linux-kernel - Re: [tip:timers/ntp] ntp: adjust SHIFT

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Date:	Tue, 02 Jun 2009 13:55:35 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Miroslav Lichvar <mlichvar@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Rik van Riel <riel@...hat.com>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/ntp] ntp: adjust SHIFT_PLL to improve NTP
 convergence

On Tue, 2009-06-02 at 18:22 +0200, Miroslav Lichvar wrote:
> On Tue, Jun 02, 2009 at 02:20:39AM +0200, Ingo Molnar wrote:
> > 
> > Not just that - but there's calibration noise during bootup that can 
> > cause randomly distributed recalibrations as well. So other hosts in 
> > a mixed environment will see inconsistencies anyway, after every 
> > bootup.
[snip]
> As for the calibration issue, would it be possible to export the
> information that an instable clocksource is used and when was the last
> time it was calibrated? Then we'd know when the drift file should not
> be trusted and let NTP calculate the frequency directly (it takes
> about 15 minutes).

Just to de-thread the issues here, the calibration noise issue really is
separate from the SHIFT_PLL convergence issue.

I'd really prefer the calibration noise issue to be resolved by the
kernel, as its really only an issue on a subset of x86 machines. The
tsc_khz= boot option I proposed earlier for folks who really care seems
to me like a good route.

The only NTPd side change to help the calibration issue that might be
useful, would be a explicit ntp option to force NTP to always calculate
the freq on startup if the drift file was present or not. Anything else
would be way too much of a hack to get around bad kernel behavior.

thanks
-john

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