Kevin Grittner wrote:
> Avoid extra locks in GetSnapshotData if old_snapshot_threshold < 0
> On a big NUMA machine with 1000 connections in saturation load
> there was a performance regression due to spinlock contention, for
> acquiring values which were never used.  Just fill with dummy
> values if we're not going to use them.

old_snapshot_threshold is PGC_POSTMASTER, so this is okay AFAICS, but
perhaps it'd be a good idea to add a oneline comment to guc.c indicating
to verify this code if there's an intention to lift that limitation --
snapshots taken before the reload would have invalid lsn/timestamp, so
the current check would simply skip the check, which would be the wrong
thing to do.

I think it's reasonable to want to enable this feature with a simple
reload, so if we ever do that it'd be good to have a pointer about that
gotcha.  (I'm not proposing you do that, only add the comment for a
future hacker.)

Álvaro Herrera      
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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