From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jeff Frost <jeff@pgexperts.com>
cc: Andres Freund <andres@2ndquadrant.com>,
 Matheus de Oliveira <matioli.matheus@gmail.com>,
 Soni M <diptatapa@gmail.com>, Heikki Linnakangas <hlinnakangas@vmware.com>,
 pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Postgres Replaying WAL slowly
In-reply-to: <571D044B-260A-4E06-ACBC-8869B8F73C0F@pgexperts.com>
References: <20140630171416.GK21422@awork2.anarazel.de>
 <CAAMgDXnT8cQbwxcNXfFKONrRVAKoGhR-3FWYSX_5gJcFrEMPMQ@mail.gmail.com>
 <8DA8C140-818D-4184-AAE6-CA37278B983C@pgexperts.com>
 <20140630183908.GT26930@awork2.anarazel.de>
 <0A7B96D4-DB10-49A8-A268-F28960F2E137@pgexperts.com>
 <F3E1168D-8D49-407E-A8B7-B3ED81C93DCB@pgexperts.com>
 <20300.1404156750@sss.pgh.pa.us>
 <C64129CF-D034-4311-AD32-09EF048C3E93@pgexperts.com>
 <CAJghg4Lq2rg7mMzRfEartAQK1c83veoW3TWroQsSvXMqBQ9h+w@mail.gmail.com>
 <B22B06CF-2BC0-4AB6-8E3C-FACDF8193F23@pgexperts.com>
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 <571D044B-260A-4E06-ACBC-8869B8F73C0F@pgexperts.com>
Comments: In-reply-to Jeff Frost <jeff@pgexperts.com>
 message dated "Mon, 30 Jun 2014 14:52:22 -0700"
Date: Mon, 30 Jun 2014 19:04:20 -0400
Message-ID: <24455.1404169460@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

Jeff Frost <jeff@pgexperts.com> writes:
>>> So it seems like we have a candidate explanation.  I'm a bit surprised
>>> that StandbyReleaseLocks would get this slow if there are only a dozen
>>> AccessExclusiveLocks in place at any one time, though.  Perhaps that
>>> was a low point and there are often many more?

> Since we turned on the monitoring for that, we had a peak of 13,550
> AccessExclusiveLocks.

Ah ... that's more like a number I can believe something would have
trouble coping with.  Did you see a noticeable slowdown with this?
Now that we've seen that number, of course it's possible there was an
even higher peak occurring when you saw the trouble.

Perhaps there's an O(N^2) behavior in StandbyReleaseLocks, or maybe
it just takes awhile to handle that many locks.

Did you check whether the locks were all on temp tables of the
ON COMMIT DROP persuasion?

			regards, tom lane


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