Messages in this thread | | | Date | Wed, 19 Jun 2013 18:00:43 +0100 | From | Morten Rasmussen <> | Subject | Re: power-efficient scheduling design |
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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 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.
> > Now for big.little and other temporary or permanent asymmetries, we may want to > have a "max performance level" type indicator, and that's fair enough > (and this can be dynamic, since it for thermal reasons this can change over time, > but on a somewhat slower timescale) > > > 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.
Morten
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