On Tue, Jun 03, 2014 at 04:59:39PM +0100, Peter Zijlstra wrote: > On Tue, Jun 03, 2014 at 01:03:54PM +0100, Morten Rasmussen wrote: > > 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.
Yes, roughly. runnable_avg goes up quite steeply if you have many tasks on a fully utilized cpu, so the actual amount of extra cpu required might be somewhat lower. I can't come up with something better, so I agree. > > > 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. I see your point, but for load balancing purposes we are more intested in the runnable_avg as it tells us about the cpu capacity requirements. I don't like to throw more terms into the mix, but you could call runnable_avg the potential task/group/cpu utilization. This is an estimate of how much utilization a task would cause if we moved it to an idle cpu. That might be quite different from running_avg on an over-utilized cpu. > > > 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. Yes, for cfs.runnable_load_avg, rq->avg.runnable_avg_sum is different. See the other reply. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
