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

On Sun, Apr 17, 2016 at 2:26 AM, Andres Freund <andres(at)anarazel(dot)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.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com