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    SubjectRe: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest]
    Rik van Riel wrote:

    >On Mon, 10 Feb 2003, Nick Piggin wrote:
    >
    >>Andrea Arcangeli wrote:
    >>
    >>
    >>>The only way to get the minimal possible latency and maximal fariness is
    >>>my new stochastic fair queueing idea.
    >>>
    >>Sounds nice. I would like to see per process disk distribution.
    >>
    >
    >Sounds like the easiest way to get that fair, indeed. Manage
    >every disk as a separately scheduled resource...
    >
    I hope this option becomes available one day.

    >
    >
    >>However dependant reads can not merge with each other obviously so
    >>you could end up say submitting 4K reads per process.
    >>
    >
    >Considering that one medium/far disk seek counts for about 400 kB
    >of data read/write, I suspect we'll just want to merge requests or
    >put adjacant requests next to each other into the elevator up to
    >a fairly large size. Probably about 1 MB for a hard disk or a cdrom,
    >but much less for floppies, opticals, etc...
    >
    Yes, but the point is _dependant reads_. This is why Andrea's approach
    alone can't solve dependant read vs write or nondependant read - while
    maintaining a good throughput.

    >
    >
    >>But your solution also does nothing for sequential IO throughput in
    >>the presence of more than one submitter.
    >>
    >
    >>I think you should be giving each process a timeslice,
    >>
    >
    >That is the anticipatory scheduler. A good complement to the SFQ
    >part of the IO scheduler. I'd really like to see both ideas together
    >in one scheduler, they sound like a winning pair.
    >
    No, anticipatory scheduling just "pauses for a bit after some reads"
    I am talking about all this nonsense about trying to measure seek vs
    throughput vs rotational latency, etc. We don't process schedule by
    how many memory accesses a process makes * processor speed / memory
    speed + instruction jumps.....

    If you want a fair disk scheduler, just give each process a disk
    timeslice, and all your seek, stream, settle, track buffer, etc
    accounting is magically done for you... For any device, and is
    100% tunable. The point is, account by _time_.

    Nick

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