[ntp:questions] Monitoring NTP - nptq -p

Spoon devnull at localhost.com
Mon Mar 26 10:26:40 UTC 2007

Richard B. gilbert wrote:
> Spoon wrote:
>> Keith E. Brandt, M.D. wrote:
>>> 'Poll' is the polling interval in seconds, which seems to vary quite 
>>> a bit on my system, so here's the first real question - how does ntp 
>>> pick the polling interval (a pointer to the docs is fine, I just 
>>> haven't been able to uncover it yet).
>> Have you played with minpoll and maxpoll?
>> http://www.eecis.udel.edu/~mills/ntp/html/confopt.html
>> minpoll minpoll
>> maxpoll maxpoll
>>     These options specify the minimum and maximum poll intervals for 
>> NTP messages, in seconds as a power of two. The maximum poll interval 
>> defaults to 10 (1,024 s), but can be increased by the maxpoll option 
>> to an upper limit of 17 (36.4 h). The minimum poll interval defaults 
>> to 6 (64 s), but can be decreased by the minpoll option to a lower 
>> limit of 4 (16 s). These option are valid only with the server and 
>> peer commands.
>> I don't know under what conditions ntpd changes the polling interval.
> Playing with MINPOLL and MAXPOLL is NOT recommended!!
> Ntpd varies the polling interval between MINPOLL and MAXPOLL in order to 
> best respond to existing conditions.  The math is over my head but it 
> has something to do with the "Allan Intercept" q.v.
> My oversimplified English explanation is that short polling intervals 
> allow large errors to be corrected quickly while the longest polling 
> interval allows small errors to be corrected very accurately.

I would have thought that short polling intervals are always better,
ignoring traffic overhead issues:

If the current "correct" interval should have been e.g. 64 seconds
instead of 16 seconds, just ignore 3 out of 4 replies.

Where is the flaw in my logic?


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