[ntp:questions] Stabilizing the drift file?

David L. Mills mills at udel.edu
Thu Nov 23 17:00:07 UTC 2006


No, you can't always measure the frequency accurately. It depends on the 
initial offset when you start and what happens after the initial 
frequency measurement. As most folks don't want to set the clock 
directly unless forced, the daemon allows an initial offset of up to the 
step threshold (128 ms) and does the best it can after the stepout 
interval (900 s). Note that the code carefully separates the frequency 
measurement from the time offset measurement, so even after an accurate 
frequency measurement the discipline has to wrangle both the time and 
frequency when the initial time is as much as 128 ms in error. This 
initial transient can take several hours to dissipate.

This could be avoided by forcefully setting the clock at initial 
startup, but in some systems the setting function can have a large error 
in and of itself. The only workaround would be to disable all 
applications until the residual offset was within the tolerance, circa 
10 ms. Setting the stepout interval something less (300 s) moves up the 
time the discipline is enabled, but brings with additional error due to 
the expected error in the offset measurements. For instance, if the 
expected error was 10 ms in a 300-s stepout interval, the maximum 
frequency error could be as high as 67 PPM. The discipline could take up 
to a day to amortize this error.


Juergen.Salm at siemens.com wrote:
> mills at udel.edu wrote :
>>When first starting ntpd without the drift file, by default the state
>>machine takes fifteen minutes to directly compute the initial frequency
>>estimate within about 1 PPM, then enables the native clock discipline
> Hi Dave.
> You statement is correct for the complete variety of platforms ntp
> supports.
> However, on a Linux system (with at least 1us time resolution) you can
> get reasonable estimates for the drift much faster.
> Have a look at my script in
> https://ntp.isc.org/bugs/show_bug.cgi?id=742
> Changing the stepout as you proposed should give the same results (on
> Linux).
> Unfortunately, I didn't understand the meaning of 'stepout' from the
> documentation. ;-(
> Bye
>  Juergen

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