Re: [chrony-users] kernel PPS troubleshooting

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On Fri, Nov 29, 2013 at 09:46:32AM -0800, Bill Unruh wrote:
> On Fri, 29 Nov 2013, Bill Unruh wrote:
> By the way, does the kernel PPS do median filtering before passing on the
> times to chrony? (Ie, taking the median of say the past 16 inputs and throwing
> away the 6 worst outliers and then retaking the median?)

The kernel doesn't filter the PPS samples in any way. In chronyd the
PPS driver fetches the latest PPS sample from the kernel once per
second and the refclock poll (16 seconds by default) runs the median

> Anyway, it should not be switching sources unless the deviation of the
> selected source exceeds the variance of the alternative (or unless the source
> has disappeared for a suitable number of poll intervals, probably related to
> how long one would expect to wait for the drift rate variance to make the
> system clock deviate by more than the second source's variance. Ie, you are
> far better off letting a clock drift unconstrained for a while than to jump to
> source which has a huge (factors of a 1000) worse variance.

The selection algorithm prefers sources with shortest distance (with
refclock that's the measured dispersion + configured delay). If there
are more sources with similar distance they will be combined together.

If a source disappears for 8 polling intervals, chronyd will select
another source even if it's much worse. I agree that could be
improved. With NMEA sources it's usually better to use the noselect
option or don't configure it at all.

Miroslav Lichvar

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