[ntp:questions] Why does ntp keep changing my conf file?

David J Taylor david-taylor at blueyonder.co.uk.invalid
Sun Sep 19 05:08:16 UTC 2010


"David Woolley" <david at ex.djwhome.demon.invalid> wrote in message 
news:i73bgf$ph$1 at news.eternal-september.org...
> David J Taylor wrote:
>
>>
>> So, if I understand you correctly, you are saying that because ntp 
>> measures and reports an offset, it will try and correct the clock in 
>> the opposite direction, so that the actual clock error is likely to be 
>> less?
>>
> So will ntpdate, but ntpdate only uses the most recent offset, whereas 
> ntpd uses (to a first approximation) an exponential rolling average of 
> the recent offsets, such that it makes effective use of quite a lot of 
> them.  That has a similar effect to using longer and longer averaging 
> periods in MRTG.  The time of the clock is much more accurate than any 
> one offset would suggest.

Understood....

> Complicating factors are that there is a non-linear finite impulse 
> response filter (best of 8) ahead of the main, infinite impulse response 
> one, the actual filter is  not just solving for an offset, but also for 
> the frequency, and the filter parameters are varied depending on an 
> estimate of the quality of the time data.

I did say it was sometimes rather esoteric!

Is there any better parameter than offset= from an ntpq -c rv command 
which one could plot?

Cheers,
David 




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