Re: [PATCH v3 09/12] Revert "sched: Put rq's sched_avg under CONFIG_FAIR_GROUP_SCHED"
From: Vincent Guittot
Date: Tue Jul 15 2014 - 05:27:50 EST
On 11 July 2014 18:13, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
> In this example using rq->avg leads to imbalance whereas unweighted load
> would not. Correct me if I missed anything.
You just miss to take into account how the imbalance is computed
> Coming back to the previous example. I'm not convinced that inflation of
> the unweighted load sum when tasks overlap in time is a bad thing. I
> have mentioned this before. The average cpu utilization over the 40ms
> period is 50%. However the true compute capacity demand is 200% for the
> first 15ms of the period, 100% for the next 5ms and 0% for the remaining
> 25ms. The cpu is actually overloaded for 15ms every 40ms. This fact is
> factored into the unweighted load whereas rq->avg would give you the
> same utilization no matter if the tasks are overlapped or not. Hence
> unweighted load would give us an indication that the mix of tasks isn't
> optimal even if the cpu has spare cycles.
> If you don't care about overlap and latency, the unweighted sum of task
> running time (that Peter has proposed a number of times) is better
> metric, IMHO. As long the cpu isn't fully utilized.
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