This contention is masked by contention elsewhere, e.g. ProcArrayLock, so the need for testing here should come once other patches ahead of this are in.
Let me explain more clearly.
Andres' patch to cache snapshots and reduce ProcArrayLock was interesting, but not initially compelling. We now have a solution that commits in batches, which will reduce the number of times the ProcArray changes - this will heighten the benefit from Andres' snapshot cache patch.
So the order of testing/commit should be
Queued commit patch
ProcArray cache patch
Clog shared commit patch (this one)
I didn't hear recent mention of Robert's chash patch, but IIRC it was effective and so we hope to see it again soon also.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services