Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Tue, Apr 12, 2016 at 12:38 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> 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.

With what settings? With or without the patch to avoid the locks when off?

> 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.

I think that we are well due for atomic access to aligned 8-byte
values. That would eliminate one potential hot spot in the
"snapshot too old" code, for sure.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company