[ltp] T60 battery

[Chris Schumann]

> From: "Laurent Gilson" <pumpkin@gmx.de>

> Put the other way round: 0.013125 W/MHz vs 0.0125 W/MHz. If 
> the calulation takes X MHz (and the RAM is lighting fast or 
> everything is in cache) 600 MHz is the way to go.

But that only covers the CPU.

RAM, display, hard drive, support chips and losses have a constant drain
too, so that needs to be computed for total efficiency.

And those losses are going to take 2.67 times as much energy (1600/600) at
the slow speed because any task will take that much more time.

For very long CPU-bound tasks, it should be apparent that you want the CPU
at full speed.

Let's try any time t:

Full speed: t seconds * (21W + 15W) = 36t J

Slow speed: t * 2.67 * (7.5 + 15)   = 60t J


Let's solve for L when P1 = P2:
p = t * (21 + L)
p = t * 2.67 * (7.5 + L)

21t + tl = 20t + 2.67tl

21t - 20t = 2.67tl - tl
t = 1.67tl
0.6 = L

So with that CPU, if your total system load is more than 0.6W (which is the
same as a hard drive at LOW POWER IDLE alone), it looks like you should go
full speed for ALL CPU-bound tasks.

I don't know a lot about laptop speed and power control, so this only works
if the constant load really is constant. If there is any "cost" to change
speed, or something else takes more power when running fast (like RAM or
support chips), then that must be addressed too; but my hunch is that
automatic CPU throttling is the best possible way to get the most
computation per Joule stored in your battery.

Chris