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Commit2201d80

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Avoid extra locks in GetSnapshotData if old_snapshot_threshold < 0
On a big NUMA machine with 1000 connections in saturation loadthere was a performance regression due to spinlock contention, foracquiring values which were never used. Just fill with dummyvalues if we're not going to use them.This patch has not been benchmarked yet on a big NUMA machine, butit seems like a good idea on general principle, and it seemed toprevent an apparent 2.2% regression on a single-socket i7 boxrunning 200 connections at saturation load.
1 parent5713f03 commit2201d80

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1 file changed

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‎src/backend/storage/ipc/procarray.c

Lines changed: 20 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -1759,14 +1759,26 @@ GetSnapshotData(Snapshot snapshot)
17591759
snapshot->regd_count=0;
17601760
snapshot->copied= false;
17611761

1762-
/*
1763-
* Capture the current time and WAL stream location in case this snapshot
1764-
* becomes old enough to need to fall back on the special "old snapshot"
1765-
* logic.
1766-
*/
1767-
snapshot->lsn=GetXLogInsertRecPtr();
1768-
snapshot->whenTaken=GetSnapshotCurrentTimestamp();
1769-
MaintainOldSnapshotTimeMapping(snapshot->whenTaken,xmin);
1762+
if (old_snapshot_threshold<0)
1763+
{
1764+
/*
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* If not using "snapshot too old" feature, fill related fields with
1766+
* dummy values that don't require any locking.
1767+
*/
1768+
snapshot->lsn=InvalidXLogRecPtr;
1769+
snapshot->whenTaken=0;
1770+
}
1771+
else
1772+
{
1773+
/*
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* Capture the current time and WAL stream location in case this
1775+
* snapshot becomes old enough to need to fall back on the special
1776+
* "old snapshot" logic.
1777+
*/
1778+
snapshot->lsn=GetXLogInsertRecPtr();
1779+
snapshot->whenTaken=GetSnapshotCurrentTimestamp();
1780+
MaintainOldSnapshotTimeMapping(snapshot->whenTaken,xmin);
1781+
}
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returnsnapshot;
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}

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