From: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
---|---|
To: | Andres Freund <andres(at)anarazel(dot)de> |
Cc: | Noah Misch <noah(at)leadboat(dot)com>, Kevin Grittner <kgrittn(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <kgrittn(at)postgresql(dot)org>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, "pgsql-committers(at)postgresql(dot)org" <pgsql-committers(at)postgresql(dot)org> |
Subject: | Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < |
Date: | 2016-04-19 14:57:31 |
Message-ID: | CAA4eK1JihZepuTasniqrHcsha0umwjeke_ay3AckpGMH4QdJ_g@mail.gmail.com |
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Lists: | pgsql-committers pgsql-hackers |
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
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