Re: [RFC PATCH 00/31] CPUFreq on ARM

[Andre Przywara <andre.przywara@linaro.org>]

Hi,

....

>> So Xen does not need to throw in its own ideas here. Which would avoid
>> some of the hard problems we encountered.
> I got all your point.
> Just question. Why does existing CPUFreq on x86 have own logic? Do we have
> something yet another on ARM that having own logic in Xen doesn't make
> any sense?

That's a good question. From quickly poking some people in #xendevel,
Julien learnt that CPUFreq on x86 might not really work well or at least
not as expected.
So the benefit is not even clear there. It just went in the tree once,
and possibly nobody ever revisited it since.
And even if there were good reasons back then, modern CPUs tend to be
quite different in terms of power characteristics.

....

>> 0. Decide whether CPUFreq justifies 1.-4. in the first place.
> Sure,
>> That sounds like a lot of work and code, so we should be sure it's worth it.
>>
>> I wonder if you could provide some input, ideally measurements on the
>> actual power savings CPUFreq provides.
> Well, I think I will be able to provide some numbers when a firmware,
> which runs on the SoC
> I am using, is ready. Actually, currently I have an emulator without
> any real freq/volt changes.

Yes, some actual numbers would very much help the case. I don't think
you need very sophisticated equipment, just running a workload once with
and once without CPUFreq and compare the power consumption would be a
good start. This could be as easy as measuring the (m)Wh consumed with
some wall-plug type power meter. I use some very cheap USB power
meter[1], which I put between the PSU and some single board computer to
get an idea on what the power consumption is. Surely not really
reliable, but better than nothing.

>> Does the wish to have CPUFreq purely come from some "tick-the-box"
>> exercise? As in: We have it on native Linux, so we need it in Xen?
> As I said before, we are interesting in purely embedded use-cases
> where power consumption is a question.
> If you know how to save power without having CPUFreq involved I would
> appreciate the pointers.

As Julien said, I guess idling and CPU offlining/CPU suspend (via PSCI)
would be a good start to look at. You could try to get some numbers on
this as well.

Cheers,
Andre.

[1]
https://www.ebay.co.uk/itm/USB-Charger-Doctor-Voltage-Current-Meter-Mobile-Battery-Tester-Power-Detector-UK/263220956905

>> What power savings can we expect from CPUFreq? Can those possible
>> savings be transferred into a virtualized environment at all? And do
>> those saving justify all the extra code in Xen?
>>
>> I think those questions need to be answered first, then we can discuss
>> about the implementation details.
> OK.
> 

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