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

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Date:	Fri, 26 Apr 2013 08:46:21 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Paul Turner <pjt@...gle.com>,
	Santosh <santosh.shilimkar@...com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Chander Kashyap <chander.kashyap@...aro.org>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <len.brown@...el.com>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Jonathan Corbet <corbet@....net>,
	Lukasz Majewski <l.majewski@...sung.com>
Subject: Re: [RFC PATCH v4 00/14] sched: packing small tasks

>>
>>
>> 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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