Re: [chrony-users] kernel PPS troubleshooting

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On Wed, 11 Dec 2013, Miroslav Lichvar wrote:

On Tue, Dec 10, 2013 at 08:29:25PM -0500, Battocchi, Scott L. wrote:
I've attached the tracking, measurements, refclocks, and sources logs trimmed to start at the 2.35 hour mark (to coincide with the graph colored by sync source in my previous mail).  I also moved the rolling header line for each log to the start of these trimmed ones and removed any subsequent headers from the remainder of the file.  They each run about 16 minutes and through multiple sync source selections.  I did not include any logs from the first two  minutes where sync=1 and dist actually changed since that seemed to be a startup artifact and not related to the rest of the long run issues.

It seems the dropping of the PPS source is caused by SHM source having
too small configured delay. The long-term stability of the SHM source
is worse than the short-term jitter, so the measured dispersion (in
one polling interval) of the SHM source is sometimes smaller than the
current offset, which means it doesn't overlap with the PPS source in
the source selection algorithm and no source is selected with the "no
majority" message.

When no source is selected, the PPS samples are ignored. If the SHM
source doesn't move to the acceptable range to overlap with the PPS
source in 8 polling intervals, the PPS source is marked as unreachable
and the SHM source is selected as the only available source.

That sounds like a bug. PPS should always be part of the selection process. It
is almost by definition the correct source. And certainly it could be argued
that the PPS should be the selected source, not the nmea. Of course some
people (me) us shm to deliver pps to chrony, so shm should not automatically
be downgraded, but a kernel pps it seems certainly should not be downgraded.

The configured delay is included in the interval used in the source
selection algorithm, so increasing the value from 0.01 to 0.4 or
larger should fix the problem.

A user should not have to do this or know this.

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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