On Sat, Jun 5, 2010 at 3:48 PM, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote: > On Sat, 5 Jun 2010 15:26:36 -0700 > Brian Swetland <swetland@xxxxxxxxxx> wrote: > >> >> I'm continually surprised by answers like this. We run on hardware >> that power gates very aggressively and draws in the neighborhood of >> 1-2mA at the battery when in the lowest state (3-5mA while the radio >> is connected to the network and paging). Waking up out of that lowest >> state and executing code every few seconds or (worse) several times a >> second) will raise your average power consumption. Being able to stay >> parked at the very bottom for minutes or hours at a time when nothing >> "interesting" is happening is very useful and can have a significant >> impact on overall battery life. > > It's relatively simple math. > > If you wake up for a burst of work, you burn power at the higher level > P1 (versus the lower power level P2), for, lets say an average time T, > with a relatively small T (few milliseconds at most). > > If you wake up X times per second (with X being a fractional number, so > can be smaller than 1) the extra power consumption factor is > > X * T * P1 > ------------------------------- > X * T * P1 + (1.0 - X * T) * P2 > > if you draw a graph of this, for real values of P and T, there's a real > point where you hit diminishing returns. > > if say T is 5 milliseconds (that's a high amount), and X is 1 > wakeup/second, then there's already a 200:1 ratio in time an power. > It is a 200:1 ratio in time not in power. > If X goes to once every 10 seconds (not unreasonable, especially since > any real device will pull email and stuff in the backgroudn), you have > 2000:1 time and power ratios... > > Unless your "on" power is insane high (and hopefully it's not, since The absolute "on" power is not relevant to the ratio, the difference between on and off power is. This can easily be 100:1. > you're not turning on the whole device obviously, you do selective > power and clock gating)... that "divide by 200 or 2000" makes the whole > problem go away.. in the "seconds" range for really low power devices. > Not in "hours" range. > If you improve the low power state, compared to the "on" state wakeup gets worse, not better, but yes the phone hardware we have now does not need to stay idle for hours to get good battery life, the msm hardware at least needs to stay idle for more than a few seconds. > > On laptops (which have much more poor powermanagement) this point is > around 40 milliseconds or so.. but on phone silicon that I've seen, > both Intel and others, this is in the 1 to 5 seconds range. > > > > > > -- > Arjan van de Ven Intel Open Source Technology Centre > For development, discussion and tips for power savings, > visit http://www.lesswatts.org > -- Arve Hjønnevåg -- 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
