Kernel PPS processing
hmurray at megapathdsl.net
Thu Jun 30 05:21:41 UTC 2016
> Local clock frequency offset, as opposed to local clock time offset.
Most NTP documentation calls that drift. Its magnitude is not very
interesting when discussing quality of time. Changes over time can be
interesting. It's usually much more interesting to look at the clock offset.
There are two sources for drift. One is crystal error. That part often
makes a good thermometer.
The other is software. If somebody gets the arithmetic a bit wrong, ntpd can
correct just like it does for the initial hardware error.
For many years, Linux had a not-good measurement of the system clock
frequency at boot time. If you rebooted, you got a different answer. It was
close, just not good enough in the low bits if you wanted good timekeeping.
Jun 2 10:34:25 fed kernel: tsc: Detected 1596.750 MHz processor
Jun 9 11:06:24 fed kernel: tsc: Detected 1596.966 MHz processor
Jun 19 11:42:22 fed kernel: tsc: Detected 1596.978 MHz processor
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