| Re: [chrony-dev] Wake from sleep on OS X |
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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/
On Wed, 2 Dec 2015, Bryan Christianson wrote:
As for keeping the RTC clock in sync, ntpd appears to do this by calling settimeofday() each time it updates its stats files.
I can add a scheduled event in the Mac OS X driver to call settimeofday() at regular intervals to force the RTC update. I would think an update every 15 minutes would not be too expensive.
One of the features of chrony is that it can keep track of the rtc and its
rate difference from 1 and its offset and use that information next time the
rtc is used. However, if you keep resetting the rtc, that does not work.
Thus if you switch your computer off for 3 days, this would allow chrony to
compensate for the rate difference of the rtc clock from unity (ie 1 sec/sec)
There is a problem with this in that the rate depends on tempreture, and the
tempurature when the rate of the rtc is detrmined ( while the computer is
running) and rate when it is off and cold can be different by a few PPM (eg
second per month). The natural rtc rate can be off by up to 100PPM (seconds
per day) Thus if you keep resetting the rtc (eg if you have the kernel's
resetting of the rtc every 11 min) you do not have the rate correction. If you
have the rate correction, the rtc can be out by many seconds. I do not know if
the OSX kenel has that 11 min mode that the linux kernel has. chrony
deliberately disables that mode on linux so it can measure the rtc clock drift
rate.
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