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    SubjectRe: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest]
    On Mon, Feb 10, 2003 at 03:44:35PM +1100, Nick Piggin wrote:
    > 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.

    note that for soem workloads the _dependant reads_ can have _dependant
    writes_ in between. I know it's not the most common case though, but I
    don't love too much to make reads that much special. But again, it makes
    perfect sense to have it as a feature, but I wouldn't like it to be only
    option, I mean there must always be a way to disable anticipatory
    scheduling like there must be a way to disable SFQ (infact for SFQ it
    makes sense to leave it disabled by default, it's only a few people that
    definitely only cares to have the lowest possible latency for mplayer or
    xmms, or for multiuser system doing lots of I/O, but those guys can't
    live without it)

    Andrea
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