On Mon, Feb 22, 2010 at 5:03 PM, Hal Murray <hmur...@megapathdsl.net> wrote: > >> That said NTP is very conservative in validating the stability of >> clock sources. I have not delved into the code, but it is obvious >> that even a refclock like a GPS receiver doesn't get any favours. Why >> should it? Who knows whether the clock is dodgy or not? > > The NMEA strings from low cost GPS units have a lot of noise/jitter. > > In particular, the SiRF units are horrible. (They are also low cost and > widely available.) The time offset has a sawtooth pattern with a long time > constant that would be nasty to filter out. Think of hanging bridges. > http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif > > >> However the times he was reporting for the offsets to drop to less >> than 1ms did look excessive. > > I've seen lots of comments about ntpd being slow to converge. I haven't > investigated carefully, but they seem credible. > > One way to get in trouble is to have a bad drift file. You can get that if > you have a warm system, shut it down, wait for it to cool off, then restart > it.
I was not trying to start a fire and I personally don't have experience with TimeKeeper which seems to be a framework to make use of the TSC as we all would like to use it (despite P/C-state invariants, there have been countless threads on all the major *NIX kernel lists that have argued to use and not use TSC for ticking). Bad drift files aside, I have now played around with both an Endrun Technologies CDMA receiver as well as some other proprietary units and I have found that no matter what ntpd reference drivers I used (Palisade/Trimple come to mine), I have seen *many* instances of ntpd doing wacky things and taking several hours to converge. I never really investigated why this was so and without looking at source code attributed to ntpd doing exactly what you described Hal: Using a very conservative approach to validate the stability of the clock. Also, TimeKeeper seems to predict time a bit if I understand what they are doing (which I don't fully) while ntpd tries to skew toward absolute time (UTC). -aps _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.