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SubjectRE: [PATCH] x86: Export tsc related information in sysfs
> > 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?


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