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    SubjectRe: slow nanosleep?
    On Wed, 8 Sep 2010, Joakim Tjernlund wrote:

    > Peter Zijlstra <peterz@infradead.org> wrote on 2010/09/08 15:00:18:
    > >
    > > On Wed, 2010-09-08 at 14:43 +0200, Thomas Gleixner wrote:
    > > > > However nanosleep with 1 ns and prctl(PR_SET_TIMERSLACK, 1) takes
    > > > > about 8 us on x86(Intel(R) Core(TM)2 Duo CPU E8500 @ 3.16GHz)
    > > > > and 20 us on my slower ppc board. Is that system call overhead
    > > > > or possibly some error?
    > > >
    > > > That's overhead I fear. We go way up to enqueue/arm the timer until we
    > > > figure out that the timeout already happened.
    > >
    > > Well, there's also the fact that his ppc board is simply dead slow,
    > > using the freq ratio: 3166/266 you'd expect (at a similar ins/clock
    > > ratio) the ppc to take 95us.
    > >
    > > So in fact the pcc taking 20us is actually quite good.
    >
    > Actually, it takes 120 us. The 20 us was when I had Thomas
    > timeout == 0 fast path patch applied(forgot to remove it).
    > Without that patch it takes about 115 us. So it seems it takes
    > 115-20=95 us to turn the timer wheel on my ppc.

    You might fire up the tracer to look where it spends that time.

    Thanks,

    tglx


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