Re: [chrony-users] kernel PPS troubleshooting

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On Fri, 29 Nov 2013, Bill Unruh wrote:

On Fri, 29 Nov 2013, Miroslav Lichvar wrote:

>  The problem in his case is that the PPS signal is occasionally
> (but far too often) off by almost .3 sec. That is rediculous. And it is > only
>  when the gps-nmea and the PPS are the only sources.

 He said chronyd was switching between the PPS and GPS sources, so the
 0.3s spike could be just the PPS-NMEA offset. The other graph with
 chronyd using NTP sources doesn't seem to have this problem.

Hm, I guess that would do it. But why would it be switching like that? If it
is doing so, then there is a problem with the chrony selection algorithm. Your
solution of having gpsd handle it all is a possible one, but chrony itself
should not be behaving that way. The nmea has a huge variance, while the PPS
variance should be tiny, and it should be being selected. Or is the PPS
exceeding its variance occasionally and chrony thinking it has gone rogue,
selects the nmea? By this time I do not remember the selection algorithm sufficiently well to be
able to say.

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?)

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.

William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1     |      and Gravity       |

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