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

From: Ani
Date: Sat Sep 05 2009 - 23:18:52 EST

On Sep 5, 3:50 pm, Lucas De Marchi <> wrote:
> 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

Hmm. This does seem to indicate that there is some kind of
relationship with SMP. So I wonder whether there is a way to turn this
'RT bandwidth accumulation' heuristic off. I did an
echo 0 > /proc/sys/kernel/sched_migration_cost
but results were identical to previous.

I figure that if I set it to zero, the regular sched-fair (non-RT)
tasks will be treated as not being cache hot and hence susceptible to
migration. From the code it looks like sched-rt tasks are always
treated as cache cold? Mind you though that I have not yet looked into
the code very rigorously. I knew the O(1) scheduler relatively well,
but I am just begun digging into the new CFS scheduler code.

On a side note, why is there no documentation explain the
sched_migration_cost tuning knob? It would be nice to have one - at
least where the sysctl variable is defined.

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