[ntp:questions] NTP 4.2.2p4 loopstats

David L. Mills mills at udel.edu
Tue Nov 14 04:09:18 UTC 2006


There are several jitter statistics now, including per-association 
jitter and system jitter. These statistics are averages of first-oder 
time differences. It became apparent that frequency differences are a 
valuable indicator of equipment malfunction, A/C failure and so on. I 
could have simply tacked on an additional field, but there would be many 
more guys than just you out for my scalp. The old statistic wasn't very 
useful anyway. Take it from me and my own monitoring stash that you 
really want the frequency differences, properly interpreted as wander or 
maybe noise, but not jitter in the normal techical interpretation.

The intended interpretation of loopstats is, as the name suggests, 
watching the discipline loop time, time differences, frequency, 
frequency differences and time constant (poll interval). From long 
experience, these xstatistics are really what you need to know. Watch 
the ;noise over a day or two; see how exquisitly sensitive it is.


TomD wrote:
> Wandering back to jg's original question...
> It appears that the jitter field which gets written to loopstats
> changed considerably between 4.2.0 and 4.2.2.
> The new value, in the global variable clock_jitter, is calculated from
> (last offset - current offset) rather than from the current offset as
> in 4.2.0.
> This new value is displayed by ntpq rv as "noise".
> The old value is still around in the global sys_jitter.     ntpq still
> displays this value as "jitter" in 4.2.2.
> The sys_jitter value still gets updated, but not by ntp_loopfilter
> anymore - which only updates clock_jitter.
> This is significant to anyone who post-processes loopstats, as the new
> values are typically smaller in magnitude.
> I can't find anything about it in the bugs, docs, or mailing list.
> Maybe I'm just not finding it.
> A quick fix (...well, a quick ugly hack actually...), for anyone who's
> loopstats post-processing has been disrupted with 4.2.2, is to change
> the calls in ntpd/ntp_loopfilter.c to pass  sys_jitter  instead of
> clock_jitter  to the 'record_loop_stats'  function.
> Note that you probably don't want to change *anything* else in
> ntp_loopfilter -  just the number which gets written to loopstats, lest
> you disrupt the calculation of poll interval, etc.
> -tom-

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