Re: More shared buffers causes lower performances

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?

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

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://postgres.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +
Index: doc/src/sgml/installation.sgml
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RCS file: /cvsroot/pgsql/doc/src/sgml/installation.sgml,v
retrieving revision 1.302
diff -c -c -r1.302 installation.sgml
*** doc/src/sgml/installation.sgml    17 Feb 2008 16:36:43 -0000    1.302
--- doc/src/sgml/installation.sgml    6 Mar 2008 21:36:39 -0000
***************
*** 1144,1157 ****
          <para>
           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.
           Also, having the tests turned on won't necessarily enhance the
           stability of your server!  The assertion checks are not categorized
           for severity, and so what might be a relatively harmless bug will
           still lead to server restarts if it triggers an assertion
!          failure.  Currently, this option is not recommended for
!          production use, but you should have it on for development work
!          or when running a beta version.
          </para>
         </listitem>
        </varlistentry>
--- 1144,1158 ----
          <para>
           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 can slow down the
!          server significantly.
           Also, having the tests turned on won't necessarily enhance the
           stability of your server!  The assertion checks are not categorized
           for severity, and so what might be a relatively harmless bug will
           still lead to server restarts if it triggers an assertion
!          failure.  This option is not recommended for production use, but
!          you should have it on for development work or when running a beta
!          version.
          </para>
         </listitem>
        </varlistentry>