Date: Tue, 10 Sep 2013 14:20:36 +0200
From: Andres Freund <andres@2ndquadrant.com>
To: Andrew Dunstan <andrew@dunslane.net>
Cc: Jeff Janes <jeff.janes@gmail.com>, Josh Berkus <josh@agliodbs.com>,
 Amit Kapila <amit.kapila@huawei.com>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Performance bug in prepared statement binding in 9.2?
Message-ID: <20130910122036.GF1024477@alap2.anarazel.de>
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Hi,

On 2013-09-09 20:38:09 -0400, Andrew Dunstan wrote:
> 
> On 08/01/2013 03:20 PM, Jeff Janes wrote:
> >On Thu, Aug 1, 2013 at 10:58 AM, Josh Berkus <josh@agliodbs.com> wrote:
> >>Amit, All:
> >>
> >>So we just retested this on 9.3b2.  The performance is the same as 9.1
> >>and 9.2; that is, progressively worse as the test cycles go on, and
> >>unacceptably slow compared to 8.4.
> >>
> >>Some issue introduced in 9.1 is causing BINDs to get progressively
> >>slower as the PARSEs BINDs get run repeatedly.  Per earlier on this
> >>thread, that can bloat to 200X time required for a BIND, and it's
> >>definitely PostgreSQL-side.
> >>
> >>I'm trying to produce a test case which doesn't involve the user's
> >>application.  However, hints on other things to analyze would be keen.
> >Does it seem to be all CPU time (it is hard to imagine what else it
> >would be, but...)
> >
> >Could you use oprofile or perf or gprof to get a profile of the
> >backend during a run?  That should quickly narrow it down to which C
> >function has the problem.
> >
> >Did you test 9.0 as well?
> 
> 
> This has been tested back to 9.0. What we have found is that the problem
> disappears if the database has come in via dump/restore, but is present if
> it is the result of pg_upgrade. There are some long-running transactions
> also running alongside this - we are currently planning a test where those
> are not present. We're also looking at constructing a self-contained test
> case.
> 
> Here is some perf output from the bad case:
> 
>    +  14.67%  postgres           [.] heap_hot_search_buffer
>    +  11.45%  postgres           [.] LWLockAcquire
>    +   8.39%  postgres           [.] LWLockRelease
>    +   6.60%  postgres           [.] _bt_checkkeys
>    +   6.39%  postgres           [.] PinBuffer
>    +   5.96%  postgres           [.] hash_search_with_hash_value
>    +   5.43%  postgres           [.] hash_any
>    +   5.14%  postgres           [.] UnpinBuffer
>    +   3.43%  postgres           [.] ReadBuffer_common
>    +   2.34%  postgres           [.] index_fetch_heap
>    +   2.04%  postgres           [.] heap_page_prune_opt

A backtrace for this would be useful. Alternatively you could recompile
postgres using -fno-omit-frame-pointer in CFLAGS and use perf record -g.

Any chance you have older prepared xacts, older sessions or something
like that around? I'd expect heap_prune* to be present in workloads that
spend significant time in heap_hot_search_buffer...

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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