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From: Ingo Molnar <mingo@elte.hu>
To: Maxime Coquelin <maxime.coquelin@stericsson.com>
Cc: linux-mm@kvack.org, linaro-mm-sig@lists.linaro.org,
	Mel Gorman <mel@csn.ul.ie>, Ankita Garg <ankita@in.ibm.com>,
	linux-kernel@vger.kernel.org, linus.walleij@stericsson.com,
	andrea.gallo@stericsson.com, vincent.guittot@stericsson.com,
	philippe.langlais@stericsson.com, loic.pallardy@stericsson.com,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [RFCv1 0/6] PASR: Partial Array Self-Refresh Framework
Date: Mon, 30 Jan 2012 14:53:41 +0100	[thread overview]
Message-ID: <20120130135341.GA3720@elte.hu> (raw)
In-Reply-To: <1327930436-10263-1-git-send-email-maxime.coquelin@stericsson.com>


* Maxime Coquelin <maxime.coquelin@stericsson.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

  parent reply	other threads:[~2012-01-30 13:54 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-30 13:33 [RFCv1 0/6] PASR: Partial Array Self-Refresh Framework Maxime Coquelin
2012-01-30 13:33 ` [RFCv1 1/6] PASR: Initialize DDR layout Maxime Coquelin
2012-01-30 13:33 ` [RFCv1 2/6] PASR: Add core Framework Maxime Coquelin
2012-01-30 13:33 ` [RFCv1 3/6] PASR: mm: Integrate PASR in Buddy allocator Maxime Coquelin
2012-01-30 15:22   ` Mel Gorman
2012-01-30 16:52     ` Maxime Coquelin
2012-01-31 12:15       ` Maxime Coquelin
2012-01-31 14:01         ` Mel Gorman
2012-01-31 18:55           ` Maxime Coquelin
2012-02-03 11:44             ` Mel Gorman
2012-01-31 12:46       ` Pekka Enberg
2012-01-30 13:33 ` [RFCv1 4/6] PASR: Call PASR initialization Maxime Coquelin
2012-01-30 13:33 ` [RFCv1 5/6] PASR: Add Documentation Maxime Coquelin
2012-02-02  3:51   ` Randy Dunlap
2012-02-02 17:12     ` Maxime Coquelin
2012-01-30 13:33 ` [RFCv1 6/6] PASR: Ux500: Add PASR support Maxime Coquelin
2012-01-30 13:53 ` Ingo Molnar [this message]
2012-01-30 14:19   ` [RFCv1 0/6] PASR: Partial Array Self-Refresh Framework Maxime Coquelin
2012-01-31 12:39     ` Ingo Molnar
2012-01-31 14:48       ` Maxime Coquelin

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