The operating system plays a key role in energy management. It controls all the devices, so it
must decide what to shutdown and when to shut it down. If it shuts down a device and that
device is needed again quickly, there may be an annoying delay while it is restarted. On the
other hand, if it waits too long to shut down a device, energy is wasted of nothing.
The trick is to find algorithms and heuristics that let the operating system make good decision
about what to shut down and when. The trouble is that "good" is highly subjective. One user
may find it acceptable that after 30 seconds of not using the conputer it takes 2 seconds for it
to respond to a keystroke. Another user may swear a blue streak under the same conditions. In
the absence of audio input, the computer can not tell these users apart.
The Display
Let us now look at the big spenders of the energy budget to see what can be done about each one.
The biggest item in everyone's energy budget is the display. To get a bright sharp image, the screen
must be backlit nad that takes substantial energy. Many operating systems attempt tp save energy
here by shuting down the display when there has been no activity for some number of minutes. Often
the user can decide what the shutdown interval is, thus pushing the trade-off between frequent blanking
of the screen and using the battery up quickly back to the user (who probably really does not want it).
Turning off the display is a sleep state because it can be regenerated (from the video RAM) almost
instantaneously when any key is struck or the pointing device is moved.