Re: More shared buffers causes lower performances

Tom Lane wrote:
> Greg Smith <gsmith(at)gregsmith(dot)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?

Docs updated with attached patch, backpatched to 8.3.X.

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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://postgres.enterprisedb.com

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