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From: Josh Berkus <josh@agliodbs.com>
To: pgsql-performance@postgresql.org
Subject: Re: High CPU usage / load average after upgrading to Ubuntu 12.04
Date: Tue, 19 Feb 2013 15:24:31 -0800
Message-ID: <512409AF.8010308@agliodbs.com> (raw)
In-Reply-To: <5123BB94.7030603@agliodbs.com>
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On 02/19/2013 09:51 AM, Josh Berkus wrote:
> On 02/18/2013 08:28 PM, Mark Kirkwood wrote:
>> Might be worth looking at your vm.dirty_ratio, vm.dirty_background_ratio
>> and friends settings. We managed to choke up a system with 16x SSD by
>> leaving them at their defaults...
> 
> Yeah?  Any settings you'd recommend specifically?  What did you use on
> the SSD system?
> 

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.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com


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