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From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: High CPU usage / load average after upgrading to Ubuntu 12.04
Date: Wed, 20 Feb 2013 12:37:38 +1300
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On 20/02/13 12:24, Josh Berkus wrote:
>
> NM, I tested lowering dirty_background_ratio, and it didn't help,
> because checkpoints are kicking in before pdflush ever gets there.
>
> So the issue seems to be that if you have this combination of factors:
>
> 1. large RAM
> 2. many/fast CPUs
> 3. a database which fits in RAM but is larger than the RAID controller's
> WB cache
> 4. pg_xlog on the same volume as pgdata
>
> ... then you'll see checkpoint "stalls" and spread checkpoint will
> actually make them worse by making the stalls longer.
>
> Moving pg_xlog to a separate partition makes this better.  Making
> bgwriter more aggressive helps a bit more on top of that.
>

We have pg_xlog on a pair of PCIe SSD. Also we running the deadline io 
scheduler.

Regards

Mark


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