Messages in this thread | | | Date | Wed, 4 Jun 2014 11:23:13 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 08/11] sched: get CPU's activity statistic |
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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.
> f you had five tasks on one cpu that each have a 25% requirement you can > get individual task runnable_avgs of up to 100% (cpu unweighted > runnable_load_avg can get up 500%, I think), but the task running_avgs > would be 20% each (total of 100%).
Yeah, more or less so indeed. I had not considered the queueing effects on runnable_avg yesterday, so good that that got raised.
That does indeed invalidate my: runnable - running := extra cpu required thing. It ends up being the extra cpu required for 0 latency but gobs of idle time, which is something else entirely. [unhandled content-type:application/pgp-signature] | |