linux-kernel - Re: [RFCv1 0/6] PASR: Partial Array Self-Refresh Framework

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Date:	Mon, 30 Jan 2012 14:53:41 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Maxime Coquelin <maxime.coquelin@...ricsson.com>
Cc:	linux-mm@...ck.org, linaro-mm-sig@...ts.linaro.org,
	Mel Gorman <mel@....ul.ie>, Ankita Garg <ankita@...ibm.com>,
	linux-kernel@...r.kernel.org, linus.walleij@...ricsson.com,
	andrea.gallo@...ricsson.com, vincent.guittot@...ricsson.com,
	philippe.langlais@...ricsson.com, loic.pallardy@...ricsson.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFCv1 0/6] PASR: Partial Array Self-Refresh Framework

* Maxime Coquelin <maxime.coquelin@...ricsson.com> wrote:

> The role of this framework is to stop the refresh of unused 
> memory to enhance DDR power consumption.

I'm wondering in what scenarios this is useful, and how 
consistently it is useful.

The primary concern I can see is that on most Linux systems with 
an uptime more than a couple of minutes RAM gets used up by the 
Linux page-cache:

 $ uptime
  14:46:39 up 11 days,  2:04, 19 users,  load average: 0.11, 0.29, 0.80
 $ free
              total       used       free     shared    buffers     cached
 Mem:      12255096   12030152     224944          0     651560    6000452
 -/+ buffers/cache:    5378140    6876956

Even mobile phones easily have days of uptime - quite often 
weeks of uptime. I'd expect the page-cache to fill up RAM on 
such systems.

So how will this actually end up saving power consistently? Does 
it have to be combined with a VM policy that more aggressively 
flushes cached pages from the page-cache?

A secondary concern is fragmentation: right now we fragment 
memory rather significantly. For the Ux500 PASR driver you've 
implemented the section size is 64 MB. Do I interpret the code 
correctly in that a continuous, 64MB physical block of RAM has 
to be 100% free for us to be able to turn off refresh and power 
for this block of RAM?

Thanks,

	Ingo
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