linux-kernel - Re: question on sched-rt group allocation cap: sched_rt_runtime

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Message-ID: <193b0f820909051540v1ad63c24xc822dbdef5bd0c1c@mail.gmail.com>
Date:	Sun, 6 Sep 2009 00:40:29 +0200
From:	Lucas De Marchi <lucas.de.marchi@...il.com>
To:	Fabio Checconi <fchecconi@...il.com>
Cc:	Anirban Sinha <ASinha@...gmasystems.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: question on sched-rt group allocation cap: sched_rt_runtime_us

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

Indeed. I've tested this same test program in a single core machine and it
produces the expected behavior:

rt_runtime_us / rt_period_us     % loops executed in SCHED_OTHER
95%                              4.48%
60%                              54.84%
50%                              86.03%
40%                              OTHER completed first


Lucas De Marchi
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