Re: power-efficient scheduling design

[Arjan van de Ven]

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

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.
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