[acallan]

Precisely. The latency of some of the more aggressive power states exceeds the resolution of most OS tick timers, so they are often aborted for no good reason. So, as I'm typing this reply, in-between keystrokes, voltage to the CPU can be killed (not just clock-gated).

[wtallis]

And it gets even better when you can postpone several low-priority interrupts for a while until a high-priority interrupt comes in, and then you wake up the CPU and handle them all at once before putting the CPU back to sleep.

[sliverstorm]

Hmm. Completely brown-out the CPU in between keypresses? I always heard cold starts are very very slow (compared to regular operation). While I guess I don't have a measure for how slow, it seems unlikely that if a user is typing at 360CPM (60WPM * ~7), that you will be able to completely shut off the CPU much.

[glhaynes]

I'm not sure about the CPU numbers either (though my understanding is that modern CPUs are super-quick to power down and back up), but I think 360 CPM is way too high: few people type at 60 WPM, most words aren't 7 letters long, and nearly no one is typing 100% of the time they're at their computer.

[sliverstorm]

Oh, I agree there is ample opportunity for shutting down the CPU. I probably spend more time just looking at the screen reading, than any other activity.

[cpeterso]

I know the Linux's default scheduler ticks are, depending on the platform, ~10 ms or less. How long are the latencies of the slower power state changes?

[wmf]

The tick is every 1 ms and C6 wakeup latency can be over 100 us, so ticks could suck over 10% of your performance (and plenty of energy).