Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity

From: Peter Zijlstra
Date: Tue Dec 15 2015 - 07:41:54 EST

On Tue, Dec 15, 2015 at 05:43:44AM +0100, Vincent Guittot wrote:
> On 14 December 2015 at 17:51, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> > No, since the WCET can and _will_ happen, its the best you can do with
> > cpufreq. If you were to set it lower you could not be able to execute
> > correctly in your 'never' tail cases.
> In the context of frequency scaling, This mean that we will never
> reach low frequency

Only if you've stuffed your machine full of deadline tasks, if you take
Luca's example of the I/B frame decoder thingy, then even the WCET for
the I frames should not be very much (albeit significantly more than B

So while the WCET is pessimistic compared to the avg case, most CPUs can
do video decoding without much effort at all, so even the WCET for the
I-frames might allow us to drop to the lowest cpufreq.

Now, if you were to decode 10 streams at the same time, different story
of course ;-)
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