[ntp:questions] Monitoring NTP - nptq -p

Richard B. gilbert rgilbert88 at comcast.net
Fri Mar 23 18:08:27 UTC 2007


Spoon wrote:
> Keith E. Brandt, M.D. wrote:
> 
>> /home/brandt 53> ntpq -p
>>       remote           refid      st t when poll reach   delay   
>> offset  jitter
>> ============================================================================== 
>>
>> *GPS_NMEA(0)     .GPS.            0 l    2   64  377    0.000    
>> 0.005   0.017
>> -ntp.your.org    130.126.24.44    3 u   47   64  377   62.065    
>> 4.289   2.750
>> +boudicca.tux.or 65.212.71.102    2 u   12   64  377   63.289   
>> -5.525   2.273
>> -dake.desynched. 132.163.4.103    2 u   18   64  377   55.516    
>> 4.039   2.726
>> -veritas.imagepi 67.128.71.76     2 u   60   64  377   51.816   
>> -8.385   2.495
>> -hudson.yutanigl 128.252.19.1     2 u   20   64  377   50.768   
>> -5.847  14.944
>> +ferocious.dairi 140.142.16.34    2 u   18   64  377  119.579   
>> -0.465  11.587
>>
>> '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.




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