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Re: More shared buffers causes lower performances

From: "Merlin Moncure" <mmoncure(at)gmail(dot)com>
To: "Guillaume Smet" <guillaume(dot)smet(at)gmail(dot)com>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>, "Stefan Kaltenbrunner" <stefan(at)kaltenbrunner(dot)cc>
Subject: Re: More shared buffers causes lower performances
Date: 2007-12-29 17:22:00
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Dec 25, 2007 7:06 PM, Guillaume Smet <guillaume(dot)smet(at)gmail(dot)com> wrote:
> While monitoring the server with vmstat, I can't see any real reason
> why it's slower. When shared_buffers has a higher value, I/O are
> lower, context switches too and finally performances. The CPU usage is
> quite similar (~50-60%). I/O doesn't limit the performances AFAICS.

Can you confirm that i/o is lower according to iostat?  One
possibility is that you are on the cusp of where your server's memory
covers the database and the higher buffers results in lower memory

If raising shared buffers is getting you more page faults to disk,
this would explain the lower figures regardless of the # of syscalls.
If your iowait is zero though the test is cpu bound and this
distinction is moot.


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