Re: PostgreSQL and Xeon MP

Guillaume Smet wrote:
> Hello,
>
> We are experiencing performances problem with a quad Xeon MP and
> PostgreSQL 7.4 for a year now.

I had a similar issue with a client the other week.

> Our context switch rate is not so high
> but the load of the server is blocked to 4 even on very high load and
> we have 60% cpu idle even in this case. Our database fits in RAM and
> we don't have any IO problem.

Actually, I think that's part of the problem - it's the memory bandwidth.

> I saw this post from Tom Lane
> http://archives.postgresql.org/pgsql-performance/2004-04/msg00249.php
> and several other references to problem with Xeon MP and I suspect our
> problems are related to this.

You should be seeing context-switching jump dramatically if it's the
"classic" multi-Xeon problem. There's a point at which it seems to just
escalate without a corresponding jump in activity.

> We tried to put our production load on a dual standard Xeon on monday
> and it performs far better with the same configuration parameters.
>
> I know that work has been done by Tom for PostgreSQL 8.1 on
> multiprocessor support but I didn't find any information on if it
> solves the problem with Xeon MP or not.

I checked with Tom last week. Thread starts below:
http://archives.postgresql.org/pgsql-hackers/2006-02/msg01118.php

He's of the opinion that 8.1.3 will be an improvement.

> My question is should we expect a resolution of our problem by
> switching to 8.1 or will we still have problems and should we consider
> a hardware change? We will try to upgrade next tuesday so we will have
> the real answer soon but if anyone has any experience or information
> on this, he will be very welcome.

--
Richard Huxton
Archonet Ltd