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

[Maxime Coquelin]

Dear Ingo,

On 01/30/2012 02:53 PM, Ingo Molnar wrote:
> * Maxime Coquelin<maxime.coquelin@xxxxxxxxxxxxxx>  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 
governor.
In the case of a mobile phone, we can imagine dropping the cache when 
system's
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 
segments.
This is the lower granularity for Partial Array Self-refresh.

Thanks for your comments,
Maxime
> Thanks,
>
> 	Ingo

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