[ntp:questions] Keeping NTP Honest Redux

David Woolley david at ex.djwhome.demon.invalid
Wed Nov 11 23:46:04 UTC 2009


Unruh wrote:
> David Woolley <david at ex.djwhome.demon.invalid> writes:
> 
>> Unruh wrote:
>>> David Woolley <david at ex.djwhome.demon.invalid> writes:
>>>
> 
>>>> Looking at the graphs, one needs the first derivative of the rate, as well.
>>> Why? If you want the details of how chrony works, I could supply it, but it uses
> 
>> Because most of the time the first derivative is almost constant, which 
>> will force an adaptive linear regression to use a very short baseline.
> 
> Sorry. If the first derivative is almost constant, chrony will grow the baseline

I think you are talking first derivative of phase (aka frequency).  This 
is first derivative of frequency, which will mean that the first 
derivative of phase is changing rapidly.

> out to its max extent (64 data points). It is only when the first derivative
> changes rapidly that the baseline is shortened. In typical use, the baseline is
> about 15 to 20 points. 
> 
> (chrony runs a test-- using the number of times that the residual changes sign-- to
> determine if the linear regression is statistically good or not. 
> )
> 
> 




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