Re: power-efficient scheduling design

[David Lang]

On Wed, 12 Jun 2013, Amit Kucheria wrote:

> On Wed, Jun 12, 2013 at 7:18 AM, Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> wrote:
> > On 6/11/2013 5:27 PM, David Lang wrote:
> > > Nobody is saying that this sort of thing should be in the fastpath of the
> > > scheduler.
> > >
> > > But if the scheduler has a table that tells it the possible states, and
> > > the cost to get from the current state to each of these states (and to get
> > > back and/or wake up to
> > > full power), then the scheduler can make the decision on what to do,
> > > invoke a routine to make the change (and in the meantime, not be fighting
> > > the change by trying to
> > > schedule processes on a core that's about to be powered off), and then
> > > when the change happens, the scheduler will have a new version of the table
> > > of possible states and costs
> > >
> > > This isn't in the fastpath, it's in the rebalancing logic.
> >
> >
> > the reality is much more complex unfortunately.
> > C and P states hang together tightly, and even C state on
> > one core impacts other cores' performance, just like P state selection
> > on one core impacts other cores.
> >
> > (at least for x86, we should really stop talking as if the OS picks the
> > "frequency",
> > that's just not the case anymore)
>
> This is true of ARM platforms too. As Daniel pointed out in an earlier
> email, the operating point (frequency, voltage) has a bearing on the
> c-state latency too.
>
> An additional complexity is thermal constraints. E.g. On a quad-core
> Cortex-A15 processor capable of say 1.5GHz, you won't be able to run
> all 4 cores at that speed for very long w/o exceeding the thermal
> envelope. These overdrive frequencies (turbo in x86-speak) impact the
> rest of the system by either constraining the frequency of other cores
> or requiring aggresive thermal management.
>
> Do we really want to track these details in the scheduler or just let
> the scheduler provide notifications to the existing subsystems
> (cpufreq, cpuidle, thermal, etc.) with some sort of feedback going
> back to the scheduler to influence future decisions?
>
> Feeback to the scheduler could be something like the following (pardon
> the names):
>
> 1. ISOLATE_CORE: Don't schedule anything on this core - cpuidle might
> use this to synchronise cores for a cluster shutdown, thermal
> framework could use this as idle injection to reduce temperature
> 2. CAP_CAPACITY: Don't expect cpufreq to raise the frequency on this
> core - cpufreq might use this to cap overall energy since overdrive
> operating points are very expensive, thermal might use this to slow
> down rate of increase of die temperature

How much data are you going to have to move back and forth between the different 
systems?

do you really only want the all-or-nothing "use this core as much as possible" 
vs "don't use this core at all"? or do you need the ability to indicate how much 
to use a particular core (something that is needed anyway for asymetrical cores 
I think)

If there is too much information that needs to be moved back and forth between 
these 'subsystems' for the 'right' thing to happen, then it would seem like it 
makes more sense to combine them.

Even combined, there are parts that are still pretty modular (like the details 
of shifting from one state to another, and the different high level strategies 
to follow for different modes of operation), but having access to all the 
information rather than only bits and pieces of the information at lower 
granularity would seem like an improvement.

David Lang
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