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SubjectRe: CFS review

* Roman Zippel <zippel@linux-m68k.org> wrote:

> [...] e.g. in this example there are three tasks that run only for
> about 1ms every 3ms, but they get far more time than should have
> gotten fairly:
>
> 4544 roman 20 0 1796 520 432 S 32.1 0.4 0:21.08 lt
> 4545 roman 20 0 1796 344 256 R 32.1 0.3 0:21.07 lt
> 4546 roman 20 0 1796 344 256 R 31.7 0.3 0:21.07 lt
> 4547 roman 20 0 1532 272 216 R 3.3 0.2 0:01.94 l

Mike and me have managed to reproduce similarly looking 'top' output,
but it takes some effort: we had to deliberately run a non-TSC
sched_clock(), CONFIG_HZ=100, !CONFIG_NO_HZ and !CONFIG_HIGH_RES_TIMERS.

in that case 'top' accounting symptoms similar to the above are not due
to the scheduler starvation you suspected, but due the effect of a
low-resolution scheduler clock and a tightly coupled timer/scheduler
tick to it. I tried the very same workload on 2.6.22 (with the same
.config) and i saw similarly anomalous 'top' output. (Not only can one
create really anomalous CPU usage, one can completely hide tasks from
'top' output.)

if your test-box has a high-resolution sched_clock() [easily possible]
then please send us the lt.c and l.c code so that we can have a look.

Ingo
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