Re: [RFC] sched: CPU topology try

[Vincent Guittot]

On 7 January 2014 14:22, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Jan 07, 2014 at 09:32:04AM +0100, Vincent Guittot wrote:
>> On 6 January 2014 17:31, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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
>
> 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.

That needs to be studied more deeply but that could be a way to have a
larger picture

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)

Vincent
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