Horton

Re: Wierd context-switching issue on Xeon

Robert Creager <robert_creager@logicalchaos.org>

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From: Robert Creager <Robert_Creager@LogicalChaos.org>
To: Bruce Momjian <pgman@candle.pha.pa.us>
Cc: pg@fastcrypt.com, Josh Berkus <josh@agliodbs.com>, Dirk_Lutzebäck <lutzeb@aeccom.com>, ohp@pyrenet.fr, Tom Lane <tgl@sss.pgh.pa.us>, Joe Conway <mail@joeconway.com>, "scott.marlowe" <scott.marlowe@ihs.com>, pgsql-performance@postgresql.org, Neil Conway <neilc@samurai.com>
Date: 2004-05-20T01:59:26Z
Lists: pgsql-performance 
When grilled further on (Wed, 19 May 2004 21:20:20 -0400 (EDT)),
Bruce Momjian <pgman@candle.pha.pa.us> confessed:

> 
> Did we ever come to a conclusion about excessive SMP context switching
> under load?
> 

I just figured out what was causing the problem on my system Monday.  I'm using
the pg_autovacuum daemon, and it was not vacuuming my db.  I've no idea why and
didn't get a chance to investigate.

This lack of vacuuming was causing a huge number of context switches and query
delays. the queries that normally take .1 seconds were taking 11 seconds, and
the context switches were averaging 160k/s, peaking at 190k/s

Unfortunately, I was under pressure to fix the db at the time so I didn't get a
chance to play with the patch.

I restarted the vacuum daemon, and will keep an eye on it to see if it behaves.

If the problem re-occurs, is it worth while to attempt the different patch
delay settings?

Cheers,
Rob

-- 
 19:45:40 up 21 days,  2:30,  4 users,  load average: 2.03, 2.09, 2.06
Linux 2.6.5-01 #7 SMP Fri Apr 16 22:45:31 MDT 2004