Messages in this thread | | | Date | Tue, 7 Jan 2014 15:37:15 +0000 | From | Morten Rasmussen <> | Subject | Re: [RFC] sched: CPU topology try |
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On Tue, Jan 07, 2014 at 02:11:22PM +0000, Vincent Guittot wrote: > On 7 January 2014 14:22, Peter Zijlstra <peterz@infradead.org> wrote: > > On Tue, Jan 07, 2014 at 09:32:04AM +0100, Vincent Guittot wrote: > >> On 6 January 2014 17:31, Peter Zijlstra <peterz@infradead.org> wrote: > >> > On Mon, Jan 06, 2014 at 02:41:31PM +0100, Vincent Guittot wrote: > >> >> IMHO, these settings will disappear sooner or later, as an example the > >> >> idle/busy _idx are going to be removed by Alex's patch. > >> > > >> > Well I'm still entirely unconvinced by them.. > >> > > >> > removing the cpu_load array makes sense, but I'm starting to doubt the > >> > removal of the _idx things.. I think we want to retain them in some > >> > form, it simply makes sense to look at longer term averages when looking > >> > at larger CPU groups. > >> > > >> > So maybe we can express the things in log_2(group-span) or so, but we > >> > need a working replacement for the cpu_load array. Ideally some > >> > expression involving the blocked load. > >> > >> Using the blocked load can surely give benefit in the load balance > >> because it gives a view of potential load on a core but it still decay > >> with the same speed than runnable load average so it doesn't solve the > >> issue for longer term average. One way is to have a runnable average > >> load with longer time window
The blocked load discussion comes up again :)
I totally agree that blocked load would be useful, but only if we get the priority problem sorted out. Blocked load is the sum of load_contrib of blocked tasks, which means that a tiny high priority task can have a massive contribution to the blocked load.
> > > > Ah, another way of looking at it is that the avg without blocked > > component is a 'now' picture. It is the load we are concerned with right > > now. > > > > The more blocked we add the further out we look; with the obvious limit > > of the entire averaging period. > > > > So the avg that is runnable is right now, t_0; the avg that is runnable + > > blocked is t_0 + p, where p is the avg period over which we expect the > > blocked contribution to appear. > > > > So something like: > > > > avg = runnable + p(i) * blocked; where p(i) \e [0,1] > > > > could maybe be used to replace the cpu_load array and still represent > > the concept of looking at a bigger picture for larger sets. Leaving open > > the details of the map p.
Figuring out p is the difficult bit. AFAIK, with blocked load in its current form we don't have any clue when a task will reappear.
> > That needs to be studied more deeply but that could be a way to have a > larger picture
Agree.
> > Another point is that we are using runnable and blocked load average > which are the sum of load_avg_contrib of tasks but we are not using > the runnable_avg_sum of the cpus which is not the now picture but a > average of the past running time (without taking into account task > weight)
Yes. The rq runnable_avg_sum is an excellent longer term load indicator. It can't be compared with the runnable and blocked load though. The other alternative that I can think of is to introduce an unweighted alternative to blocked load. That is, sum of load_contrib/priority.
Morten
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