linux-kernel - Re: power-efficient scheduling design

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Message-ID: <51C4C6C8.1050008@linux.intel.com>
Date:	Fri, 21 Jun 2013 14:34:00 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Catalin Marinas <catalin.marinas@....com>
CC:	Morten Rasmussen <morten.rasmussen@....com>,
	David Lang <david@...g.hm>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"alex.shi@...el.com" <alex.shi@...el.com>,
	"corbet@....net" <corbet@....net>,
	"peterz@...radead.org" <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"efault@....de" <efault@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"pjt@...gle.com" <pjt@...gle.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: power-efficient scheduling design

On 6/21/2013 2:23 PM, Catalin Marinas wrote:
>>
>> oops sorry I misread your mail (lack of early coffee I suppose)
>>
>> I can see your point of having a thing for "did we ask for all the performance
>> we could ask for" prior to doing a load balance (although, for power efficiency,
>> if you have two tasks that could run in parallel, it's usually better to
>> run them in parallel... so likely we should balance anyway)
>
> Not necessarily, especially if parallel running implies powering up a
> full cluster just for one CPU (it depends on the hardware but for
> example a cluster may not be able to go in deeper sleep states unless
> all the CPUs in that cluster are idle).

I guess it depends on the system

the very first cpu needs to power on
* the core itself
* the "cluster" that you mention
* the memory controller
* the memory (out of self refresh)

while the second cpu needs
* the core itself
* maybe a second cluster

normally on Intel systems, the memory power delta is quite significant
which then means the efficiency of the second core is huge compared to
running things in sequence.
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