Re: [RFC] sched: unused cpu in affine workload

[Rik van Riel]

On Mon, 2016-04-04 at 15:23 +0200, Peter Zijlstra wrote:
> On Mon, Apr 04, 2016 at 11:38:44AM +0200, Ingo Molnar wrote:
> > 
> > We'd upgrade that to O(nr_cpus^2), which is totally unrealistic
> > with 16,000 CPUsÂ
> > even in a slowpath - but it would probably cause problems even with
> > 120 CPUs. ItÂ
> > will get quadratically worse as the number of CPUs in a system
> > increases on itsÂ
> > current exponential trajectory ...
> The arbitrary affinity thing is I think a packing problem, which is
> NP
> hard IIRC.

An optimal solution is NP hard.

Heuristics that "move tasks with pressure" may be
much more doable, and lead to perfectly satisfactory
results, especially if most migrations happen within
a socket (and the same shared L3 cache).

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