| Re: [chrony-dev] automatic RTC trimming |
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On Tue, Feb 14, 2012 at 09:37:35AM -0800, Bill Unruh wrote:
> On Mon, 13 Feb 2012, Miroslav Lichvar wrote:
> >- track long-term drift: always keep a maximum number of samples
> > (disable the runs test), increase the maximum measurement interval
> > or do RTC sampling only on reference updates (NTP polling), decouple
> > RTC and system clock drifts (don't slew RTC samples on changes in
> > system clock frequency)
>
> I do not know how you can decouple the rtc and system drifts since the only
> time which the rtc can be compared with is the system time. If you change the
> notion of what the system time is, you also have to change the notion of what
> the rtc time is.
Decouple only their frequencies. Currently, when there is a change in
the system clock frequency, the RTC samples are slewed according to
that change. This makes sense if their sample spans are similar, but
probably not much if the RTC samples are much older. If the frequency
adjustments are ignored, I think we'll get an accurate long-term RTC
drift.
> >- add an option to periodically trim RTC
>
> Seems reasonable. Of course this means that one has to import into chrony, the
> notion as to whether the rtc is on UTC of local time. As it is , the rtc can
> be off by 8 hours, say (here in BC) and chrony will not object. But if chrony
> trims the rtc, it has to know what to trim it to. Sys time ( meaning use of
> UTC), local time, of some other time the user might want his rtc to run at.
There already is the rtconutc option for that. But to have the
automatic trimming option enabled by default in a distribution, it
would have to work in both UTC/local times automatically. I think the
approach used in the 11-minute kernel sync mode which makes no
assumption about timezone and corrects RTC only within +-15 minutes
might be useful here too.
--
Miroslav Lichvar
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