[ntp:questions] Tighter regulation?
Mischanko, Edward T
Edward.Mischanko at arcelormittal.com
Fri May 24 05:02:36 UTC 2013
> > On 2013-05-21, Mischanko, Edward T <Edward.Mischanko at arcelormittal.com>
> >> 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.
[Mischanko, Edward T]
This is exactly what I am saying!! This needs to be corrected; I consider it a bug.
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 biased 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 realizes it needs to do aggressive correction.
[Mischanko, Edward T]
It takes too long to figure out it needs a more aggressive correction.
If I leave maxpoll at the default of 1024 seconds, my clock drifts outside
of 5 milliseconds consistently.
Too much assumption is made that everyone will have the perfect computer and
the perfect network when configuring these various filters. What works on
the blackboard does not always work in reality. My computer has a
-19 precision but it can't keep time inside 1 millisecond with default
Settings; go figure.
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