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    SubjectRe: [patch] i386, numaq: enable TSCs again
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    On Fri, 2007-05-25 at 11:16 -0700, Dave Hansen wrote:
    > On Fri, 2007-05-25 at 10:41 +0200, Ingo Molnar wrote:
    > > * William Lee Irwin III <wli@holomorphy.com> wrote:
    > > > > yes, that's what i meant under 'slightly async'. Some AMD CPUs are
    > > > > like that too and sched_clock() now handles that fine. So we should
    > > > > try my patch.
    > > >
    > > > Sorry, then. I took slight to mean something else. In any event I was
    > > > only quantifying things. I've no opinion whatsoever on the impact of
    > > > the code on NUMA-Q, only some recall of its operating characteristics.
    > >
    > > there's no need to apologize at all! Thanks for reminding us about the
    > > time-scale and nature of the TSC drift on NUMAQ. I was worried that
    > > maybe the TSC was totally unusable for some reason - but that's
    > > fortunately not the case. So we now have one quirk less, hopefully :-)
    >
    > Last I remember, it was totally useless for timekeeping, but was useful
    > for cpu-local time measurements.
    >
    > John, it's still useless for time, right? Does sched_clock() really fix
    > it?

    Yea, on multi-node NUMAQ the TSC shouldn't be used for timekeeping.

    However it should be fine for sched_clock(), or other cpu-local
    measurements as the TSCs are constant (no cpufreq, no deep sleep
    states).

    -john

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