Re: power-efficient scheduling design

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2013-06-21 at 14:34 -0700, Arjan van de Ven wrote:
> On 6/21/2013 2:23 PM, Catalin Marinas wrote:
> >>
> >> oops sorry I misread your mail (lack of early coffee I suppose)
> >>
> >> I can see your point of having a thing for "did we ask for all the performance
> >> we could ask for" prior to doing a load balance (although, for power efficiency,
> >> if you have two tasks that could run in parallel, it's usually better to
> >> run them in parallel... so likely we should balance anyway)
> >
> > Not necessarily, especially if parallel running implies powering up a
> > full cluster just for one CPU (it depends on the hardware but for
> > example a cluster may not be able to go in deeper sleep states unless
> > all the CPUs in that cluster are idle).
> 
> I guess it depends on the system

Sort-of. We have something similar with threads on ppc. IE, the core can
only really stop if all threads are. From a Linux persepctive it's a
matter of how we define the scope of that 'cluster' Catalin is talking
about. I'm sure you do too.

Then there is the package, which adds MC etc...

> the very first cpu needs to power on
> * the core itself
> * the "cluster" that you mention
> * the memory controller
> * the memory (out of self refresh)
> 
> while the second cpu needs
> * the core itself
> * maybe a second cluster
> 
> normally on Intel systems, the memory power delta is quite significant
> which then means the efficiency of the second core is huge compared to
> running things in sequence.

What's your typical latency for bringing an MC back (and memory out of
self refresh) ? IE. Basically bringing a package back up ?

Cheers,
Ben.

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