Re: power-efficient scheduling design

[Morten Rasmussen <morten.rasmussen@arm.com>]

On Wed, Jun 19, 2013 at 06:08:29PM +0100, Arjan van de Ven wrote:
> On 6/19/2013 10:00 AM, Morten Rasmussen wrote:
> > On Wed, Jun 19, 2013 at 04:39:39PM +0100, Arjan van de Ven wrote:
> >> On 6/18/2013 10:47 AM, David Lang wrote:
> >>
> >>>
> >>> It's bad enough trying to guess the needs of the processes, but if you also are reduced to guessing the capabilities of the cores, how can anything be made to work?
> >>
> >> btw one way to look at this is to assume that (with some minimal hinting)
> >> the CPU driver will do the right thing and get you just about the best performance you can get
> >> (that is appropriate for the task at hand)...
> >> ... and don't do anything in the scheduler proactively.
> >
> > If I understand correctly, you mean if your hardware/firmware is fully
> 
> hardware, firmware and the driver
> 
> > in control of the p-state selection and changes it fast enough to match
> > the current load, the scheduler doesn't have to care? By fast enough I
> > mean, faster than the scheduler would notice if a cpu was temporarily
> > overloaded at a low p-state. In that case, you wouldn't need
> > cpufreq/p-state hints, and the scheduler would only move tasks between
> > cpus when cpus are fully loaded at their max p-state.
> 
> with the migration hint, I'm pretty sure we'll be there today typically.

A hint when a task is moved to a new cpu is too late if the migration
shouldn't have happened at all. If the scheduler knows that the cpu is
able to switch to a higher p-state it can decide to wait for the p-state
change instead of migrating the task and waking up another cpu.

> we'll notice within 10 msec regardless, but the migration hint will take
> the edge of those 10 msec normally.

I'm not sure if 10 msec is fast enough for the scheduler to not notice.
Real use-case studies will tell.

> 
> I would argue that the "at their max p-state" in your sentence needs to go away.
> since you don't know what you actually are except in hindsight.
> And even then you don't know if you could have gone higher or not.

Yes. What I meant was that if your p-state selection is responsive
enough the scheduler would only see the cpu as overloaded when it is in
its highest available p-state. That may determined dynamically by power,
thermal, and other factors.

> 
> 
> >> the hints I have in mind are not all that complex; we have the biggest issues today
> >> around task migration (the task migrates to a cold cpu... so a simple notifier chain
> >> on the new cpu as it is accepting a task and we can bump it up), real time tasks
> >> (again, simple notifier chain to get you to a predictably high performance level)
> >> and we're a long way better than we are today in terms of actual problems.
> >>
> >> For all the talk of ondemand (as ARM still uses that today)... that guy puts you in
> >> either the lowest or highest frequency over 95% of the time. Other non-cpufreq solutions
> >> like on Intel are bit more advanced (and will grow more so over time), but even there,
> >> in the grand scheme of things, the scheduler shouldn't have to care anymore with those
> >> two notifiers in place.
> >
> > You would need more than a few hints to implement more advanced capacity
> > management like proposed for the power scheduler. I believe that Intel
> > would benefit as well from guiding the scheduler to idle the right cpu
> > to enable deeper idle states and/or enable turbo-boost for other cpus.
> 
> that's an interesting theory.
> I've yet to see any way to actually have that do something useful.
> 
> yes there is some value in grouping a lot of very short tasks together.
> not a lot of value, but at least some.
> 
> and there is some value in the grouping within a package (to a degree) thing.
> 
> (both are basically "statistically, sort left" as policy)
> 

The proposed task packing patches have shown significant benefits for
scenarios with many short tasks. This is a typical scenario on android.

Morten