Re: [chrony-dev] 1.24-pre1-- sudden change in rtc slope

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Mon, Dec 21, 2009 at 01:19:24PM -0800, Bill Unruh wrote:
> Strange behaviour of the rtc slope with the new version of chronyi (1.24pre1). I have
> instituted it across a number of machines, and on many of them, suddenly the
> rtc slope has changed hugely. For example on one machine, with the old chrony
> 1.23 the rtc slope was 7.5, it is now 206. PPM Nothing was done to the system
> except change the version of the chrony that was running.

Is the slope stable or just much more noisy? I would find it very
strange if the new chrony caused RTC to run suddenly slower or faster.

Probably the only related change between 1.23 and 1.24-pre1 is the
commit 9c9530, you can try revert it and see if it changes back.

On the machines where I have enabled rtc tracking I saw no such
changes in the slope. The only problem I'm seeing is the HPET
emulation.

A machine without HPET running 1.24-pre1:

   10.340139 1       10.340180        16.346  16   7  240
   10.344105 1       10.344135        16.346  17   7  240
   10.347974 1       10.347976        16.331  17   7  240
   10.351923 1       10.351929        16.331  17   7  240
   10.355886 1       10.355886        16.331  18   8  240
   10.359840 1       10.359840        16.332  19   8  240
   10.363786 1       10.363790        16.331  20   9  240

A machine with HPET and synced to PPS refclock:

   -2.089519 1       -2.087215         3.294  12   6  120
   -2.091335 1       -2.089193         2.106  13   6  120
   -2.092934 1       -2.089439         0.622  13   5  120
   -2.094780 1       -2.094780       -14.679   8   4  120
   -2.097859 1       -2.095693       -14.683   9   5   60
   -2.099156 1       -2.096917       -15.169  10   5   60
   -2.105748 1       -2.101276       -18.610  11   5  120
   -2.092038 1       -2.100617       -15.167  12   6  120
   -2.093754 1       -2.101713       -14.246  13   6  120
   -2.095239 1       -2.095196        -0.680   9   4  120
   -2.098260 1       -2.097954        -4.709   9   4   60
   -2.099887 1       -2.095276         3.832   9   4   60

I think this means it's not just a noise added by the RTC emulation,
but something with a pattern which the linear regression fails to
smooth out. Ideally the code would be able to detect it and try to use
more points in the regression or use longer intervals between
measurements.

Ideas?

-- 
Miroslav Lichvar

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