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

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Date:	Fri, 03 Aug 2007 09:47:17 +0200
From:	John Sigler <linux.kernel@...e.fr>
To:	linux-kernel@...r.kernel.org
CC:	linux-rt-users@...r.kernel.org, johnstul@...ibm.com,
	tglx@...utronix.de, questions@...ts.ntp.org
Subject: Re: System clock frequency offset changes drastically across reboots

[ Expanding recipients list to include NTP mailing 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?

Is there, perhaps, more appropriate a forum to discuss such matters?

Regards.

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