[ntp:questions] Polling interval in FreeBSD vs. Windows

David Woolley david at ex.djwhome.demon.invalid
Tue Jan 18 19:58:17 UTC 2011


unruh wrote:
> more responsive to frequecy shifts. Also ntpd's averaging time is NOT of
> the order of 100 poll intervals ( which you would need to get your order

Fair comment.  However, it is still measuring the difference between a 
filtered estimate of true time and an instantaneous measurement.  It is 
not measuring the difference between true time and the system clock 
time, which is how a lot of people seem to treat it.

> of magnitude) Especially as ntpd uses only one of every 8 polls, the
> actual statistical improvement is less than 3, not an order of
> magnitude. 
> 
> No. a fast poll also corrects wander faster. ntpd is horribly slow at
> correcting wander, but doing it quickly corrects it faster. What it does

Depends whether the wander is in the clock or the measurement.


>> loop time constant will report higher offsets, but the time will 
>> actually be more correct.
> 
> sorry, but that is just wrong. The offsets are the best estimate of the
> "correct time" Higher offsets means higher errors. 

The offsets are the difference between the best estimate of the correct 
time, before it is updated, and the time from a specific, selected 
measurement.
> 
> 
>> This may be complicated in that I'm not sure that the loop time constant 
>> is actually clamped by the poll rate, so it is possible that setting a 
>> high rate over samples without changing the overall behaviour.  Maybe it 
>> is clamped by maxpoll?
> 
> No, it is determined by the number of readings, not by time. More
> readings in a given time means shorter time constant. Fewer readeings,

I am pretty certain that they are decoupled internally, and the degree 
of over-sampling can vary.  Essentially, though, it is the time constant 
that is tuned, and the poll interval has to over sample that and the 1 
out of 8 filter.

> longer. ntpd throws away 7 out of 8 readings, so the actual poll
> interval of used reading is 8 times as long as the set poll interval,
> and the time constant is about 5 times that if I remember correctly. 
> 
> 




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