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    SubjectRe: [RESEND][RFC] BFQ I/O Scheduler
    On Thu, Apr 17 2008, Paolo Valente wrote:
    > Jens Axboe ha scritto:
    > >
    > >
    > >I was thinking about that too. Generally I've been opposed to doing
    > >scheduling decisions on anything but time, since that is always
    > >relevant. When to hand out slices and to what process, that algorithm is
    > >really basic in CFQ and could do with an improvement.
    > >
    > >
    > Maybe there is also another middle-ground solution. I'll try to sketch
    > it out:
    > . use sectors instead of time
    > . impose a penalty to each thread in proportion to the distance between
    > its disk requests
    > . reduce the maximum budget of each thread as a function of this seek
    > penalty so as to prevent the thread from stealing more than a given time
    > slice (the simple mechanism to limit per-thread budget is already
    > implemented in bfq).
    >
    > By doing so, both fairness and time isolation should be guaranteed.
    > Finally, this policy should be safe in that, given the maximum time used
    > by a seeky thread to consume its maximum budget on a reference disk, the
    > time used on any faster disk should be shorter.
    >
    > Does it seem reasonable?

    Not for CFQ, that will stay time based. The problem with #2 above is
    that it then quickly turns to various heuristics, which is just
    impossible to tune for general behaviour. Or it just falls apart for
    other real life situations.

    --
    Jens Axboe



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