On Sat, 5 Jun 2010 15:26:36 0700
Brian Swetland wrote:
>
> I'm continually surprised by answers like this. We run on hardware
> that power gates very aggressively and draws in the neighborhood of
> 12mA at the battery when in the lowest state (35mA 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.
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
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.
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
