linux-kernel - Re: [PATCH] tsc_khz= boot option to avoid TSC calibration variance

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Date:	Wed, 13 May 2009 12:23:35 +0300
From:	Petri Kaukasoina <kaukasoina704m93hd7@....fi>
To:	Ulrich Windl <ulrich.windl@...uni-regensburg.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Clark Williams <williams@...hat.com>, zippel@...ux-m68k.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] tsc_khz= boot option to avoid TSC calibration variance

On Wed, May 13, 2009 at 09:59:59AM +0200, Ulrich Windl wrote:
> On 13 May 2009 at 0:42, Thomas Gleixner wrote:
> 
> > Honestly all this is just the futile attempt to fix the flaws of NTP
> > via (super)user interaction.
> 
> This has nothing to do with flaws in NTP (AFAIK).

Correct. When ntp is NOT RUNNING at all, if the TSC calibration gives a MHz
value differing e.g. 100 ppm from the previous value, the clock will run at
a different speed, 100 ppm making a difference of 17 seconds a day. On the
other hand, if ntp is running, it will eventually compensate the offset. But
it takes quite a long time to settle, IMO too long for a quality time
server.

The real changes in the clock frequency between bootups are in the order of
1 ppm, but the TSC calibration can be even 100 ppm off randomly.

-Petri
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