linux-kernel - Re: [RFC PATCH 2/3] sched: power aware load balance,

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Date:	Mon, 12 Nov 2012 11:05:20 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Preeti Murthy <preeti.lkml@...il.com>
CC:	rob@...dley.net, mingo@...hat.com, peterz@...radead.org,
	suresh.b.siddha@...el.com, arjan@...ux.intel.com,
	vincent.guittot@...aro.org, tglx@...utronix.de,
	gregkh@...uxfoundation.org, andre.przywara@....com, rjw@...k.pl,
	paul.gortmaker@...driver.com, akpm@...ux-foundation.org,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	cl@...ux.com, pjt@...gle.com,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 2/3] sched: power aware load balance,

On 11/12/2012 02:49 AM, Preeti Murthy wrote:
> Hi Alex
> I apologise for the delay in replying .

That's all right. I often also busy on other Intel tasks and have no
time to look at LKML. :)
> 
> On Wed, Nov 7, 2012 at 6:57 PM, Alex Shi <alex.shi@...el.com> wrote:
>> On 11/07/2012 12:37 PM, Preeti Murthy wrote:
>>> Hi Alex,
>>>
>>> What I am concerned about in this patchset as Peter also
>>> mentioned in the previous discussion of your approach
>>> (https://lkml.org/lkml/2012/8/13/139)
>>> is that:
>>>
>>> 1.Using nr_running of two different sched groups to decide which one
>>> can be group_leader or group_min might not be be the right approach,
>>> as this might mislead us to think that a group running one task is less
>>> loaded than the group running three tasks although the former task is
>>> a cpu hogger.
>>>
>>> 2.Comparing the number of cpus with the number of tasks running in a sched
>>> group to decide if the group is underloaded or overloaded again faces
>>> the same issue.The tasks might be short running,not utilizing cpu much.
>>
>> Yes, maybe nr task is not the best indicator. But as first step, it can
>> approve the proposal is a correct path and worth to try more.
>> Considering the old powersaving implement is also judge on nr tasks, and
>> my testing result of this. It may be still a option.
> Hmm.. will think about this and get back.
>>>
>>> I also feel before we introduce another side to the scheduler called
>>> 'power aware',why not try and see if the current scheduler itself can
>>> perform better? We have an opportunity in terms of PJT's patches which
>>> can help scheduler make more realistic decisions in load balance.Also
>>> since PJT's metric is a statistical one,I believe we could vary it to
>>> allow scheduler to do more rigorous or less rigorous power savings.
>>
>> will study the PJT's approach.
>> Actually, current patch set is also a kind of load balance modification,
>> right? :)
> It is true that this is a different approach,in fact we will require
> this approach
> to do power savings because PJT's patches introduce a new 'metric' and not a new
> 'approach' in my opinion, to do smarter load balancing,not power aware
> load balancing per say.So your patch is surely a step towards power
> aware lb.I am just worried about the metric used in it.
>>>
>>> It is true however that this approach will not try and evacuate nearly idle
>>> cpus over to nearly full cpus.That is definitely one of the benefits of your
>>> patch,in terms of power savings,but I believe your patch is not making use
>>> of the right metric to decide that.
>>
>> If one sched group just has one task, and another group just has one
>> LCPU idle, my patch definitely will pull the task to the nearly full
>> sched group. So I didn't understand what you mean 'will not try and
>> evacuate nearly idle cpus over to nearly full cpus'
> No, by 'this approach' I meant the current load balancer integrated with
> the PJT's metric.Your approach does 'evacuate' the nearly idle cpus
> over to the nearly full cpus..

Oh, a misunderstand on 'this approach'. :) Anyway, we are all clear
about this now.

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