Re: System clock frequency offset changes drastically across reboots

John Sigler
2007-07-30T08:00:38Z

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John Sigler wrote:

> I'm using 2.6.20.7-rt8 on a P3.

But some ntpd users have reported the problem with mainline kernels and
different versions.

> I've noticed that the frequency offset of my system clock (computed
> either by ntpd, or by hand) changes drastically across reboots.
>
> By drastically, I mean +/- 60 ppm every time I reboot.
>
> Apparently this is caused by some imprecision in the frequency
> estimation done in calculate_cpu_khz() (arch/i386/kernel/tsc.c)
>
> My brutally crude work-around was to skip the computation altogether and
> hard-code the "correct" value for cpu_khz.
>
> http://linux.kernel.free.fr/cpu_khz.adlink
>
> Has this issue been investigated more thoroughly by other users?

Regards.