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Subject2.6.1 "clock preempt"?

i've started to test the 2.6 series of kernels and observed a
strange thing: with moderate background load, the system clock
(i.e. time) seems to slow down to about 60% of normal speed
and the normally reliable ntp process (v4.2.0)

details: working interactively with a couple of background dummy
processes (*1), my system clock slowed down approx 90 mins over
a period of approx 4 hrs real time.

(*1) infinite loop: 1 rip, 1 encode -- both run at nice 10

the kernel logs show messages on the form:

localhost kernel: Losing too many ticks!
localhost kernel: TSC cannot be used as a timesource.
(Are you running with SpeedStep?)
localhost kernel: Falling back to a sane timesource.
localhost kernel: set_rtc_mmss: can't update from 5 to 58

without the background load, the system keeps perfect time.

==> any ideas of what could be going on would be appreciated.

hardware details
Intel P4 2.53gz on Supermicro P4SAA mobo (Intel 7205 chipset)
1gb memory; multiple drives; Promise Ultra/133 raid controller
kernel 2.6.1 w/APIC and PREEMPT options turned on.
Fedora Core 1 + selected development packages (verified on 2.4).

more info available on request.

Steinar Hauan, dept of ChemE --
Carnegie Mellon University, Pittsburgh PA, USA

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