From: Merlin Moncure <mmoncure@gmail.com>
To: Jim C. Nasby <jim@nasby.net>
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: Tue, 14 Nov 2006 15:11:40 -0500
Message-ID: <b42b73150611141211y4e9b8b48o995aceb68c520387@mail.gmail.com> (raw)
In-Reply-To: <20061114165022.GS90133@nasby.net>
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>
<20061114165022.GS90133@nasby.net>
On 11/14/06, Jim C. Nasby <jim@nasby.net> wrote:
> On Tue, Nov 14, 2006 at 09:17:08AM -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.
>
> It depends on your workload. If you're really sort-heavy, then having
> memory available for that will be hard to beat. Otherwise, having a
> large shared_buffers setting can really help cut down on switching back
> and forth between the kernel and PostgreSQL.
>
> BTW, shared_buffers of 16384 is pretty low by today's standards, so that
> could be why you're not seeing much difference between that and 8192.
> Try upping it to 1/4 - 1/2 of memory and see if that changes things.
Can you think of a good way to construct a test case that would
demonstrate the difference? What would be the 'best case' where a
high shared buffers would be favored over a low setting?
merlin
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To: pgsql-performance@postgresql.org
Cc: mmoncure@gmail.com, jim@nasby.net, cosimo@streppone.it, dev@archonet.com
Subject: Re: Context switch storm
In-Reply-To: <b42b73150611141211y4e9b8b48o995aceb68c520387@mail.gmail.com>
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