[ntp:questions] NTP server with Garmin GPS 18 LVC

unruh unruh at invalid.ca
Sat Dec 21 02:25:49 UTC 2013

On 2013-12-20, Adrian P <ntp at pascalau.ro> wrote:
> On 19 December 2013 01:16, unruh <unruh at invalid.ca> wrote:
>> So it repeats approx every 1.5  hours, quite regularly (slightly longer
>> period at night than during the day). Are you doing
>> something on the machine every two hours? 9 minutes is a long time (
>> over 30 time periods) so disk activity or other interrupt delays does
>> not seem a reasonable explanation. I really do not have any idea why
>> there would be this extremely regular long term delay
> Nope, this is a fresh FreeBSD 9.2 installation. Just the base system
> and the kernel were installed. I also compiled a new kernel with PPS
> support. Only ntpd is running, and of course sshd for remote
> connection.
> In the end what is this clock offset? I suppose it is the difference
> between my laptop's internal clock and the precise timing provided by
> the GPS's PPS signal, right? When the laptop's internal clock remains
> behind, I have a negative clock offset, and when the laptop's internal
> clock rushes ahead I have a positive clock offset.... NTP will always
> tries to align the internal clock of the laptop with the precise
> timing received from the GPS's PPS signal.

I am never sure of the sign-- is a positive offset the computer clock
ahead of the time as delivered by ntp from the time source, or the time
source ahead of the computer clock. 

Heating could be a possibility, but what you have is a very sudden rate
change by to order of 1PPM. Ie, the fan could come on for a few minutes
per hour and suddenly cool the cpu changing its rate by 1PPM or so, and
then switch off for anothr hour or so. Is that what it does? does the
fan come on for a few minutes per hour about every hour and a half?
Also, the positive spike afterwards would then seem to indicate your
clock warming up again. Seems unlikely. But look at the clock rate graph
for the same period.

> Just wondering, could it be that the heating of the laptop causes
> these spikes? The laptop heats itself, then the fans start working for
> a while, until the temperature goes down. Then the laptop starts to
> heat again, and so on...

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