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

From: Arjan van de Ven
Date: Fri Apr 26 2013 - 11:46:35 EST

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'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 assume that on your ARM systems, memory goes into self refresh during system idle just as it does on x86

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