linux-kernel - Re: CFQ read performance regression

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Message-ID: <1271865911.24780.292.camel@tucsk.pomaz.szeredi.hu>
Date:	Wed, 21 Apr 2010 18:05:11 +0200
From:	Miklos Szeredi <mszeredi@...e.cz>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Suresh Jayaraman <sjayaraman@...e.de>
Subject: Re: CFQ read performance regression

Jens, Corrado,

Here's a graph showing the number of issued but not yet completed
requests versus time for CFQ and NOOP schedulers running the tiobench
benchmark with 8 threads:

http://www.kernel.org/pub/linux/kernel/people/mszeredi/blktrace/queue-depth.jpg

It shows pretty clearly the performance problem is because CFQ is not
issuing enough request to fill the bandwidth.

Is this the correct behavior of CFQ or is this a bug?

This is on a vanilla 2.6.34-rc4 kernel with two tunables modified:

read_ahead_kb=512
low_latency=0 (for CFQ)

Thanks,
Miklos


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