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

unruh unruh at wormhole.physics.ubc.ca
Tue Jan 18 19:26:53 UTC 2011


On 2011-01-18, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> Miroslav Lichvar wrote:
>
>> 
>> Yes, when the jitter is too low or the clock too unstable. Ideally,
>> ntp would run a statistic and adjust it in runtime. Chrony counts
>
> It does.  I forget the exact metric, but look for the term poll adjust.
>
>> number of runs of residuals with same sign, perhaps it could work for
>
>> 
>> Linking them makes sense if you want to keep things simple and robust.
>> The problem is that even the minimum allowed poll 3 is too long in
>> some situations and that it wastes network bandwidth.
>
> Reducing the poll interval without reducing the time constant will 
> simply result in oversampling.  The time constant will still determine 
> the loop behaviour.  (It may make things worse if network delay 
> transients no longer fit within the 8 sample filter.)

If the error is random error, then oversampling is exactly what you
want. It knocks down the error by the square root of the oversampling. 

>
>
>> 
>> Define error and network wander.
>
> It looks like you are actually measuring error, but most people assume 
> that what ntpd reports as offset is the error.
>
> Network wander is when, for example, an asymmetric load is applied 
> temporarily.  With 3.1 kHz modems, that would result in such a large 
> wander that ntpd would start ignoring the server entirely, so was rather 
> benign.  With lower speed DSL, it results in clock steps. With higher 
> speed connections, the peak to peak wander may be under 128ms.

In none of my tests-- both ethernet in the university, or adsl/cable
modem from home ( over commercial telephone/cable tv lines with their
huge variability of load) have I ever seen such behaviour. 


>> 
>> Please note that the RMS offset is from the actual clock offset
>> maintained by the simulator, not the one reported by ntp.
>> 




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