Re: [11/11] system 1: Saving energy using DVFS

From: Kalle Jokiniemi
Date: Tue Jan 21 2014 - 14:01:50 EST


On 20.01.2014 20:25, Sebastian Reichel wrote:
On Mon, Jan 20, 2014 at 06:54:32PM +0100, Pavel Machek wrote:
On Mon 2014-01-20 17:10:29, Catalin Marinas wrote:
On Mon, Jan 20, 2014 at 04:49:26PM +0000, Pavel Machek wrote:
To save energy, the higher frequencies should be avoided and only used
when the application performance requirements can not be satisfied
otherwise (e.g. spread tasks across more cpus if possible).

I argue this is untrue for any task where user waits for its
completion with screen on. (And that's quite important subset).

Lets take Nokia n900 as an example.


Sleeping CPU: 2mA
Screen on: 230mA
CPU loaded: 250mA

Now, lets believe your numbers and pretend system can operate at 33%
of speed with 11% power consumption.

Lets take task that takes 10 seconds on max frequency:

~ 10s * 470mA = 4700mAs

You suggest running at 33% speed, instead; that means 30 seconds on
low requency.

CPU on low: 25mA (assumed).

~ 30s * 255mA = 7650mAs

Hmm. So race to idle is good thing on Intel machines, and it is good
thing on ARM design I have access to.

Race to idle doesn't mean that the screen goes off as well. Let's say
the screen stays on for 1 min and the CPU needs to be running for 10s
over this minute, in the first case you have:

No, it does not. I just assumed user is continuing to use his
machine. Obviously, waiting 60 seconds with screen on will make the
difference look smaller. But your solution still means user has to
wait longer _and_ you consume more battery doing so.

And this is for any task where user waits for result with screen
on. Like rendering a webpage. Like opening settings screen. Like
installing application.

There are not too many background tasks on a cellphone.

But hey, maybe you are right and running at lowest possible frequency
is right. Please provide concrete numbers like I did.

So what about using the display status information for power
management? Basically always using the lowest frequency should be ok
on phones if the display is disabled?

Well, not really. There are a looot of devices running linux kernel, and there are always devices and use cases that can't operate if you hardcode something like that.

It is good to know what the problematic use cases are, but it usually does not end well if you optimize for specifics. End users are unpredictable in ways they utilize their devices :)

And these days audio playback power optimization in smart phones is mostly for the product spec marketing purposes anyways :P

I think the discussion has been going into right direction:
- find what data we have to make better decisions
- find ways to utilize that data

And then in the end the big smart phone manufacturers will twist that to their use cases in horrible ways to meet product specs on tight schedules :D But at least the starting point will be closer to target.

Even on N900 dvfs is beneficial for audio playback. But it was not because of CPU consuming less, it was because the peripheral bus frequency was tied to CPU frequency. There we ended up removing a very low 125MHz cpu operating point so that the device ran at highest possible 250MHz CPU rate (to idle quickly) while still keeping the lower peripheral bus speed.

So we raced to idle and used DVFS.. how nice :)

- Kalle

-- Sebastian

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