Re: what to revert

On 2016-05-03 20:57:13 +0200, Tomas Vondra wrote:
> On 05/03/2016 07:41 PM, Andres Freund wrote:
> >On 2016-05-03 11:46:23 -0500, Kevin Grittner wrote:
> >>>was immediately addressed by another round of benchmarks after you
> >>>pointed it out.
> >>
> >>Which showed a 4% maximum hit before moving the test for whether it
> >>was "off" inline.
> >
> >>(I'm not clear from the posted results whether that was before or
> >>after skipping the spinlock when the feature was off.)
> >
> >They're from after the spinlock issue was resolved. Before that the
> >issue was a lot worse (see mail linked two messages upthread).
> >
> >
> >I'm pretty sure that I said that somewhere else at least once: But to
> >be absolutely clear, I'm *not* really concerned with the performance
> >with the feature turned off. I'm concerned about the performance with
> >it turned on.
>
> If you tell me how to best test it, I do have a 4-socket server sitting idly
> in the corner (well, a corner reachable by SSH). I can get us some numbers,
> but I haven't been following the snapshot_too_old so I'll need some guidance
> on what to test.

I think it'd be cool if you could test the effect of the feature in
read-only (and additionally read-mostly?) workload with various client
counts and snapshot_too_old values. For the latter maybe -1, 0, 10, 60
or such? I've done so (accidentally comparing 0 and 1 instead of -1 and
1) on a two socket machine in:
www.postgresql.org/message-id/20160413171955.i53me46fqqhdlztq@alap3.anarazel.de

It'd be very interesting to see how big the penalty is on a bigger box.