linux-kernel - Re: power-efficient scheduling design

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Message-ID: <CAHkRjk5G7XJxzjcWa4BqRxz0AtAAh_-Eomzk4+dQRaFk1=HQqA@mail.gmail.com>
Date:	Fri, 21 Jun 2013 22:23:25 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arjan van de Ven <arjan@...ux.intel.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 21 June 2013 16:38, Arjan van de Ven <arjan@...ux.intel.com> wrote:
> On 6/21/2013 1:50 AM, Morten Rasmussen wrote:
>> A hint when a task is moved to a new cpu is too late if the migration
>> shouldn't have happened at all. If the scheduler knows that the cpu is
>> able to switch to a higher p-state it can decide to wait for the p-state
>> change instead of migrating the task and waking up another cpu.
>
> 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).

--
Catalin
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