Re: shared_buffers performance

On Mon, 14 Apr 2008, Gaetano Mendola wrote:

> I'm using postgres 8.2.3 on Red Hat compiled with GCC 3.4.6.

8.2.3 has a performance bug that impacts how accurate pgbench results are;
you really should be using a later version.

> http://img84.imageshack.us/my.php?image=totalid7.png
> as you can see using 64MB as value for shared_buffers I'm obtaining
> better results.

I'm assuming you've read my scaling article at
http://www.westnet.com/~gsmith/content/postgresql/pgbench-scaling.htm
since you're using the graph template I suggest there.

If you look carefully at your results, you are getting better results for
higher shared_buffers values in the cases where performance is memory
bound (the lower scale numbers).  Things reverse so that more buffers
gives worse performance only when your scale >100.  I wouldn't conclude
too much from that.  The pgbench select test is doing a low-level
operation that doesn't benefit as much from having more memory available
to PostgreSQL instead of the OS as a real-world workload will.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD