Re: [RFCv1 0/6] PASR: Partial Array Self-Refresh Framework
From: Maxime Coquelin
Date: Tue Jan 31 2012 - 09:49:25 EST
On 01/31/2012 01:39 PM, Ingo Molnar wrote:
* Maxime Coquelin<maxime.coquelin@xxxxxxxxxxxxxx> wrote:
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.
Is this "page-cache flush governor" some existing code?
How does it work and does it need upstream patches?
For now, such a governor has not been implemented.
I use the dedicated ProcFS interface to test the framework (echo 3 >
/proc/sys/vm/drop_caches).
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.
Ok. I guess phones/appliances can generally live with a
relatively large movable zone as they don't have serious
memory pressure issues.
Actually, current high-end smartphones and tablets have 1GB DDR.
Smartphones and tablets arriving later this year should have up to 2GB DDR.
For example, my Android phone running for 2 days has only 230MB are used
in idle once the page-caches dropped.
So I think having a 1GB movable zone on a 2GB DDR phone is conceivable.
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.
Ok, so do you see real, consistent power savings with a large
movable zone, with page cache governor patches applied (assuming
it's a kernel mechanism) and CONFIG_COMPACTION=y enabled, on an
upstream kernel with all these patches applied?
I don't have consistent figures for now as it is being prototyped.
From the DDR datasheet I gathered, the DDR power savings is about 33%
when half of the die is in self-refresh, compared to the full die in
self-refresh.
Thanks for your comments,
Maxime
Thanks,
Ingo
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