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    SubjectRe: scheduler vs hardware? (was Re: another i7 (linux) bug?)
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    On Fri, 2010-01-22 at 09:51 +0100, Peter Zijlstra wrote:
    > On Fri, 2010-01-22 at 08:19 +0100, Mike Galbraith wrote:
    > > > if I run a cpu intensive process with the lowest priority (19
    > > > from man nice) I obtain much better performance that with the
    > > > highest priority available (-20 from man nice).
    > > >
    > > > For example the same file is processed by lame in 8.7 seconds
    > > > at the lowest priority, and in 12 seconds at the highest
    > > > priority. Before posting a bug I wold like to understand if
    > > > this is a problem related to the i7 mobile (my processor is a
    > > > i7 Q720).
    > > >
    > > > As far as I tested on the same laptop series (dell studio 15),
    > > > with the same kernel this problem does not exists.
    > >
    > > So you only see this on the i7. That's odd. Can you try 33-rc5?
    > >
    > > Posting a reliable reproducer would be nice. It'd also be nice to see
    > > what all is running when you see this, and where.
    >
    > Using a sample from: http://lame.sourceforge.net/quality.php
    >
    > My laptop does:
    >
    >
    > # time nice -n 19 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    > real 0m3.273s
    > user 0m3.217s
    > sys 0m0.022s
    >
    >
    > # time nice -n 0 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    > real 0m1.121s
    > user 0m1.102s
    > sys 0m0.013s
    >
    >
    > # time nice -n -20 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    > real 0m1.112s
    > user 0m1.093s
    > sys 0m0.018s
    >
    >
    >
    > On a Nehalem class server machine it does:
    >
    >
    > # time nice -n 19 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    > real 0m0.932s
    > user 0m0.917s
    > sys 0m0.005s
    >
    >
    > # time nice -n 0 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    > real 0m0.927s
    > user 0m0.922s
    > sys 0m0.003s
    >
    >
    > # time nice -n -20 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    > real 0m0.919s
    > user 0m0.914s
    > sys 0m0.005s

    Weird. Here there is zip squat difference, as expected with 1 thread.

    time nice -n 19 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    real 0m0.912s
    user 0m0.908s
    sys 0m0.000s

    time nice -n 0 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    real 0m0.912s
    user 0m0.904s
    sys 0m0.004s

    time nice -n -20 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

    real 0m0.912s
    user 0m0.904s
    sys 0m0.004s

    (bah, who needs a nehalem;)

    -Mike



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