Re: [chrony-dev] Wake from sleep on OS X

[Bryan Christianson <bryan@xxxxxxxxxxxxx>]

> On 2/12/2015, at 10:40 AM, Bill Unruh <unruh@xxxxxxxxxxxxxx> wrote:
> 
> 
> 
> 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.

I didn't know that - thanks. 

In light of that, I am not sure how the RTC should be synchronised. As far as I can tell there is no equivalent to hwclock to force synchronisation and that it is dependent on the user either setting the time manually or running ntpd

-- 
Bryan Christianson
bryan@xxxxxxxxxxxxx




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