linux-kernel - Re: [RFC PATCH 0/5] enable runnable load avg in load balance

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Date:	Sun, 18 Nov 2012 16:35:48 +0800
From:	Alex Shi <lkml.alex@...il.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Alex Shi <alex.shi@...el.com>, mingo@...hat.com,
	peterz@...radead.org, pjt@...gle.com, vincent.guittot@...aro.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] enable runnable load avg in load balance

On Sun, Nov 18, 2012 at 3:12 AM, Preeti U Murthy
<preeti@...ux.vnet.ibm.com> wrote:
> Hi Alex,
>
> On 11/17/2012 06:34 PM, Alex Shi wrote:
>> This patchset try to consider runnable load avg when do cpu load comparison
>> in load balance.
>>
>> I had seen preeti's enabling before patch finished, but I still think considing
>> runnable load avg on rq is may a more natrual way.
>>
>> BTW, I am thinking if 2 times decay for cpu_load is too complicate? one for
>> runnable time, another for CPU_LOAD_IDX. I think I missed the decay reason
>> for CPU_LOAD_IDX. Could anyone like do me favor to give some hints of this?
>
> The decay happening for CPU_LOAD_IDX is *more coarse grained* than the
> decay that __update_entity_runnable_avg() is performing.While
> __update_cpu_load() decays the rq->load.weight *for every jiffy*(~4ms)
> passed so far without update of the load,
> __update_entity_runnable_avg() decays the rq->load.weight *for every
> 1ms* when called from update_rq_runnable_avg().
>
> Before the introduction of PJT's series,__update_cpu_load() seems to be
> the only place where decay of older rq load was happening(so as to give
> the older load less importance in its relevance),but with the
> introduction of PJT's series since the older rq load gets decayed in
> __update_entity_runnable_avg() in a more fine grained fashion,perhaps
> you are right,while the CPU_LOAD_IDX gets updated,we dont need to decay
> the load once again here.


If cpu_load is just a coarse decay, we can remove it. but it has
different meaning for busy_idx, forkexec_idx, idle_idx, newidle_idx.
each of them has different degree decay. that is the key part, but I
has no idea of their value come from.

Thanks!
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