On 2/11/2015 2:12 AM, Harlan Stenn wrote: > William Unruh writes: >> > On 2015-02-10, Terje Mathisen <terje.mathi...@tmsw.no> 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... >
Well, it indicates that it can be changed, not how easy it is nor what it entails nor the consequences. As I recall, the choice of PPM limit directly affects the maximum error budget. In Mills book he explains that the control loop is carefully crafted to simplify the calculations and to be provably correct and stable. With today's systems I am not sure simplifying implementation at the expense of clarity is necessary and provably correct is a goal that most of us worry about. Dr. Mills crafted a wonderful piece of software, amazing for its time, but he no longer actively engages us much at all. I understand, that isn't his fault. But no one who does actively engage really understands it or knows how to improve it. Unruh has a point, we don't know if there isn't a better way built on statistical analysis. Perhaps a hybrid between the two approaches would be better still. But we don't even know the consequences of changing a single constant with any degree of certainty. -- Brian Utterback Solaris RPE, Oracle Corporation. Ph:603-262-3916, Em:brian.utterb...@oracle.com _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
