[time-nuts] Linux timekeeping / jiffy source

[Mike S]

Anyone familiar with Linux kernel timekeeping?

I've recently upgraded a server to an AMD 890FX/SB850 based 
motherboard. After doing so, I observed a large (in time-nut terms) 
inconsistency in system timing, as seen in the rate of the system's 
Time Of Day clock.

Sync'ing to a local GPS locked NTP server, I see up to an 80 ppm spread 
between reboots, which I've documented at 
http://www.flatsurface.com/AMD_SB850/index.html . I'm running kernel 
2.6.32 (Debian squeeze).

I think that kernel timekeeping ("jiffies") are linked to the "8254" 
timer in the SB850 south bridge, but maybe it's the HPET in the 890FX 
north bridge. Anyone know how to tell which the kernel is using for 
timekeeping?

Also, is it possible to restart the Linux kernel without a full reboot 
(avoiding BIOS initialization), to see if it's a kernel or BIOS issue? 
I don't believe a simple change of runlevel restarts the kernel from 
scratch.

I haven't seen this inconsistency on previous Intel or Serverworks 
based motherboards, but I've seen this behavior on 890FX/SB850 
motherboards from two different manufacturers (although I think both 
use Award BIOS).