"David L. Mills" <[EMAIL PROTECTED]> writes: >Root,
>You could have saved a lot of time sweating the code if you looked at >the briefings. See especially the before and after time series and note >the 10 dB improvement in S/N. I am sorry, but looking at the code is far simpler than trying to find something in the documentation. Code I can use grep on, your documents I cannot. >You might not have noticed a couple of crucial issues in the clock >filter code. I did notice them all. Thus my caveate. However throwing away 80% of the precious data you have seems excessive. >1. The basic principle is to select the samples that have not been >delayed in queues, leaving hopefully the ones delayed only by >propagation. It's better to discard the others that can only result in >less accurate measurements. >2. The filter samples are correlated only if the total span of the >filter is less than the Allan intercept, generally assumed in the order >of 2000 s. For poll intervals above that, the samples become >uncorrelated, so fewer samples are used. >3. Never use an old sample, only new onese. It often happens that the >last used minimum delay sample is older than the most recent sample. The >result is that the filter can toss out up to seven samples before >finding a new candidate. The clock discipline loop is specifically >designed to deal with that, which is one reason the time constant is >larger than you might like. >Dave >root wrote: >> OK, having looked at the code, I see what it is doing. It essentially >> takes the measurement with the shortest delay in the past 8 measurements. >> If that measurement happens to be the current one, it is actually used >> in the clock loop. (Yes, I know this characterisation of the clock filter is >> cruder >> than reality). >> >> >> This makes the smaller variance of chrony even more impressive, since in my >> tests, evey single measurement of offset was used to calculate the variance >> in the case of chrony, while >> for ntp, only those "smallest" values as reported in peerstats were used. >> This also >> explains why the roundtrip variance in ntp was so much better than chrony's. >> >> snip >> >> >> >> _______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
