Re: More shared buffers causes lower performances

[Magnus Hagander <magnus(at)hagander(dot)net>]

On Thu, Dec 27, 2007 at 01:10:29AM -0500, 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?

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