[ntp:questions] Linux clock glitch - drift jump on reboot

Michael L. Semon mlsemon33 at verizon.net
Fri Mar 2 03:11:52 UTC 2007

Hal Murray wrote:
> I'm running Fedora Core 6 on a 2.8 GHz P4/Xeon (2 hyperthreaded CPUs).
> I'm running a 2.6.19 kernel - my own config file, no changes to the
> official sources.
> I just updated a whole bunch of software (yum update) and rebooted
> my system.  (same kernel)
> NTP's drift went from 151 ppm to 115 ppm.
> Has anybody seen anything like that?  Any hints on where I should
> start looking?
> I'm pretty sure I saw something like this a while ago, probably in
> the other direction. but I didn't investigate.  I think I have
> enough log files to find it if that is likely to help.  There may
> have been a kernel switch.

This happened to me once.  I think I was using acpi_pm as my clocksource 
in the Linux kernel, then I compiled ACPI out of the Linux kernel and 
rebooted.  Oops.

Each Linux clocksource has its own frequency.  On my PC, it's about 33.2 
PPM for acpi_pm, 35.5 PPM for pit, and 95.3 for tsc.  Whether tsc works 
differently on a 2-CPU system, I don't know.  If ntpd was stable on 151 
PPM, and now it's stable on 115 PPM, maybe that's what happened.


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