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

On Tue, Apr 19, 2016 at 8:41 PM, Kevin Grittner <kgrittn@gmail.com> wrote:
>
> On Tue, Apr 19, 2016 at 9:57 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> > On Sun, Apr 17, 2016 at 2:26 AM, Andres Freund <andres@anarazel.de> wrote:
> >>
> >> On 2016-04-16 16:44:52 -0400, Noah Misch wrote:
> >> > That is more controversial than the potential ~2% regression for
> >> > old_snapshot_threshold=-1.  Alvaro[2] and Robert[3] are okay releasing
> >> > that way, and Andres[4] is not.
> >>
> >> FWIW, I could be kinda convinced that it's temporarily ok, if there'd be
> >> a clear proposal on the table how to solve the scalability issue around
> >> MaintainOldSnapshotTimeMapping().
> >
> > It seems that for read-only workloads, MaintainOldSnapshotTimeMapping()
> > takes EXCLUSIVE LWLock which seems to be a probable reason for a performance
> > regression.  Now, here the question is do we need to acquire that lock if
> > xmin is not changed since the last time value of
> > oldSnapshotControl->latest_xmin is updated or xmin is lesser than equal to
> > oldSnapshotControl->latest_xmin?
> > If we don't need it for above cases, I think it can address the performance
> > regression to a good degree for read-only workloads when the feature is
> > enabled.
>
> Thanks, Amit -- I think something along those lines is the right
> solution to the scaling issues when the feature is enabled.
>

I have tried attached patch along the above lines and it seems that it addresses performance regression to a good degree when feature is enabled at moderate client-count like 32, but still more needs to be done for somewhat higher client-count like 64.

Performance data is for median of 3, 5 min runs of read-only workload -

pgbench -c $client_count -j $client_count -T 300 -M prepared -S postgres

o_s_t - old_snapshot_threshold

 

If you think that attached patch is correct functionality wise, then I think we can go-ahead with it and then investigate what more can be improved.  I think newly introduced spinlocks might be the reason of performance degradation at higher client-count, if that turns out to be true, then I think we can replace them with atomics, once Andres's patch for completing the 64-bit atomics implementation is committed.

m/c details used for performance testing