Re: More shared buffers causes lower performances - Mailing list pgsql-performance

From Magnus Hagander
Subject Re: More shared buffers causes lower performances
Date
Msg-id 20071227084759.GB23218@svr2.hagander.net
Whole thread Raw
In response to Re: More shared buffers causes lower performances  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-performance
On Thu, Dec 27, 2007 at 01:10:29AM -0500, Tom Lane wrote:
> 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?

Sounds like a good idea. We got bit by the same thing when doing some
benchmarks on the MSVC port (and with we I mean Dave did the work, and several
people couldn't understand why the numbers sucked)

//Magnus

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