Messages in this thread | | | Date | Fri, 26 Apr 2013 16:02:27 +0530 | From | Preeti U Murthy <> | Subject | Re: [RFC PATCH v3 3/6] sched: pack small tasks |
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Hi Peter,
On 04/26/2013 03:48 PM, Peter Zijlstra wrote: > On Wed, Mar 27, 2013 at 03:51:51PM +0530, Preeti U Murthy wrote: >> Hi, >> >> On 03/26/2013 05:56 PM, Peter Zijlstra wrote: >>> On Fri, 2013-03-22 at 13:25 +0100, Vincent Guittot wrote: >>>> +static bool is_buddy_busy(int cpu) >>>> +{ >>>> + struct rq *rq = cpu_rq(cpu); >>>> + >>>> + /* >>>> + * A busy buddy is a CPU with a high load or a small load with >>>> a lot of >>>> + * running tasks. >>>> + */ >>>> + return (rq->avg.runnable_avg_sum > >>>> + (rq->avg.runnable_avg_period / (rq->nr_running >>>> + 2))); >>>> +} >>> >>> Why does the comment talk about load but we don't see it in the >>> equation. Also, why does nr_running matter at all? I thought we'd >>> simply bother with utilization, if fully utilized we're done etc.. >>> >> >> Peter, lets say the run-queue has 50% utilization and is running 2 >> tasks. And we wish to find out if it is busy. We would compare this >> metric with the cpu power, which lets say is 100. >> >> rq->util * 100 < cpu_of(rq)->power. >> >> In the above scenario would we declare the cpu _not_busy? Or would we do >> the following: >> >> (rq->util * 100) * #nr_running < cpu_of(rq)->power and conclude that it >> is just enough _busy_ to not take on more processes? > > That is just confused... ->power doesn't have anything to do with a per-cpu > measure. ->power is a inter-cpu measure of relative compute capacity.
Ok.
> > Mixing in nr_running confuses things even more; it doesn't matter how many > tasks it takes to push utilization up to 100%; once its there the cpu simply > cannot run more.
True, this is from the perspective of the CPU. But will not the tasks on this CPU get throttled if, you find the utilization of this CPU < 100% and decide to put more tasks on it?
Regards Preeti U Murthy
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