Re: [chrony-dev] Question / Feature suggestion - trimrtc on start?

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Wed, Jul 20, 2011 at 05:24:35AM -0700, Bill Unruh wrote:
> trimrtc is supposed to occur such that the algorithm to determine the rtc
> drift rate compensates for the change in rtc caused by the trinrtc (all
> entries in the prior rtc measurement table are shifted by the same amt that
> the rtc is).

When the RTC is trimmed, the old samples are dropped and the
measurement starts from scratch. The reason behind this seems to be
that RTC can't be set very precisely (only to half a second?).

I think the proposed option could behave similarly to running hwclock
--adjust on boot.

> >A related question is then - given I want chrony to learn a reasonable
> >estimate of RTC drift, plus I want to run trimrtc periodically, what is
> >a sensible frequency to set cron to call trimrtc?  Hourly, daily,
> >weekly?  What is the shortest sensible trim that will still lead to a
> >moderate estimate of RTC drift being learned?

The samples are collected in 15-480 second intervals and the maximum
number of samples is 64. I haven't done any experiments on how does
number of RTC samples affect the error in the drift estimate. I'd
probably go with the daily frequency.

I don't think there are many users of this feature. Generally, the -s
option (or hwclock --adjust) can't be used by default as there is no
guarantee that it's the only thing touching RTC. In my packages, I set
the rtcsync option instead (the 11-minute kernel RTC synchronization).

-- 
Miroslav Lichvar

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