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    SubjectRe: [RFC patch] Use IPI_shortcut for lapic timer broadcast
    On Mon, 29 Jun 2009 16:43:16 +0800
    Luming Yu <luming.yu@gmail.com> wrote:

    > > Even a Pentium-5 100 MHz dual box was able to do cross-CPU IPIs
    > > within 10-20 microseconds more than a decade ago - so 50-100 usecs
    > > latency on a modern platform is totally out of this planet and will
    > > hurt Linux performance big time. And the worst thing about it is
    > > that none of the usual performance metrics will really show _why_
    > > performance is tanking ...
    > >
    >
    >
    > Please note this is deep-C-state related.
    > C state does add extra latency.. but I don't know how much...

    C states normally only add on the "wait for" side, not on the "send"
    side.

    (RHEL5 is rather old, so it may have done things a bit different)

    Maybe it is time for mainline to not allow a !NO_HZ config....


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