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To: Greg Smith <gsmith@gregsmith.com>
cc: Guillaume Smet <guillaume.smet@gmail.com>,
	pgsql-performance <pgsql-performance@postgresql.org>,
	Stefan Kaltenbrunner <stefan@kaltenbrunner.cc>,
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Subject: Re: More shared buffers causes lower performances 
In-reply-to: <Pine.GSO.4.64.0712261816520.16448@westnet.com> 
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Comments: In-reply-to Greg Smith <gsmith@gregsmith.com>
	message dated "Wed, 26 Dec 2007 18:35:33 -0500"
Date: Thu, 27 Dec 2007 01:10:29 -0500
Message-ID: <29359.1198735829@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Greg Smith <gsmith@gregsmith.com> writes:
> On Wed, 26 Dec 2007, Guillaume Smet wrote:
>> beta RPMs are by default compiled with --enable-debug and
>> --enable-cassert which doesn't help them to fly fast...

> Got that right.  Last time I was going crazy after running pgbench with 
> those options and not having realized what I changed, I was getting a 50% 
> slowdown on results that way compared to without the debugging stuff. 
> Didn't realize it scaled with shared_buffers though.

See AtEOXact_Buffers().  There are probably any number of other
interesting scaling behaviors --- in my tests, AllocSetCheck() is
normally a major cycle-eater if --enable-cassert is set, and that costs
time proportional to the number of memory chunks allocated by the query.

Currently the docs say that --enable-cassert

         Enables <firstterm>assertion</> checks in the server, which test for
         many <quote>cannot happen</> conditions.  This is invaluable for
         code development purposes, but the tests slow things down a little.

Maybe we ought to put that more strongly --- s/a little/significantly/,
perhaps?

			regards, tom lane