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

On Tue, Apr 19, 2016 at 8:44 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Tue, Apr 19, 2016 at 11:11 AM, Kevin Grittner <kgrittn(at)gmail(dot)com>
wrote:
> > On Tue, Apr 19, 2016 at 9:57 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
>
> >> 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. For
> > now I'm focusing on the back-patching issues and the performance
> > regression when the feature is disabled, but I'll shift focus to
> > this once the "killer" issues are in hand.
>
> Maybe Amit could try his idea in parallel.
>

Okay, will look into it.

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