Re: [chrony-users] kernel PPS troubleshooting

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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.

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