Date: Thu, 19 Oct 2006 10:28:31 -0500
From: "Jim C. Nasby" <jim@nasby.net>
To: Tobias Brox <tobias@nordicbet.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Swappiness setting on a linux pg server
Message-ID: <20061019152831.GT71084@nasby.net>
References: <20061019135428.GD20328@oppetid.no>
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On Thu, Oct 19, 2006 at 03:54:28PM +0200, Tobias Brox wrote:
> 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 think it'd be much better to experiment with using much larger
shared_buffers settings. The conventional wisdom there is from 7.x days
when you really didn't want a large buffer, but that doesn't really
apply with the new buffer management we got in 8.0. I know of one site
that doubled their performance by setting shared_buffers to 50% of
memory.

Something else to consider is that many people will put pg_xlog on the
same drives as the OS (and swap). It's pretty important that those
drives not have much activity other than pg_xlog, so any swap activity
would have an even larger than normal impact on performance.

> 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.
-- 
Jim Nasby                                            jim@nasby.net
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)