On 2015-02-11, Harlan Stenn <[email protected]> wrote: > William Unruh writes: >> On 2015-02-10, Terje Mathisen <[email protected]> wrote: >> > William Unruh wrote: >> >> 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. > > Is not! Is so! ... > >> If it can be changed from 100 to 500 >> after complaints, it indicates that the number was not picked to >> optimise anything. > > I'm not sure that logically follows... > >> 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. > > So what's the difference between following a jump and following a > system that applies changes faster than the 500ppm that NTP is designed > for? > > And given that reality bites, what are the tradeoffs between re-synching > after a step and slowly dealing with a know offset error? > > We're talking about choices, and the different effects of these choices. > > It's one thing if a system rarely steps. It's a bit different if those > steps happen more frequently.
Yes. And it is either equally rare that the system will go over 500PPM, but sometimes a computer can have a large "natural" drift, (even over 500PPM) and that will drastically reduce the "headroom" to deal with unusual situations. (ie, if the computers normal drift is 400PPM, that means that the effective cap is only 100PPM, not 500). stepping is much worse than high PPM since it is infinite PPM. Note that were ntpd designed for 5000 PPM then anything else could follow it since it could also do 5000 PPM. Yes, we are talking about choices. And all I was saying was that this particular choice was somewhat arbitrary. > > H _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
