[Beowulf] Idle core on overloaded CPU?

Carsten Aulbert carsten.aulbert at aei.mpg.de
Fri Oct 24 07:25:09 EDT 2008


Hi, (I send this yesterday already to LKML, so sorry if you receive this
twice),

on Intel X3220 CPU based systems (4 physical cores) I came across the
following thing (Debian etch, with vanilla kernel 2.6.25.9):

starting the following:
$ screen -d -m stress -c 2
$ nice -19 screen -d -m stress -c 4

This causes two cores to be 100% busy in user state and one core to be
busy in 100% nice state. However, the remaining core is idle. Even
watching it over longer stretches of time, the situation remains static.

I guess it's a kernel misconfiguration on my part (config available here:
https://n0.aei.uni-hannover.de/linux/kernel/config-2.6.25.9-nodes
), but if not could this be a scheduling bug?

Thanks for any hint

Cheers

Carsten

PS: In the mean time we have made similar tests on a dual CPU, dual core
opteron machine with the same results (on a different kernel, 2.6.24.x).
Most bizarre was this result (thanks Steffen!):
6 jobs with nice 19, one with nice 0:
Core 1: 100% with nice 0
Core 2: 100% with single nice 19
Core 3: 2*50% (nice 19)
Core 4: 3*33% (nice 19)
That's definitely something I would not expect.
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