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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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