Re: [RFC PATCH v4 00/14] sched: packing small tasks

From: Vincent Guittot
Date: Fri Apr 26 2013 - 11:56:38 EST

On 26 April 2013 17:46, Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> wrote:
>>> so I got to ask the hard question; what percentage of system level (not
>>> just
>>> cpu level)
>>> power consumption gain can you measure (pick your favorite workload)...
>> I haven't system level figures for my patches but only for the cpu
>> subsystem. If we use the MP3 results in the back of my mail, they show
>> an improvement of 37 % (113/178) for the CPU subsystem of the
>> platform. If we assume that the CPU subsystem contributes 25% of an
>> embedded system power consumption (this can vary across platform
>> depending of the use of HW accelerator but it should be a almost fair
>> percentage), the patch can impact the power consumption on up to 9%.
> sadly the math tends to not work quite that easy;
> memory takes significantly more power when the system is not idle than when
> it is idle for example. [*]
> so while reducing cpu power by making it run a bit longer (at lower
> frequency or
> slower core or whatever) is a pure win if you only look at the cpu, but it
> may
> (or may not) be a loss when looking at a whole system level.

I agree. That's why the default packing mode of the patches is trying
to not increase (too much) the scheduling latency of tasks but only to
force tasks to use the same cpu when ever possible in order to
minimize the number CPU/cluster that are used

> I've learned the hard way that you cannot just look at the cpu numbers; you
> must look
> at the whole-system power when playing with such tradeoffs.
> That does not mean that your patch is not useful; it very well can be, but
> without having looked at whole-system power that's a very dangerous
> conclusion to make.
> So.. if you get a chance, I'd love to see data on a whole-system level...
> even for just one workload
> and one system
> (playing mp3 sounds like a quite reasonable workload for such things indeed)

I will try to get such figures

> [*] I assume that on your ARM systems, memory goes into self refresh during
> system idle just as it does on x86
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