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On Mon, 23 Apr 2012, Ed W wrote:
On 17/04/2012 17:07, Bill Unruh wrote:
Why not do the hwclock stuff (it is not clear to me that the hwclock is
not
more accurate than chrony at estimating the rate error of the rtc
nowadays)
I wanted to make use of the estimated drift feature of chrony, ie estimating
what the real time is based on RTC + drift. I don't believe I can get that
from hwclock?
Actually, there is at least one version of hwclock that does give that to you.
It will estimate the average drift since it turned on the clock. Now, the
problem with the rtc drift is that it depends on temperature, and the drift
estimate either by chrony or hwclock, measures it when the rtc is hot, but you
want to use it for estimating the drift when the computer is off (ie cold) so
you can only compenstate for the drift to a few PPM.
See http://giraffe-data.com/software/about_hwclock.html
for a brief comment about the various versions of hwclock. You will have to
check to see which version you have. (One of the problems wih GPL is people
can hijack the name of a program, produce something that is not very good but
keep calling it by the old name.)
My application will be highly disconnected, so whilst I assume my RTC
accuracy will freely wobble based on temperature, etc, my hope is that it
will keep things closer. Very roughly we seem to be able to reduce the error
in the estimate rtc clock range to between 1% and 10% of where it would be
without such a correction. A bit better...
Sounds reasonable.
Mind you the rtc should not take 8 sec to read (it will take a couple, so
perhaps most of that time is chrony starting up), so it is not clear what
is
happening.
The time difference is almost completely the stuff I mentioned. I have
patched it out in my current build just for testing and chrony startup is now
near enough instant. (measured at 0.13s)
Kind regards
Ed W
--
William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
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