Re: System clock frequency offset changes drastically across reboots

From: John Sigler
Date: Mon Jul 30 2007 - 04:00:40 EST


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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.
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