On 2015-02-10, Terje Mathisen <terje.mathi...@tmsw.no> wrote: > William Unruh wrote: >> On 2015-02-10, Jochen Bern <jochen.b...@linworks.de> wrote: >>> On 02/10/2015 06:15 AM, catherine.wei1...@gmail.com wrote: >>>> However, when I wait for several minutes, the time can be adjusted to >>>> the right time. I couldn't see the gradual changes of offset. Is that >>>> normal? >>> >>> Assuming that you're using a minimalistic configuration: Yes. >>> >>> ntpd would take almost three months to *gradually* eliminate (slew) one >>> hour of offset, so as soon as the >>> offset-from-hell-that-struck-us-out-of-the-blue-sky was confirmed, it >>> gave up all hope for the universe and "just set the clock" hard (step). >> >> No. It only does that for "offsets from Hades". The Ones from Hell, ntpd >> abandons all hope and quits. ( Hades is 128ms to 1000 sec, Hell is >> >1000 sec) >> Ie, for <128ms, ntp will try to slew the clock ( at a max of 500PPM- as >> far as I can see a completely arbitrary limit Mills decided on decades > > The 500 ppm limit is not at all arbitrary! > > In fact, it was originally just 100 ppm, but when too many systems > turned up with a system clock which was a bit too far out, Prof Mills > redid the control loop to allow a 500 ppm range. > > It could have been a lot more, but the ultimate stability of the control > loop is supposed to be better this way. > > My own control theory math was back around 1980, so I have forgotten > most of it. :-( >
As you state, it is arbitrary. If it can be changed from 100 to 500 after complaints, it indicates that the number was not picked to optimise anything. And as far as I can see, 500 or 5000 makes little difference to the control loop. Yes, it is harder for other systems to follow one with a large drift, but it is even harder to follow one that jumps, which is what we get now. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
