linux-kernel - Re: Documentation on CFQ iosched parameters

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Message-ID: <20080923093218.GY26460@kernel.dk>
Date:	Tue, 23 Sep 2008 11:32:18 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Martin Steigerwald <ms@...mix.de>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Aaron Carroll <aaronc@...ato.unsw.edu.au>
Subject: Re: Documentation on CFQ iosched parameters

On Tue, Sep 23 2008, Martin Steigerwald wrote:
> Am Dienstag, 23. September 2008 schrieb Aaron Carroll:
> > 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.
> 
> [...]
> 
> Thanks. As I do not find documentation about CFQ at all, I think I need to 
> create a new file from scratch... how about 
> Documentation/block/cfq-iosched.txt?

That would be great!

> For that I need some more information about CFQ. I found the following 
> resources:
> 
> - CFQ V3 aka CFQ TS: http://lwn.net/Articles/143474/, 
> http://lwn.net/Articles/114770/ and posts by Jens, 
> http://lwn.net/Articles/113869/
> 
> - CFQ V2: Post by Jens, http://lwn.net/Articles/101029/
> 
> - CFQ: Post by Jens, http://lwn.net/Articles/22429/

You can mention the links for v1 and v2, but as they have even little
historic relevance, don't spend more time on that.

> - Documentation/block/as-iosched.txt / deadline-iosched.txt (as for how an IO 
> scheduler can be documented)
> 
> Would that be what I should read in order to write such an documentation? 

Yes

> How about the relevance of CFQ and CFQ v2 stuff? What of it would be good to 
> mention? I would like to limit documentation to what is still relevant for 
> the current implementation of the IO scheduler. Should I mention its 
> inheritance from SFQ - maybe in a small introduction and historic overview?

Even the very first CFQ posted used per-process queues, so there's
really very little inheritance from SFQ at all. So I don't think it's
particularly interesting to mention. But if you want, go ahead :-)


-- 
Jens Axboe

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