Message-ID: <53280.75.37.29.21.1174242150.squirrel@webmail.asterdata.net>
In-Reply-To: <45FD5A2D.2090307@boreham.org>
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	<45FD5A2D.2090307@boreham.org>
Date: Sun, 18 Mar 2007 11:22:30 -0700 (PDT)
Subject: Re: Performance Tuning and Disk Cache
From: "Rangarajan Vasudevan" <ranga@asterdata.com>
To: pgsql-performance@postgresql.org
Reply-To: ranga@asterdata.com
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If you are running on a Linux kernel, try /proc/sys/vm/drop_caches. I
believe the appropriate command is "echo 3 > /proc/sys/vm/drop_caches".
Since Postgres has its own cache of data, the above followed by a PG
restart should do what you are looking for.

Ranga


> Barry Moore wrote:
>
>> I have a very slow query that I'm trying to tune.  I think my
>> performance tuning is being complicated by the system's page cache.
>>
>> If a run the query after the system has been busy with other tasks
>> for quite a long time then the query can take up to 8-10 minutes to
>> complete.  If I then rerun the same query it will complete in a
>> couple of seconds.
>>
>> Does anyone know how I can repeatedly run the same query in the
>> "worst case scenario" of no postgres data in the disk cache (e.g.,
>> clear the page cache or force it to be ignored)?
>
> In my experience the only 100% reliable way to do this is to reboot the
> machine.
>
>
>
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