[ntp:questions] Slow convergence of NTP with GPS/PPS

Unruh unruh-spam at physics.ubc.ca
Tue Oct 21 22:54:29 UTC 2008

hal-usenet at ip-64-139-1-69.sjc.megapath.net (Hal Murray) writes:

>>> The driftfile also sometimes seems to do more harm than good - especially
>>> after a reboot.
>>Some kernels do a calibration of clock against RTC clock. This will make
>>driftfile misleading.

>There is a bug in the Linux calibration routine for the TSC mode
>clock.  It doesn't get a consistent answer.  I hacked the code
>to loop and print the answer.  It was a splatter.  None were far
>off unless you are a time keeping geek.  It's easy to see the
>different drift results and startup transients are "interesting".

>clocksource=acpi_pm on the boot command line might help.

The recent kernels, especially if you have HPET enabled-- not used, just
enabled, are a complete disaster as far as the rtc is concerned. They poll
the rtc with something like a 16ms poll interval, since the second
transition interrupt is then grabbed by the HPET bios routing and not
delivered anywhere. This does not affect the system clock, but does affect
the rtc reading and setting. In fact at present, the rtc on a linux system
is essentially useless for any kind of time keeping or time setting. 

>These are my opinions, not necessarily my employer's.  I hate spam.

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