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

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Wed, 20 Jul 2011, Ed W wrote:

> Possibly more succinctly:
>
> a) I want to step rtc to track real time (since it's entwined with booting)

You could do the 11 min mode. That would mean that the offset was always very
very close to zero when the device is switched off, but the rate is unknown.
But, if Lichvar is right, you might never get a good rate anyway because the
rate checking occurs too rarely. (On my system rtc is checked 400 times a day
roughly, so that is about once every 5 min)

The rate I get on a bunch of my machines  is around 15PPM or 2 sec/day 
for the drift rate and it fluctuates by about 1 or 2 PPM. which suggests you
cannot get the cold drift rate to better than about 5 PPM anyway. So it is not
clear how much of an advantage the drigfft rate buys you.

> b) The device may not be running for days on end (so rtctrim only every
> 24 hours might rarely/never get called)


> c) No expectation of regular clean shutdowns (can't trimrtc on shutdown)

If you do the 11 min rule, or run trimrtc every 15 min or so, it should not
make any difference.

> d) Dialup, so can't ignore rtc and assume we are getting a network
> connection straight after boot
>
>
> So the only options to call trimrtc seem to be:
>
> 1) On boot.  Seems highly safe if done early in chrony startup since it
> should simply emulate a fixed step in time
> 2) Occasionally every X hours, where I'm hoping for some feedback on a
> good number for X.  I have to assume that my device will often run for
> days on end, but often only for a few mins/hours at a time.

> I was hoping 1) would be an easy win and 2) could be left as a best
> efforts option which happens occasionally.  After all if we are being
> rebooted more often than every X hours, then we get to step the RTC more
> often via 1)
>
> Cheers
>
> Ed W
>
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