On Tue, Jun 03, 2014 at 01:03:54PM +0100, Morten Rasmussen wrote: > On Wed, May 28, 2014 at 05:39:10PM +0100, Vincent Guittot wrote: > > On 28 May 2014 17:47, Morten Rasmussen <morten.rasmus...@arm.com> wrote: > > > On Wed, May 28, 2014 at 02:15:03PM +0100, Vincent Guittot wrote: > > >> On 28 May 2014 14:10, Morten Rasmussen <morten.rasmus...@arm.com> wrote: > > >> > On Fri, May 23, 2014 at 04:53:02PM +0100, Vincent Guittot wrote:
> > > I agree that the task runnable_avg_sum is always affected by the > > > circumstances on the cpu where it is running, and that it takes this > > > history with it. However, I think cfs.runnable_load_avg leads to less > > > problems than using the rq runnable_avg_sum. It would work nicely for > > > the two tasks on two cpus example I mentioned earlier. We don't need add > > > > i would say that nr_running is an even better metrics for such > > situation as the load doesn't give any additional information. > > I fail to understand how nr_running can be used. nr_running doesn't tell > you anything about the utilization of the cpu, just the number tasks > that happen to be runnable at a point in time on a specific cpu. It > might be two small tasks that just happened to be running while you read > nr_running. Agreed, I'm not at all seeing how nr_running is useful here. > An unweighted version of cfs.runnable_load_avg gives you a metric that > captures cpu utilization to some extend, but not the number of tasks. > And it reflects task migrations immediately unlike the rq > runnable_avg_sum. So runnable_avg would be equal to the utilization as long as there's idle time, as soon as we're over-loaded the metric shows how much extra cpu is required. That is, runnable_avg - running_avg >= 0 and the amount is the exact amount of extra cpu required to make all tasks run but not have idle time. > Agreed, but I think it is quite important to discuss what we understand > by cpu utilization. It seems to be different depending on what you want > to use it for. I understand utilization to be however much cpu is actually used, so I would, per the existing naming, call running_avg to be the avg utilization of a task/group/cpu whatever. > We have done experiments internally with rq runnable_avg_sum for > load-balancing decisions in the past and found it unsuitable due to its > slow response to task migrations. That is why I brought it up here. So I'm not entirely seeing that from the code (I've not traced this), afaict we actually update the per-cpu values on migration based on the task values. old_rq->sum -= p->val; new_rq->sum += p->val; like,.. except of course totally obscured.
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