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    SubjectRe: 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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