +4282,15 @@ IssuePendingWritebacks(WritebackContext *context)
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/*
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* Check whether the given snapshot is too old to have safely read the given
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* page from the given table. If so, throw a "snapshot too old" error.
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* Implement slower/larger portions of TestForOldSnapshot
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*
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* This test generally needs to be performed after every BufferGetPage() call
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* that is executed as part of a scan. It is not needed for calls made for
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* modifying the page (for example, to position to the right place to insert a
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* new index tuple or for vacuuming). It may also be omitted where calls to
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* lower-level functions will have already performed the test.
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*
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* Note that a NULL snapshot argument is allowed and causes a fast return
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* 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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*
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* For best performance, keep the tests that are fastest and/or most likely to
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* 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
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* big for that.
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*/
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void
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TestForOldSnapshot(Snapshot snapshot, Relation relation, Page page)
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TestForOldSnapshot_impl(Snapshot snapshot, Relation relation)
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{
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Assert(relation != NULL);
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if (old_snapshot_threshold >= 0
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&& (snapshot) != NULL
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&& (snapshot)->satisfies == HeapTupleSatisfiesMVCC
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&& !XLogRecPtrIsInvalid((snapshot)->lsn)
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&& PageGetLSN(page) > (snapshot)->lsn
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&& !IsCatalogRelation(relation)
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if (!IsCatalogRelation(relation)
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&& !RelationIsAccessibleInLogicalDecoding(relation)
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&& (snapshot)->whenTaken < GetOldSnapshotThresholdTimestamp())
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ereport(ERROR,

‎src/include/storage/bufmgr.h

Copy file name to clipboardExpand all lines: src/include/storage/bufmgr.h
+31-1Lines 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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extern void AtProcExit_LocalBuffers(void);
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extern void TestForOldSnapshot(Snapshot snapshot, Relation relation, Page page);
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extern void TestForOldSnapshot_impl(Snapshot snapshot, Relation relation);
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/* in freelist.c */
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extern BufferAccessStrategy GetAccessStrategy(BufferAccessStrategyType btype);
@@ -257,6 +257,36 @@ extern void FreeAccessStrategy(BufferAccessStrategy strategy);
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#ifndef FRONTEND
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/*
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* Check whether the given snapshot is too old to have safely read the given
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* page from the given table. If so, throw a "snapshot too old" error.
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*
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* This test generally needs to be performed after every BufferGetPage() call
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* that is executed as part of a scan. It is not needed for calls made for
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* modifying the page (for example, to position to the right place to insert a
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* new index tuple or for vacuuming). It may also be omitted where calls to
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* lower-level functions will have already performed the test.
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*
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* Note that a NULL snapshot argument is allowed and causes a fast return
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* 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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*
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* For best performance, keep the tests that are fastest and/or most likely to
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* exclude a page from old snapshot testing near the front.
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*/
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static inline void
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TestForOldSnapshot(Snapshot snapshot, Relation relation, Page page)
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{
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Assert(relation != NULL);
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if (old_snapshot_threshold >= 0
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&& (snapshot) != NULL
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&& (snapshot)->satisfies == HeapTupleSatisfiesMVCC
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&& !XLogRecPtrIsInvalid((snapshot)->lsn)
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&& PageGetLSN(page) > (snapshot)->lsn)
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TestForOldSnapshot_impl(snapshot, relation);
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}
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#endif /* FRONTEND */
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#endif /* BUFMGR_H */

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