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Re: Performance increase with elevator=deadline

From: Florian Weimer <fweimer(at)bfk(dot)de>
To: Jeff <threshar(at)real(dot)jefftrout(dot)com>
Cc: Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at>, <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Performance increase with elevator=deadline
Date: 2008-04-15 13:27:20
Message-ID: 82od8bi6yv.fsf@mid.bfk.de (view raw or flat)
Thread:
Lists: pgsql-performance
* Jeff:

> Using 4 of these with a dataset of about 30GB across a few files
> (Machine has 8GB mem) I went from around 100 io/sec to 330 changing to
> noop.   Quite an improvement.  If you have a decent controller CFQ is
> not what you want.   I tried deadline as well and it was a touch
> slower.  The controller is a 3ware 9550sx with 4 disks in a raid10.
>
> I'll be trying this out on the big array later today.  I found it
> suprising this info wasn't more widespread (the use of CFQ on a good
> controller).

3ware might be a bit special because the controller has got very deep
queues on its own, so many assumptions of the kernel I/O schedulers do
not seem to apply.  Toying with the kernel/controller queue depths
might help, but I haven't done real benchmarks to see if it's actually
a difference.

A few days ago, I experienced this: On a machine with a 3ware
controller, a simple getxattr call on a file in an uncontended
directory took several minutes because a PostgreSQL dump process was
running in the background (and some other activity of a legacy
database which caused frequent fdatasync calls).  Clearly, this is
unacceptable, and I've since switched to the deadline scheduler, too.
So far, this particular behavior hasn't occurred again.

-- 
Florian Weimer                <fweimer(at)bfk(dot)de>
BFK edv-consulting GmbH       http://www.bfk.de/
Kriegsstra├če 100              tel: +49-721-96201-1
D-76133 Karlsruhe             fax: +49-721-96201-99

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