[ntp:questions] Tighter regulation?
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.
More information about the questions