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    SubjectRe: Linux disk performance.
    On 12/18/06, Erik Mouw <mouw@nl.linux.org> wrote:
    <...snip...>
    > >
    > > But isn't O_DIRECT supposed to bypass buffering in Kernel?
    >
    > It is.
    >
    > > Doesn't it directly write to disk?
    >
    > Yes, but it still uses an IO scheduler.
    >

    Ok. but i also tried with noop to turnoff disk scheduling effects.
    There was still timing differences. Usually i get 3100 microseconds
    but upto 20000 microseconds at certain intervals. I am just using
    gettimeofday between two writes to read the timing.



    > In your first message you mentioned you were using an ancient 2.6.10
    > kernel. That kernel uses the anticipatory IO scheduler. Update to the
    > latest stable kernel (2.6.19.1 at time of writing) and it will default
    > to the CFQ scheduler which has a smoother writeout, plus you can give
    > your process a different IO scheduling class and level (see
    > Documentation/block/ioprio.txt).

    Thanks... i will try with CFQ.



    Nick Piggin:
    > but
    > they look like they might be a (HZ quantised) delay coming from
    > block layer plugging.

    Sorry i didn´t understand what you mean.

    To minimise scheduling effects i tried giving it maximum priority.


    --
    ---------------------------------------------------------------
    regards
    Manish Regmi

    ---------------------------------------------------------------
    UNIX without a C Compiler is like eating Spaghetti with your mouth
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