XO power consumption under 8.2-759

[John Gilmore]

I spent a few hours tonight testing the power consumption of a G1G1 XO
under 8.2-759.  I didn't test any of the tricky autosuspend/resume
stuff -- just how much power can be saved by various user actions.
Both the answers, and how you can do measurements like this on your
own XO (using the power-measurement chip in the battery) are here:

  http://wiki.laptop.org/go/XO_Power_Draw

Rough summary: Normal power consumption of about 6 watts. By dimming
the screen, you can save up to a watt. By turning the screen all the
way off ("xset dpms force off"), you can save another 0.5 watt (total
1.5 watts for screen).  By default, X or Ohm dims to monochrome a
minute after you stop interacting with the laptop, and shuts off the
screen after 20 minutes.

By turning the WiFi chip off (with Extreme Power Management in the
Sugar control panel), you can save up to a watt.  By using a WiFi
access point rather than a mesh, you can save up to 0.2 watts. None of
these techniques involve turning off power to the CPU (the "Automatic
Power Management" option in the Sugar control panel), which still
tickles some bugs, particularly with respect to presence and sharing.

When you close the lid of the XO (*), it suspends the CPU, and also
turns off the WiFi chip unless you're on the Mesh.  Turning off the
WiFi is new in 8.2, and saves huge amounts of power.  My test laptop
was able to remain closed for almost 48 hours without exhausting its
battery or losing the ability to wake up in a second.  (However, due
to a bug, it takes it 90 seconds to re-associate with your selected
WiFi access point.)

I don't know why my power measurements are much higher than Richard's
from February (he got 3.9W to 4.9W where I got 5.9W).  I tend to trust
his more, since mine are one-shot samples, but my dimmer measurements
do tend to corroborate each other (i.e. none of them got down to 4.9W
until very very dim, yet he says the backlight was on full during his
test).

Further power testing, by anybody, would be welcome!  Please corroborate
these numbers, and/or test what Automatic Power Management does for 
power consumption and/or laptop run-times.  8.2 will be the first public
release in which we allow ordinary users to enable the automatic suspend
on idle (one of the OLPC hardware's unique features), and we haven't
yet characterized how it affects the laptop's average run time before the
battery runs out.

        John

(*) Some bugs occasionally prevent the XO from properly going to sleep
when you close it.  Sometimes it won't power off the CPU; other times
it won't power off the screen and backlight.  These are being worked
on for a 8.2 or a future release.  To be sure your laptop won't run
out of power quickly due to these bugs, check that the power LED is
blinking, and look into a USB port to see that the backlight is off.
If not, opening the laptop, waiting five seconds, and closing it again
often works.