Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On 2016-04-12 16:49:25 +0000, Kevin Grittner wrote:
> 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.

FWIW, I could see massive regressions with just 64 connections.

I'm a bit scared of having an innoccuous sounding option regress things
by a factor of 10. I think, in addition to this fix, we need to actually
solve the scalability issue here to a good degree.  One way to do so is
to apply the parts of 0001 in
http://archives.postgresql.org/message-id/20160330230914.GH13305%40awork2.anarazel.de
defining PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY and rely on that. Another
to apply the whole patch and simply put the lsn in an 8 byte atomic.

- Andres