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

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

> Dear Ingo,
>
> On 01/30/2012 02:53 PM, Ingo Molnar wrote:
> >* 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?
>
> 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?

> > 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.

> > 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?

Thanks,

Ingo


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