[ntp:questions] Server offset included in served time?

Unruh unruh-spam at physics.ubc.ca
Wed Sep 17 08:01:14 UTC 2008

David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>Unruh wrote:
>> David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>> Not sure what you mean. Do you mean that chrony averages out the offsets,
>> so that the actual estimated offset of the clock is less than the offsets
>> measured because of statistical averaging, then yes. 

>Yes.  And that would be true of any algorithm that was well matched to 
>the noise characteristics.
>> Temperature causes true changes in the clock rate. 

>Exactly.  And to that extent, projecting the new clock rate back to 
>adjust the apparent historic offsets is wrong.

Which is why chrony tests its fit to see whether or not a linear fit to the
data is a good statistical fit or not. (It tests the number of times that the data
changes sign around the fit and if it does not do so often enough, assumes
that a straight line is not a good fit, and reduces the number of historic
points it saves. Ie, if a linear drift is not a good fit to the data, it
 assumes that the changes in drift are more important source of error  than the random
measurement/transmission noise and  adjusts the fit model. Effectively it
is constantly trying to figure out where the minimum allan variance lies
where random offset noise and drift noise are of equal size. )

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