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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Craig Ringer <craig@postnewspapers.com.au>
Cc: Matthew Wakeling <matthew@flymine.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Pooling in Core WAS: Need help in performance tuning.
Date: Tue, 27 Jul 2010 16:40:02 -0400
Message-ID: <13654.1280263202@sss.pgh.pa.us> (raw)
In-Reply-To: <AANLkTinnxNM2vOwB6-tkBhCQ=6+xChOfdjrofDEpxYxk@mail.gmail.com>
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	<AANLkTimNJZlX7kFH_r0-BB_K3NGorU7BHd85SxXJbuGJ@mail.gmail.com>
	<4C38BF03.6060503@agliodbs.com>
	<19244.1278787866@sss.pgh.pa.us>
	<alpine.DEB.2.00.1007121044330.2534@aragorn.flymine.org>
	<4C3AF564.5030204@postnewspapers.com.au>
	<AANLkTikG_QqNXmGaAtnChGHyVAhdSosoE5M3MXwDKMH8@mail.gmail.com>
	<20100722212954.GA16758@anarazel.de>
	<AANLkTinnxNM2vOwB6-tkBhCQ=6+xChOfdjrofDEpxYxk@mail.gmail.com>

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Jul 22, 2010 at 5:29 PM, Andres Freund <andres@anarazel.de> wrote:
>>> The problem is harder for us because a backend can't switch identities
>>> once it's been assigned to a database.  I haven't heard an adequate
>>> explanation of why that couldn't be changed, though.

>> Possibly it might decrease the performance significantly enough by
>> reducing the cache locality (syscache, prepared plans)?

> Those things are backend-local.  The worst case scenario is you've got
> to flush them all when you reinitialize, in which case you still save
> the overhead of creating a new process.

"Flushing them all" is not zero-cost; it's not too hard to believe that
it could actually be slower than forking a clean new backend.

What's much worse, it's not zero-bug.  We've got little bitty caches
all over the backend, including (no doubt) some caching behavior in
third-party code that wouldn't get the word about whatever API you
invented to deal with this.

			regards, tom lane



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