From: Richard Huxton <dev@archonet.com>
To: Cosimo Streppone <cosimo@streppone.it>
Cc: Postgresql Performance list <pgsql-performance@postgresql.org>
Subject: Re: Context switch storm
Date: Fri, 03 Nov 2006 13:29:28 +0000
Message-ID: <454B4438.9080105@archonet.com> (raw)
In-Reply-To: <454B3EDC.3090502@streppone.it>
References: <efc1d7c07cf.454afe8d@brturbo.com.br>
<454B3B75.6@archonet.com>
<454B3EDC.3090502@streppone.it>
Cosimo Streppone wrote:
> Richard Huxton wrote:
>
>> creimer@brturbo.com.br wrote:
>>>
>>> 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 ?
All versions suffer to a degree - they just push the old Xeon in the
wrong way. However, more recent versions *should* be better than older
versions. I believe some work was put in to prevent contention on
various locks which should reduce context-switching across the board.
> I seem to have the same exact behaviour for an OLTP-loaded 8.0.1 server
upgrade from 8.0.1 - the most recent is 8.0.9 iirc
> 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.
Are you seeing a jump in context-switching in top? You'll know when you
do - it's a *large* jump. That's the key diagnosis. Otherwise it might
simply be your configuration settings aren't ideal for that workload.
> 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.
That sounds like it. Query time increases across the board as all the
clients fail to get any data back.
> When the load increases, *random* database queries show this "slowness",
> even if they are perfectly planned and indexed.
>
> Is there anything we can do?
Well, the client I saw it with just bought a dual-opteron server and
used their quad-Xeon for something else. However, I do remember that 8.1
seemed better than 7.4 before they switched. Part of that might just
have been better query-planning and other efficiences though.
--
Richard Huxton
Archonet Ltd
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: dev@archonet.com, cosimo@streppone.it
Subject: Re: Context switch storm
In-Reply-To: <454B4438.9080105@archonet.com>
* 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