Re: suspend blockers & Android integration

[Ingo Molnar]

* Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:

> On Thu, 3 Jun 2010 19:26:50 -0700 (PDT)
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > If the system is idle (or almost idle) for long times, I would heartily 
> > recommend actively shutting down unused cores. Some CPU's are hopefully 
> > smart enough to not even need that kind of software management, but I 
> > suspect even the really smart ones might be able to take advantage of the 
> > kernel saying: "I'm shutting you down, you don't have to worry about 
> > latency AT ALL, because I'm keeping another CPU active to do any real 
> > work".
> 
> sadly the reality is that "offline" is actually the same as "deepest C 
> state". At best.
> 
> As far as I can see, this is at least true for all Intel and AMD cpus.
> 
> And because there's then no power saving (but a performance cost), it's 
> actually a negative for battery life/total energy.
> 
> (lots of experiments inside Intel seem to confirm that, it's not just 
> theory)

Well, the scheme would only be useful if it's _NOT_ just a deep C4 state, but 
something that prevents tasks from being woken to that CPU for a good period 
of time. Hot-unplugging that CPU achieves that (the runqueues are pulled), so 
i think in Linus's idea makes sense in principle.

[ Or have you done deep-idle experiments to that effect as well? ]

I suspect it all depends on the cost: and our current hot-unplug and 
hot-replug code is all but cheap ...

	Ingo
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