|| ||Maxime Coquelin <maxime.coquelin-AT-stericsson.com> |
|| ||Ingo Molnar <mingo-AT-elte.hu> |
|| ||Re: [RFCv1 0/6] PASR: Partial Array Self-Refresh Framework |
|| ||Mon, 30 Jan 2012 15:19:45 +0100|
|| ||"linux-mm-AT-kvack.org" <linux-mm-AT-kvack.org>,
Mel Gorman <mel-AT-csn.ul.ie>,
Linus WALLEIJ <linus.walleij-AT-stericsson.com>,
Andrea GALLO <andrea.gallo-AT-stericsson.com>,
Vincent GUITTOT <vincent.guittot-AT-stericsson.com>,
Philippe LANGLAIS <philippe.langlais-AT-stericsson.com>,
Loic PALLARDY <loic.pallardy-AT-stericsson.com>,
Andrew Morton <akpm-AT-linux-foundation.org>,
Linus Torvalds <torvalds-AT-linux-foundation.org>,
Thomas Gleixner <tglx-AT-linutronix.de>,
Peter Zijlstra <a.p.zijlstra-AT-chello.nl>|
|| ||Article, Thread
On 01/30/2012 02:53 PM, Ingo Molnar wrote:
> * Maxime Coquelin<firstname.lastname@example.org> 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?
You're right Ingo, page-cache fills up the RAM.
This framework is to be used in combination with a page-cache flush
In the case of a mobile phone, we can imagine dropping the cache when
screen is off for a while, in order to preserve user's experience.
> A secondary concern is fragmentation: right now we fragment
> memory rather significantly.
Yes, I think fragmentation is the main challenge.
This is the same problem faced for Memory Hotplug feature.
The solution I see is to add a significant Movable zone in the system and
use the Compaction feature from Mel Gorman.
The problem of course remains for the Normal zone.
> 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?
Current DDR (2Gb/4Gb dies) used in mobile platform have 64MB banks and
This is the lower granularity for Partial Array Self-refresh.
Thanks for your comments,
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