On Tue, Dec 15, 2015 at 05:43:44AM +0100, Vincent Guittot wrote:

On 14 December 2015 at 17:51, Peter Zijlstra [off-list ref] wrote:

quoted

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
frames).

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 ;-)