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.
--
Cheers,
David
Web: http://www.satsignal.eu
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