Date: Thu, 19 Oct 2006 15:54:28 +0200
From: Tobias Brox <tobias@nordicbet.com>
To: pgsql-performance@postgresql.org
Subject: Swappiness setting on a linux pg server
Message-ID: <20061019135428.GD20328@oppetid.no>
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Organization: Group Nordicbet
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I just came to think about /proc/sys/swappiness ...

When this one is set to a high number (say, 100 - which is maximum), the
kernel will aggressively swap out all memory that is not beeing
accessed, to allow more memory for caches.  For a postgres server, OS
caches are good, because postgres relies on the OS to cache indices,
etc.  At the other hand, for any production server it's very bad to
experience bursts of iowait when/if the swapped out memory becomes
needed - particularly if the server is used for interactive queries,
like serving a web application.

I know there are much religion on this topic in general, I'm just
curious if anyone has done any serious thoughts or (even better!)
experimenting with the swappiness setting on a loaded postgres server.

I would assume that the default setting (60) is pretty OK and sane, and
that modifying the setting would have insignificant effect.  My
religious belief is that, however insignificant, a higher setting would
have been better :-)

We're running linux kernel 2.6.17.7 (debian) on the postgres server, and
our memory stats looks like this:
         total       used       free     shared    buffers cached
Mem:      6083M      5846M      236M          0       31M   5448M
-/+ buffers/cache:    366M     5716M
Swap:     2643M         2M     2640M

In addition to the postgres server we're running some few cronscripts
and misc on it - nothing significant though.