Re: [chrony-users] kernel PPS troubleshooting

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On Fri, 29 Nov 2013, Miroslav Lichvar wrote:

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

Ah. OK.

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.

It looks in the source code as if it grabs a new source as soon as the source
disappears, but that was really not a very good look I had at the code.

If only only had nmea and pps, one needs the nmea at least at start up to get
the time to within a half second or so, but thereafter of course it probably
should not be used unless the PPS disappears for quite a while ( in which case
the nmea is liable to be not very good either)

Certainly it would be good to find out what was happening with his clock

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