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SubjectRe: CFS review
Hi,

On Wed, 1 Aug 2007, Ingo Molnar 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.

I used my old laptop for these tests, where tsc is indeed disabled due to
instability. Otherwise the kernel was configured with CONFIG_HZ=1000.

> 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.

Well, it magnifies the rounding problems in CFS.
I mainly wanted to test a little the behaviour of CFS and I thought a saw
patch which enabled the use of TSC in these cases, so I didn't check
sched_clock().

Anyway, I want to point out that this wasn't the main focus of what I
wrote.

bye, Roman
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