linux-kernel - Re: [Results] [RFC PATCH v4 00/40] mm: Memory Power Management

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Date:	Thu, 26 Sep 2013 11:06:36 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>, mgorman@...e.de,
	dave@...1.net, hannes@...xchg.org, tony.luck@...el.com,
	matthew.garrett@...ula.com, riel@...hat.com,
	srinivas.pandruvada@...ux.intel.com, willy@...ux.intel.com,
	kamezawa.hiroyu@...fujitsu.com, lenb@...nel.org, rjw@...k.pl,
	gargankita@...il.com, paulmck@...ux.vnet.ibm.com,
	svaidy@...ux.vnet.ibm.com, isimatu.yasuaki@...fujitsu.com,
	santosh.shilimkar@...com, kosaki.motohiro@...il.com,
	linux-pm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, maxime.coquelin@...ricsson.com,
	loic.pallardy@...ricsson.com, thomas.abraham@...aro.org,
	amit.kachhap@...aro.org
Subject: Re: [Results] [RFC PATCH v4 00/40] mm: Memory Power Management

>>>>
>>>
>>> Arjan, are you referring to the fact that Intel/SNB systems can exploit
>>> memory self-refresh only when the entire system goes idle? Is that why
>>> this
>>> patchset won't turn out to be that useful on those platforms?
>>
>> no we can use other things (CKE and co) all the time.
>>
>
> Ah, ok..
>
>> just that we found that statistical grouping gave 95%+ of the benefit,
>> without the cost of being aggressive on going to a 100.00% grouping
>>
>
> And how do you do that statistical grouping? Don't you need patches similar
> to those in this patchset? Or are you saying that the existing vanilla
> kernel itself does statistical grouping somehow?

so the way I scanned your patchset.. half of it is about grouping,
the other half (roughly) is about moving stuff.

the grouping makes total sense to me.
actively moving is the part that I am very worried about; that part burns power to do
(and performance).... for which the ROI is somewhat unclear to me
(but... data speaks. I can easily be convinced with data that proves one way or the other)

is moving stuff around the 95%-of-the-work-for-the-last-5%-of-the-theoretical-gain
or is statistical grouping enough to get > 95% of the gain... without the cost of moving.


>
> Also, I didn't fully understand how NUMA policy will help in this case..
> If you want to group memory allocations/references into fewer memory regions
> _within_ a node, will NUMA policy really help? For example, in this patchset,
> everything (all the allocation/reference shaping) is done _within_ the
> NUMA boundary, assuming that the memory regions are subsets of a NUMA node.
>
> Regards,
> Srivatsa S. Bhat
>

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