[ntp:questions] Tighter regulation?

David Woolley david at ex.djwhome.demon.invalid
Tue May 21 20:38:27 UTC 2013


> On 2013-05-21, Mischanko, Edward T <Edward.Mischanko at arcelormittal.com> wrote:
> 
>> My concern is that too much data is being thrown away when polling
>> above 256 seconds and that allows excessive wandering of my clock.
> 

If too much data is being thrown away, it would be because the poll 
adjust algorithm has chosen too high a poll interval (or you have too 
high a minpoll) for the noise statistics.  The samples are, effectively, 
put through an IIR digital low pass filter, with a bandwidth that is 
sufficiently narrow that variations across the 8 samples are largely 
suppressed.  However, if you include all the samples, their overall 
average is likely to be biassed away from a zero error, and as that is 
likely to not change, or only change slowly from one group of 8 to the 
next, it will introduce an error that is not filtered out.

It is not throwing out anything like the equivalent of 7 out of eight 
samples, because the contributions of the variations between samples are 
high frequency noise, which is probably being attenuated by a factor 
significantly more than 8.

The way that ntpd chooses the poll interval is by looking at the offset 
and jitter (exponential average of sample to sample variations).  If the 
offset consistently exceeds the jitter, it means that part of the offset 
cannot be accounted for by the measurement noise, and more aggressive 
correction is needed.  Unfortunately, if something make the crystal 
frequency change suddenly, after a long stable period, it can take some 
time before it realises it needs to do aggressive correction.



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