From: Cosimo Streppone <cosimo@streppone.it>
To: Richard Huxton <dev@archonet.com>
Cc: Postgresql Performance list <pgsql-performance@postgresql.org>
Subject: Re: Context switch storm
Date: Fri, 03 Nov 2006 14:06:36 +0100
Message-ID: <454B3EDC.3090502@streppone.it> (raw)
In-Reply-To: <454B3B75.6@archonet.com>
References: <efc1d7c07cf.454afe8d@brturbo.com.br>
<454B3B75.6@archonet.com>
Richard Huxton wrote:
> creimer@brturbo.com.br wrote:
>> Hi,
>>
>> We've migrated one of our servers from pg 7.4 to 8.1 and from times
>> to times (4 hours) the server start doing a lot of context switching
>> and all transactions become very slow.
>>
>> The average context switching for this server as vmstat shows is 1
>> but when the problem occurs it goes to 250000.
>
> You'll tend to see it when you have multiple clients and most queries
> can use RAM rather than disk I/O. My understanding of what happens is
> that PG requests data from RAM - it's not in cache so the process gets
> suspended to wait. The next process does the same, with the same result.
> You end up with lots of processes all fighting over what data is in
> the cache and no-one gets much work done.
Does this happen also with 8.0, or is specific to 8.1 ?
I seem to have the same exact behaviour for an OLTP-loaded 8.0.1 server
when I raise `shared_buffers' from 8192 to 40000.
I would expect an increase in tps/concurrent clients, but I see an average
performance below a certain threshold of users, and when concurrent users
get above that level, performance starts to drop, no matter what I do.
Server logs and io/vm statistics seem to indicate that there is little
or no disk activity but machine loads increases to 7.0/8.0.
After some minutes, the problem goes away, and performance returns
to acceptable levels.
When the load increases, *random* database queries show this "slowness",
even if they are perfectly planned and indexed.
Is there anything we can do?
--
Cosimo
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To: pgsql-performance@postgresql.org
Cc: cosimo@streppone.it, dev@archonet.com
Subject: Re: Context switch storm
In-Reply-To: <454B3EDC.3090502@streppone.it>
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