David, The original model implemented in the Alpha kernel does not step the clock backward unless the step is greater than two seconds. Rather, it stops the clock and advances one microsecond at each read. This applies whether NTP slews or steps. Various ports of that code have broken this model in every possible way.
The 500-PPM slew once was common in the ubiquitous Unix kernel. The value was chosen as a compromise between short slew time for relatively small adjustments and moderate resolution during the slew interval. This works out to 5 microseconds per tick with a 100-Hz clock and a 5-us jitter. In truth this could be changed to anything you want, as long as the value is fixed. Some kernelmongers, including SGI and Linux, have put up fancy code designed to reduce the slew time for large adjustments. This inserts and additional pole in the clock discipline impulse response which results in unstable behavior for adjustments over half a second or so. The default step threshold is 128 ms; the -x command line option sets it to 600 s and does nothing else. The 600-s value was chosen as the expected accuracy with eyeball and wristwatch. If the extra pole is not there, the original response is preserved over that range and largely independent of the slew value itself. Say you change from 5 us per tick to 1 ms per tick or 100 ms/s. This would amortize a 600-s adjustment in almost two hours and reduce the resolution to 1 ms. If your extended network requires synchronization to better than one second, in all but the last second of that slew the network would not be synchronized. Dave David Woolley wrote: > Unruh wrote: > >> >> Not at 500PPM limit but if you use the tick adjustment, it is more than >> enough time. (The tick adjust limits out at 100,000PPM) >> > I believe ntpd assumes that it is constant. Having a large tickadj > causes poor resolution when using the user space discipline. > > I suspect that Dr Mills would say that a high slew rate also compromises > the system behaviour when you cascasde multiple strata. _______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
