Messages in this thread | | | Date | Mon, 24 May 2010 15:30:47 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86: Export tsc related information in sysfs |
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On 05/24/2010 03:04 PM, Dan Magenheimer wrote: >>> Is that still the case? I thought newer versions of NTP could deal >> with >>> large values. Inaccuracies of way more than 500 ppm are everyday. >> >> That's scary. >> >> Yea, in the kernel the ntp freq correction tops out at 500ppm. Almost >> all the systems I see tend to fall in the +/- 200ppm range (if there's >> not something terribly wrong with the hardware). >> >> So maybe things aren't so bad out there? Or is that wishful thinking? > > Since Brian's concern is at boot-time at which point there is no > network or ntp, and assuming that it would be unwise to vary tsc_khz > dynamically on a clocksource==tsc machine (is it?), would optionally > lengthening the TSC<->PIT calibration beyond 25ms result in a more > consistent tsc_khz between boots? Or is the relative instability > an unavoidable result of skew between the PIT and the fixed constant > PIT_TICK_RATE combined with algorithmic/arithmetic error? Or is > the jitter of the (spread-spectrum) TSC too extreme? Or ??? > > If better more consistent calibration is possible, offering > that as an optional kernel parameter seems better than specifying > a fixed tsc_khz (stamped or user-specified) which may or may > not be ignored due to "too different from measured tsc_khz". > Even an (*optional*) extra second or two of boot time might > be perfectly OK if it resulted in an additional five or six > bits of tsc_khz precision. > > Thoughts, Brian?
Making the calibration time longer should give a more precise result, but of course at the expense of longer boot time.
A longer sample would make sense if the goal is to freeze it into a kernel command line variable, but the real question is how many people would actually do that (and how many people would then suffer problems because they upgraded their CPU/mobo and got massive failures on post-boot.)
-hpa
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