Re: [PATCH v2 08/11] sched: get CPU's activity statistic

[Morten Rasmussen]

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.
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