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    SubjectRe: [RFC] scheduler: improve SMP fairness in CFS

    * Tong Li <tong.n.li@intel.com> wrote:

    > Thanks for the patch. It doesn't work well on my 8-way box. Here's the
    > output at two different times. It's also changing all the time.

    you need to measure it over longer periods of time. Its not worth
    balancing for such a thing in any high-frequency manner. (we'd trash the
    cache constantly migrating tasks back and forth.)

    > [time 2]
    > 3721 tongli 20 0 7864 108 16 R 91 0.0 1:44.67 loop
    > 3720 tongli 20 0 7864 108 16 R 89 0.0 1:40.55 loop
    > 3722 tongli 20 0 7864 108 16 R 86 0.0 1:41.40 loop
    > 3723 tongli 20 0 7864 108 16 R 82 0.0 1:38.94 loop
    > 3715 tongli 20 0 7864 108 16 R 82 0.0 1:43.09 loop
    > 3717 tongli 20 0 7864 108 16 R 79 0.0 1:46.38 loop
    > 3718 tongli 20 0 7864 108 16 R 79 0.0 1:40.72 loop
    > 3714 tongli 20 0 7864 108 16 R 77 0.0 1:40.17 loop
    > 3719 tongli 20 0 7864 108 16 R 73 0.0 1:35.41 loop
    > 3716 tongli 20 0 7864 108 16 R 61 0.0 1:38.66 loop

    the runtime ranges from 1:38.66 to 1:46.38 - that's a +-3% spread which
    is pretty acceptable for ~90 seconds of runtime. Note that the
    percentages you see above were likely done over a shorter period that's
    why you see them range from 61% to 91%.

    > I'm running 2.6.23-rc1 with your patch on a two-socket quad-core
    > system. Top refresh rate is the default 3s.

    the 3s is the problem: change that to 60s! We no way want to
    over-migrate for SMP fairness, the change i did gives us reasonable
    long-term SMP fairness without the need for high-rate rebalancing.

    Ingo
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