From: Cosimo Streppone <cosimo@streppone.it>
To: Merlin Moncure <mmoncure@gmail.com>
Cc: Postgresql Performance list <pgsql-performance@postgresql.org>
Cc: Jim Nasby <jim@nasby.net>
Cc: Richard Huxton <dev@archonet.com>
Subject: Re: Context switch storm
Date: Tue, 14 Nov 2006 22:43:20 +0100
Message-ID: <455A3878.20702@streppone.it> (raw)
In-Reply-To: <b42b73150611141211y4e9b8b48o995aceb68c520387@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>
<20061114165022.GS90133@nasby.net>
<b42b73150611141211y4e9b8b48o995aceb68c520387@mail.gmail.com>
Merlin wrote:
> 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.
>> > [...]
>>
>> BTW, shared_buffers of 16384 is pretty low by today's standards
>
> Can you think of a good way to construct a test case that would
> demonstrate the difference?
Not sure of actual relevance, but some time ago I performed
(with 8.0) several pg_bench tests with 1,5,10,20 concurrent
clients with same pg configuration except one parameter for
every run.
In one of these tests I run pgbench with shared_buffers starting
at 1024 and doubling it to 2048, ..., until 16384.
I found the best performance in terms of transactions per second
around 4096/8192.
That said, I don't know if pgbench stresses the database
like my typical oltp application does.
And also, I suspect that shared_buffers should not be
evaluated as an absolute number, but rather as a number relative to
maximum main memory (say 1/2 the total ram, 1/3, 2/3, ...).
--
Cosimo
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To: pgsql-performance@postgresql.org
Cc: cosimo@streppone.it, mmoncure@gmail.com, jim@nasby.net, dev@archonet.com
Subject: Re: Context switch storm
In-Reply-To: <455A3878.20702@streppone.it>
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