From: Heikki Linnakangas <heikki@enterprisedb.com>
To: Csaba Nagy <nagy@ecircle-ag.com>
Cc: Gregory Stark <stark@enterprisedb.com>
Cc: Decibel! <decibel@decibel.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Linux mis-reporting memory
Date: Fri, 21 Sep 2007 11:34:53 +0100
Message-ID: <46F39E4D.1080401@enterprisedb.com> (raw)
In-Reply-To: <1190369325.4661.178.camel@PCD12478>
References: <20070920230400.GM95343@decibel.org>
<16213.1190329934@sss.pgh.pa.us>
<87r6ksa1jb.fsf@oxford.xeocode.com>
<1190363401.4661.116.camel@PCD12478>
<87r6ksuzf8.fsf@oxford.xeocode.com>
<1190369325.4661.178.camel@PCD12478>
Csaba Nagy wrote:
> On Fri, 2007-09-21 at 10:43 +0100, Gregory Stark wrote:
>> The other possibility is that Postgres just hasn't even touched a large part
>> of its shared buffers.
>
> But then how do you explain the example I gave, with a 5.5GB table
> seq-scanned 3 times, shared buffers set to 12 GB, and top still showing
> almost 100% memory as cached and no SWAP "used" ? In this case you can't
> say postgres didn't touch it's shared buffers - or a sequential scan
> won't use the shared buffers ?
Which version of Postgres is this? In 8.3, a scan like that really won't
suck it all into the shared buffer cache. For seq scans on tables larger
than shared_buffers/4, it switches to the bulk read strategy, using only
a few buffers, and choosing the starting point with the scan
synchronization facility.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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To: pgsql-performance@postgresql.org
Cc: heikki@enterprisedb.com, nagy@ecircle-ag.com, stark@enterprisedb.com, decibel@decibel.org
Subject: Re: Linux mis-reporting memory
In-Reply-To: <46F39E4D.1080401@enterprisedb.com>
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