Re: More shared buffers causes lower performances

[Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>]

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?

			regards, tom lane