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authorPeter Zijlstra <a.p.zijlstra@chello.nl>2008-10-17 19:27:04 +0200
committerIngo Molnar <mingo@elte.hu>2008-10-20 14:05:04 +0200
commitf9c0b0950d5fd8c8c5af39bc061f27ea8fddcac3 (patch)
tree288537eac8d5e03970422ac0c705617e551f544d /kernel/sched_features.h
parenta4c2f00f5cb848af7a8c816426b413c8e41834df (diff)
sched: revert back to per-rq vruntime
Vatsa rightly points out that having the runqueue weight in the vruntime calculations can cause unfairness in the face of task joins/leaves. Suppose: dv = dt * rw / w Then take 10 tasks t_n, each of similar weight. If the first will run 1 then its vruntime will increase by 10. Now, if the next 8 tasks leave after having run their 1, then the last task will get a vruntime increase of 2 after having run 1. Which will leave us with 2 tasks of equal weight and equal runtime, of which one will not be scheduled for 8/2=4 units of time. Ergo, we cannot do that and must use: dv = dt / w. This means we cannot have a global vruntime based on effective priority, but must instead go back to the vruntime per rq model we started out with. This patch was lightly tested by doing starting while loops on each nice level and observing their execution time, and a simple group scenario of 1:2:3 pinned to a single cpu. Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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