From: Simon Riggs <simon@2ndquadrant.com>
To: Merlin Moncure <mmoncure@gmail.com>
Cc: Cosimo Streppone <cosimo@streppone.it>
Cc: Richard Huxton <dev@archonet.com>
Cc: Postgresql Performance list <pgsql-performance@postgresql.org>
Subject: Re: Context switch storm
Date: Wed, 15 Nov 2006 09:07:16 +0000
Message-ID: <1163581636.27956.141.camel@silverbirch.site> (raw)
In-Reply-To: <b42b73150611140617m446e801bj7ec834bc661c138a@mail.gmail.com>
References: <efc1d7c07cf.454afe8d@brturbo.com.br>
<454B3B75.6@archonet.com>
<454B3EDC.3090502@streppone.it>
<454B4438.9080105@archonet.com>
<455991B0.6070209@streppone.it>
<b42b73150611140617m446e801bj7ec834bc661c138a@mail.gmail.com>
On Tue, 2006-11-14 at 09:17 -0500, Merlin Moncure wrote:
> On 11/14/06, Cosimo Streppone <cosimo@streppone.it> wrote:
> > I must say I lowered "shared_buffers" to 8192, as it was before.
> > I tried raising it to 16384, but I can't seem to find a relationship
> > between shared_buffers and performance level for this server.
>
> My findings are pretty much the same here. I don't see any link
> between shared buffers and performance. I'm still looking for hard
> evidence to rebut this point. Lower shared buffers leaves more
> memory for what really matters, which is sorting.
In 8.0 there is a performance issue such that bgwriter will cause a
performance problem with large shared_buffers setting. That in itself
could lead to some fairly poor measurements of the value of
shared_buffers.
In 7.4 and prior releases setting shared_buffers higher was counter
productive in many ways, so isn't highly recommended.
In general, setting shared_buffers higher works for some workloads and
doesn't for others. So any measurements anybody makes depend upon the
workload and the size of the database. The more uniformly/randomly you
access a large database, the more benefit you'll see from large
shared_buffers. 8.1 benefits from having a higher shared_buffers in some
cases because it reduces contention on the buffer lwlocks; 8.2 solves
this issue.
Even in 8.2 ISTM that a higher shared_buffers setting wastes memory with
many connected users since the PrivRefCount array uses memory that could
have been used as filesystem cache.
--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: simon@2ndquadrant.com, mmoncure@gmail.com, cosimo@streppone.it, dev@archonet.com
Subject: Re: Context switch storm
In-Reply-To: <1163581636.27956.141.camel@silverbirch.site>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox