[LinuxPPS] Re: comfortable switch of clocksource ?

[Bernhard Schiffner]

Am Freitag, 8. Dezember 2006 05:11 schrieb Udo van den Heuvel:
> Bernhard Schiffner wrote:
> > Yes, it is!
> > (found in kernel/time/clocksource.c)
> >
> > root at bs4:~#
> > cat /sys/devices/system/clocksource/clocksource0/current_clocksource
> > acpi_pm
>
> [cut]
>
> Which clocksource out of acpi_pm jiffies tsc pit is the best?
>
> Udo

See
linux/include/linux/clocksource.h
rating
(lines 27 ff.)

The rating depends of the diver-authors estimation.
It can be influenced: see "TSC marked as unstable" while booting.
The kernel tries to use the highest rated clocksource.

See also
kernel/time/clocksource.c
void clocksource_reselect(void)
(line 179 ff.)

My personal favorite in a pps-environment is TSC.
It offers the highest resolution (GHz!) but can have some problems with SMP 
and / or while speedstepping.
It's not driven by a GHz-cristal but multiplied MHz, that means some Jitter is 
possible.
Ultmost source seems a TSC driven by atomic clock (they are GHz!), combined 
with pps controled system-start.
Modern processors count TSC "normal" while other cpu-units can reduce there 
speed on demand.

If I'am right, these times TSC is not automatically selected, because of some 
ACPI C2 entries (not their _use_ yet).

Bernhard