Messages in this thread | | | Date | Wed, 4 Jun 2014 11:25:59 +0100 | From | Morten Rasmussen <> | Subject | Re: [PATCH v2 08/11] sched: get CPU's activity statistic |
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On Wed, Jun 04, 2014 at 10:35:15AM +0100, Vincent Guittot wrote: > On 4 June 2014 11:23, Peter Zijlstra <peterz@infradead.org> wrote: > > On Wed, Jun 04, 2014 at 09:55:42AM +0100, Morten Rasmussen wrote: > >> Both running_avg and runnable_avg are affected by other tasks on the > >> same cpus, but in different ways. They are equal if you only have one > >> task on a cpu. If you have more, running_avg will give you the true > >> requirement of the tasks until the cpu is fully utilized. At which point > >> the task running_avg will drop if you add more tasks (the unweighted sum > >> of task running_avgs remains constant). > >> > >> runnable_avg on the other hand, might be affected as soon as you have > >> two task running on the same cpu if they are runnable at the same time. > >> That isn't necessarily a bad thing for load-balancing purposes, because > >> tasks that are runnable at the same time are likely to be run more > >> efficiently by placing them on different cpus. You might view as at sort > >> of built in concurrency factor, somewhat similar to what Yuyang is > >> proposing. runnable_avg increases rapidly when the cpu is over-utilized. > > > > Agreed. > > > >> > I'm not sure I see how 100% is possible, but yes I agree that runnable > >> > can indeed be inflated due to this queueing effect. > >> > >> You should only be able to get to 75% worst case for runnable_avg for > >> that example. The total running_avg is 50% no matter if the tasks > >> overlaps or not. > > > > Yes, 75% is what I ended up with. > > Can you explain how you reach 75% as it depends on the runtime and a > runtime longer than 345ms will end to a 100% load whatever the > idletime was previously ?
If the busy period of each task is long enough that the first one to run runs to completetion before the other task is scheduled you get 25% and 50%. But as I said in my other reply, you can get higher task runnable if the tasks busy period is long enough that you switch between them before the first one completes.
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