Messages in this thread | | | Subject | Re: [patch] i386, numaq: enable TSCs again | From | john stultz <> | Date | Fri, 25 May 2007 11:23:26 -0700 |
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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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