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

[Ed W <lists@xxxxxxxxxxxxxx>]

Hi, Suppose I have a desire that chrony conditions my RTC, say because
the device is rebooted daily, internet isn't always on and the "off"
time might be significant and measured in days.  Lets also assume that
system is not shutdown cleanly most of the time (hey, lets just call it
the home router...)

Now, what are the negative implications of running "trimrtc" on boot?
The docs suggest that chrony internally updates itself to factor in the
rtc change? We can assume that it happens before any iburst or other
network connectivity is finished, so we aren't disturbing any
in-progress computation?  Additionally I'm naively assuming that the
previously calculated drift rates for the RTC are preserved, so even if
we immediately turn the machine off again, then when we turn on (some
days later) it will apply a sensible offset to the previously updated RTC?

Assuming the above then running trimrtc at chrony startup seems safe? As
such could we please have an option to do this automatically?
"trimrtconstart" or similar

The logic is that modern kernels set the clock from the RTC for you and
then on some machines I see a clock skew detected once chrony wakes up
and computes a better time by incorporating the known offset. One
solution would be to run chrony earlier in the boot (what does Redhat
do?).  My suggestion at least means that the next reboot might start
closer to the correct time (clearly depends a lot on reboot frequency).

(The point of this change is not about increasing accuracy, it's only
about decreasing the gap between RTC and clock, mainly to decrease the
opportunity for boot scripts to use the wrong one)



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?

Thanks

Ed W


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