Re: question on sched-rt group allocation cap: sched_rt_runtime_us

From: Anirban Sinha
Date: Sat Sep 05 2009 - 20:47:51 EST

> You say you pin the threads to a single core: how many cores does your
> system have?

The results I sent you were on a dual core blade.

> If this is the case, this behavior is the expected one, the scheduler
> tries to reduce the number of migrations, concentrating the bandwidth
> of rt tasks on a single core. With your workload it doesn't work well
> because runtime migration has freed the other core(s) from rt bandwidth,
> so these cores are available to SCHED_OTHER ones, but your SCHED_OTHER
> thread is pinned and cannot make use of them.

But, I ran the same routine on a quadcore blade and the results this time were:

rt_runtime/rt_period % of iterations of reg thrd against rt thrd

0.20 46%
0.25 18%
0.26 7%
0.3 0%
0.4 0%
(rest of the cases) 0%

So if the scheduler is concentrating all rt bandwidth to one core, it should be effectively 0.2 * 4 = 0.8 for this core. Hence, we should see the percentage closer to 20% but it seems that it's more than double. At ~0.25, the regular thread should make no progress, but it seems it does make a little progress.
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