On Tue, Jan 18, 2011 at 02:11:12PM +0000, David Woolley wrote: > Miroslav Lichvar wrote: > >Yes, when the jitter is too low or the clock too unstable. Ideally, > >ntp would run a statistic and adjust it in runtime. Chrony counts > > It does. I forget the exact metric, but look for the term poll adjust.
The allan_xpt variable is 11 by default, and can be changed only by the tinker allan command. (which is what was used to force ntpd to enable FLL in the clknetsim tests) > >Linking them makes sense if you want to keep things simple and robust. > >The problem is that even the minimum allowed poll 3 is too long in > >some situations and that it wastes network bandwidth. > > Reducing the poll interval without reducing the time constant will > simply result in oversampling. The time constant will still > determine the loop behaviour. It will improve the accuracy when your Allan intercept is high (e.g. you have a thermally stabilized oscillator). Reducing time constant is probably the more interesting case, but this needs to be done carefully to avoid undersampling, the constant probably could be reduced only to half or quarter, then it would have to switch to FLL. > (It may make things worse if network > delay transients no longer fit within the 8 sample filter.) Good point. That's another advantage of reducing time constant independently from poll interval. > Network wander is when, for example, an asymmetric load is applied > temporarily. With 3.1 kHz modems, that would result in such a large > wander that ntpd would start ignoring the server entirely, so was > rather benign. With lower speed DSL, it results in clock steps. > With higher speed connections, the peak to peak wander may be under > 128ms. Ok, on the clknetsim page are some tests with temporary asymmetric delays too. It's actually a good example where increasing time constant would allow dropping more than 8 consecutive samples, instead of mangling them with huffpuff to avoid undersampling. -- Miroslav Lichvar _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
