[LinuxPPS] ntptime status - relaunch of that issue

[Michael Meier]

> I played around with different hardware and realised, that then faster a
> machine is, then less the offset / jitter is.
> For example on an amd64 X2 5000+ jitter/offset is about 0,100 - 0,010
> +/- while with the Pentium M processor the offset/jitter rises up far
> beyond 1,000...
> I think, the Pentium M (1700MHz) should be way enough for a ntp server?

The CPU Power surely is enough, but you might hit an architectural 
problem with the Pentium M: It has a timestamp counter [1] that does NOT 
increase monotonically, even when all power saving features are 
disabled. Since the TSC is the default and preferred timesource of the 
kernel, that will cause some confusion. The kernel will detect that the 
TSC is crap after some time, and switch to another source after some 
time if it has some available, but the TSC really is the best you can 
get. The only thing that comes close to it is HPET (High Precision Event 
Timer), and that is not available everywhere. And naturally, if ntpd 
cannot get the local time from the kernel with a sufficiently high 
resolution, things will start to jitter.
You should check what your kernel currently uses as timesource (cat 
/sys/devices/system/clocksource/clocksource0/current_clocksource), and 
what sources it has available 
(/sys/devices/system/clocksource/clocksource0/current_clocksource).
You can also check in the kernel logs, the kernel usually tells you when 
it changes the timesource and why.

[1] http://en.wikipedia.org/wiki/Time_Stamp_Counter
-- 
Michael Meier, HPC Services
Friedrich-Alexander-Universitaet Erlangen-Nuernberg
Regionales Rechenzentrum Erlangen
Martensstrasse 1, 91058 Erlangen, Germany
Tel.: +49 9131 85-28973, Fax: +49 9131 302941
michael.meier at rrze.uni-erlangen.de
www.rrze.uni-erlangen.de/hpc/