linux-kernel - Re: [RFC PATCH v2 00/15][Sorted-buddy] mm: Memory Power Management

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Message-ID: <51A593F1.5090200@linux.vnet.ibm.com>
Date:	Wed, 29 May 2013 11:06:49 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Phillip Susi <psusi@...ntu.com>
CC:	Simon Jeons <simon.jeons@...il.com>, akpm@...ux-foundation.org,
	mgorman@...e.de, matthew.garrett@...ula.com, dave@...1.net,
	rientjes@...gle.com, riel@...hat.com, arjan@...ux.intel.com,
	srinivas.pandruvada@...ux.intel.com,
	maxime.coquelin@...ricsson.com, loic.pallardy@...ricsson.com,
	kamezawa.hiroyu@...fujitsu.com, lenb@...nel.org, rjw@...k.pl,
	gargankita@...il.com, paulmck@...ux.vnet.ibm.com,
	amit.kachhap@...aro.org, svaidy@...ux.vnet.ibm.com,
	andi@...stfloor.org, wujianguo@...wei.com, kmpark@...radead.org,
	thomas.abraham@...aro.org, santosh.shilimkar@...com,
	linux-pm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 00/15][Sorted-buddy] mm: Memory Power Management

On 05/29/2013 01:38 AM, Phillip Susi wrote:
> 
> On 4/19/2013 3:12 AM, Srivatsa S. Bhat wrote:
>> But going further, as I had mentioned in my TODO list, we can be
>> smarter than this while doing compaction to evacuate memory regions
>> - we can choose to migrate only the active pages, and leave the
>> inactive pages alone. Because, the goal is to actually consolidate
>> the *references* and not necessarily the *allocations* themselves.
> 
> That would help with keeping references compact to allow use of the
> low power states, but it would also be nice to keep allocations
> compact, and completely power off a bank of ram with no allocations.
> 

That is a very good point, thanks! But one of the differences we have to
keep in mind is that powering off a bank requires intervention from the
OS (ie., OS should initiate the power-off, because we lose the contents
on power-off) whereas going to lower power states can be mostly done
automatically by the hardware (because it is content-preserving).

But powering-off unused banks of RAM (using techniques such as PASR -
Partial Array Self Refresh) can give us more power-savings than just
entering lower power states. So yes, keeping allocations consolidated
has that additional advantage. And the sorted-buddy design of the page
allocator helps us achieve that.

Thanks a lot for your inputs, Phillip!

Regards,
Srivatsa S. Bhat

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