linux-kernel - Re: Documentation on CFQ iosched parameters

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Date:	Tue, 23 Sep 2008 11:00:29 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Aaron Carroll <aaronc@...ato.unsw.edu.au>
Cc:	Martin Steigerwald <ms@...mix.de>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: Documentation on CFQ iosched parameters

On Tue, Sep 23 2008, Aaron Carroll wrote:
> Martin Steigerwald wrote:
> >Hi!
> >
> >I am searching documentation about CFQ io scheduler. I can't find it in 
> >linux 2.6.26 Documentation directory.
> >
> >I found about these in german[1]:
> >
> >back_seek_max:16384
> >back_seek_penalty:2
> >fifo_expire_async:250
> >fifo_expire_sync:123
> >quantum:4
> >
> >But I am completely missing about these:
> >
> >slice_async:40
> 
> Base length of an asynchronous queue timeslice (that is, how long the
> queue has to dispatch requests each round).  The actual timeslice
> length is scaled by the I/O priority.
> 
> >slice_async_rq:2
> 
> The base number of requests per round for asynchronous queues.  Like
> slice_async, the actual maximum is a function of slice_async_rq and I/O
> priority.
> 
> >slice_idle:6
> 
> How long to wait for processes to produce more I/O before switching
> queues. This is for anticipation of sequential I/O, and more even disk
> time distribution for processes doing back to back synchronous I/Os.
> 
> >slice_sync:100
> 
> Same as slice_async, but for synchronous requests.

Nothing further to add, Aaron nailed them.

-- 
Jens Axboe

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