[ntp:questions] Has anyone thought about this?

Brian Utterback brian.utterback at oracle.com
Thu Apr 10 13:44:36 UTC 2014


On 4/10/2014 3:22 AM, Terje Mathisen wrote:
>
> The maximum ntpd slew is ± 500 ppm, which means that the absolute 
> maximum possible slew between UTC and the local clock would be 1000 
> ppm (i.e. the clock is maximally bad, at +500 ppm, and we are 
> currently slewing at -500 ppm), in which case the maximum error 
> component from this would be 1/1000th of the actual time delta. (In 
> real operating systems the actual errors are several orders of 
> magnitude less! Typical clock frequency adjustments due to temperature 
> cycling are in the single ppm range, but even a few tens of ppm gives 
> relative errors in the 1e-4 to 1e-5 range, which doesn't impact the 
> control loop at all. 

I am pretty sure that the ± 500 ppm is absolute and is already the sum 
of the frequency correction and the current clock slewing. But one of 
the reasons for having a maximum in the first place is to put a cap on 
the error introduced because if the instantaneous frequency corrections 
taking place at the time the timestamps are taken. This is all covered 
in chapter 11, Analysis of Errors, in the first edition of Das Buch 
(Computer Network Time Synchronization, Mills, 2006). I am pretty sure 
that it is also in the 2nd ed, but I don't have access to that one.

Brian Utterback


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