linux-kernel - Re: System clock frequency offset changes drastically across reboots

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Message-ID: <46AD9A74.8070302@free.fr>
Date:	Mon, 30 Jul 2007 09:59:48 +0200
From:	John Sigler <linux.kernel@...e.fr>
To:	linux-kernel@...r.kernel.org,
	linux-rt-users <linux-rt-users@...r.kernel.org>
CC:	johnstul@...ibm.com, tglx@...utronix.de
Subject: Re: System clock frequency offset changes drastically across reboots

[ Expanding recipients list ]

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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