[ntp:questions] What is the NTP recovery time from 16s step in GPSserver?

David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Nov 1 17:41:03 UTC 2012

On 01/11/2012 13:36, David Woolley wrote:
> David Taylor wrote:
>> That's the offset as stated by NTP.
> That's not the time error.  Under ideal circumstances, it is several
> times higher than the actual error, but with a low value like that, it
> could easily be orders of magnitude worse (e.g. if there were interrupt
> latencies in the 100s of microseconds and up, or, for network time, if
> there were asymmetric delays in the milliseconds).
> At 250 microseconds, do they all have the same sign? The claim for
> chrony would be that it gets to the state where the offsets are balanced
> about zero, much faster.

Well, it's the best data I have available to plot, and seems to indicate 
well how changes in configuration of hardware and software affect 
timekeeping.  The device shows NTP offsets in the order of 10 
microseconds from other PPS-driven network sources.

I believe the interrupt latency on that 700 MHz processor is around 
30-50 microseconds with the Raspberry Pi hardware.  For what I need it's 
more than adequate - an absolute error of less than a millisecond would 
not be noticed by the applications I run.  Using different software 
isn't going to affect the interrupt latency unless it had some way of 
measuring that with external hardware.
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