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Subject: Re: Linux 2.6.29-rc6
From: john stultz <johnstul@us.ibm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Jesper Krogh <jesper@krogh.cc>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Len Brown <len.brown@intel.com>
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Date: Thu, 26 Feb 2009 14:59:42 -0800
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On Thu, 2009-02-26 at 14:40 -0800, Linus Torvalds wrote:
> 
> On Thu, 26 Feb 2009, john stultz wrote:
> > 
> > I'll kick up some of my own testing between these two releases to see if
> > I can't find something similar.
> 
> Since the PIT timer read is possibly hw-dependent, it might be that you 
> can't necessarily reproduce it on some random hardware.
> 
> How sensitive is ntpd to (stable) drift? IOW, if we get the calibration 
> wrong, the TSC should still hopefully be very _stable_, it's just that the 
> initial guesstimate for the frequency is off and ntp would have to correct 
> for that.

NTP can adjust the clock about +/-500ppm (so a 1000ppm range). Past that
it starts throwing errors.

Part of the issue is that if the drift value changes in between boots,
NTPd can take a while to settle down on the right freq. I suspect that's
whats happening here, and should the box be left alone for a few hours
(maybe overnight) NTPd will find the new drift correction the issue will
go away.

Thomas tripped over this a little while back when the
NTP_INTERVAL_LENGTH change landed, but I think that was prior to 2.6.26,
so its probably the calibration changes discussed, but I wanted to see
if there were any other slight changes that might be contributing to the
issue as well.

thanks
-john