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Re: Potential memory usage issue

From: Bill Moran <wmoran(at)collaborativefusion(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Cc: David Brain <dbrain(at)bandwidth(dot)com>
Subject: Re: Potential memory usage issue
Date: 2007-03-22 12:50:37
Message-ID: 20070322085037.54780906.wmoran@collaborativefusion.com (view raw or flat)
Thread:
Lists: pgsql-performance
In response to David Brain <dbrain(at)bandwidth(dot)com>:
> 
> I recently migrated one of our large (multi-hundred GB) dbs from an 
> Intel 32bit platform (Dell 1650 - running 8.1.3) to a 64bit platform 
> (Dell 1950 - running 8.1.5).  However I am not seeing the performance 
> gains I would expect

What were you expecting?  It's possible that your expectations are
unreasonable.

In our testing, we found that 64bit on the same hardware as 32bit only
gave us a 5% gain, in the best case.  In many cases the gain was near
0, and in some there was a small performance loss.  These findings seemed
to jive with what others have been reporting.

> - I am suspecting that some of this is due to 
> differences I am seeing in reported memory usage.
> 
> On the 1650 - a 'typical' postmaster process looks like this in top:
> 
> 5267 postgres  16   0  439m 427m 386m S  3.0 21.1   3:31.73 postmaster
> 
> On the 1940 - a 'typical' postmaster process looks like:
> 
> 10304 postgres  16   0 41896  13m  11m D    4  0.3   0:11.73 postmaster
> 
> I currently have both systems running in parallel so the workloads will 
> be approximately equal.  The configurations of the two systems in terms 
> of postgresql.conf is pretty much identical between the two systems, I 
> did make some changes to logging, but nothing to buffers/shared memory 
> config.
> 
> I have never seen a postmaster process on the new system consume 
> anywhere near as much RAM as the old system - I am wondering if there is 
> something up with the shared memory config/usage that is causing my 
> performance issues.  Any thoughts as to where I should go from here?

Provide more information, for one thing.  I'm assuming from the top output
that this is some version of Linux, but more details on that are liable
to elicit more helpful feedback.

We run everything on FreeBSD here, but I haven't seen any difference in
the way PostgreSQL uses memory on ia32 FreeBSD vs. amd64 FreeBSD.  Without
more details on your setup, my only suggestion would be to double-verify
that your postgresql.conf settings are correct on the 64 bit system.

-- 
Bill Moran
Collaborative Fusion Inc.

wmoran(at)collaborativefusion(dot)com
Phone: 412-422-3463x4023

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