Re: Latency problems with simple queries

From: Jeff Davis
Subject: Re: Latency problems with simple queries
Date: ,
Msg-id: 1310049317.3012.153.camel@jdavis
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In response to: Latency problems with simple queries  (Adrian Schreyer)
List: pgsql-general

On Thu, 2011-07-07 at 12:13 +0100, Adrian Schreyer wrote:
> I randomly get latency/performance problems even with very simple
> queries, for example fetching a row by primary key from a small table.
> Since I could not trace it back to specific queries, I decided to give
> LatencyTOP (http://www.latencytop.org/) a go. Soon after running a
> couple of queries, I saw this in latencytop whilst a query was hanging
> in postgres:
>
> Cause                           Maximum          Percentage
> Writing a page to disk    19283.9 msec    99.7

What IO scheduler and filesystem are you using?

I think that CFQ has some problems for database workloads. It would be
easy to test: just switch to deadline and/or noop for a while and see if
the problem persists.

Also, I have heard of a few strange things with ext4, but they have
probably fixed those issues and it would be much harder for you to test.
But it might be worth searching for issues/bugs with your particular
version of the filesystem.

Regards,
    Jeff Davis



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