To whom it may concern:
We observed that the PTP reported "clockcheck: clock jumped" events when we
added some CPU/Memory load by using stress-ng.
It seemed to us that the problem was triggered by the uncertainty of the
mono_interval = (int64_t)mono_ts - cc->last_mono_ts at this function where
mono_ts is the local time when this routine is called.
There is some uncertainty when the function is called in particular when the
system is busy and therefore there will be a large variance of mono_interval.
In our test case, the PTP will receive the sync event every 120/130ms, the
difference of receive timestamp is very stable, but there could be large
difference at the mono_interval and it triggers false clock jump event.
// light load
ptp4l[1984.369]: interval 130021595 mono_interval 130022231
ptp4l[1984.489]: interval 119977963 mono_interval 119997411
ptp4l[1984.619]: interval 130005312 mono_interval 129992626
ptp4l[1984.739]: interval 120002293 mono_interval 119996434
ptp4l[1984.870]: interval 130287464 mono_interval 130400107
ptp4l[1984.989]: interval 119719964 mono_interval 119621813
ptp4l[1985.119]: interval 129993071 mono_interval 129975709
// two bad intervals
ptp4l[1985.262]: interval 119997966 mono_interval 142649231
ptp4l[1985.369]: interval 130029848 mono_interval 107389768
ptp4l[1985.369]: clockcheck: clock jumped forward or running faster than
expected!
//heavy load
ptp4l[1090.977]: interval 119990608 mono_interval 109828955
ptp4l[1091.110]: interval 130000707 mono_interval 133090775
ptp4l[1091.226]: interval 120002438 mono_interval 116894219
ptp4l[1091.373]: interval 130025626 mono_interval 146662543
ptp4l[1091.500]: interval 120022987 mono_interval 126504891
ptp4l[1091.607]: interval 129964127 mono_interval 106861426
ptp4l[1091.608]: clockcheck: clock jumped forward or running faster than
expected!
Is it expected behavior?
Best regards,
Eric
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