* 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 -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
