On 2013-04-26, Steve Kostecke <[email protected]> wrote: > On 2013-04-26, Biebaut Sven <[email protected]> wrote: > >>ATTRIBUTION MISSING said: >> >>>Biebaut Sven wrote: >> >>>> If I drop the idea of the RTC as a reference clock, am I correct in >>>+ stating that, when there is no external synchronisation: >>>> - my local clock and my RTC will drift away from each other, but at >>>+ least my RTC will be closer to the mark (the DS3231 is chosen for its >>>+ precision) >> >>>Probably not. ntpd will continue to apply first order frequency >>>correction to the local clock. >> >> Ah, I did not realise that. So a system with ntpd but without an external >> reference clock would still be more accurate than a system without ntpd >> at all ? > > Only if ntpd has had access to a real local reference clock, or to > remote time servers, for a long enough period of time to calculate the > frequency correction. And the ambient conditions and system load match > that which existed when the correction was determined. > Yes, this is important. You need 5-10 hours of connection for ntpd to settle down to its "best" timekeeping. If the system load changes, it heats up the computer making it worse. There do exist versions of ntpd which have incorporated temperature compensation-- they measure the temp of the system using any of the on-board termometers, and derive the sensititivity of the onboard clock to temperature changes. They then use that to continually alter the rate of the clock to compensate for those temperature drifts. So those may be a better way to go if your system is being installed in a thermally fluctuating environment. Noone has created such an addon for chrony yet, but it might be even more effective as chrony is more sensitive to determining short term frequency changes than is ntpd. (ie, if you daily have a big job that runs for an hour and increases the internal temperature, ntpd will be very slow to respond to the frequency change caused by that temp spike, and thus to being able to measure the effect of the temp on the frequency. ntpd takes hours ( or once it has settled down to poll 10, days) to notice that things have changed. (poll 10 means 2^10 sec= 10^3 sec=1/3 hour, and since ntpd throws away 7 of 8 polls, that means it is effectively making a measurement every 3 hours. Then it only slowly responds to maintain stability of the local clock.)
chrony will respond much faster. It would be interesting for someone to install a temperature compensation into chrony to see how it responded. But that does not help you. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
