5.9 POWER MANAGEMENT
The first general-purpose electronic computer, the ENIAC, had 18,000
vacuum tubes and consumed 140,000 watts of power. As a result, it ran up a non-
trivial electricity bill. After the invention of the transistor, power usage dropped
dramatically and the computer industry lost interest in power requirements. How-
ever, nowadays power management is back in the spotlight for several reasons,
and the operating system is playing a role here.
Let us start with desktop PCs. A desktop PC often has a 200-watt power sup-
ply (which is typically 85% efficient, that is, loses 15% of the incoming energy to
heat). If 100 million of these machines are turned on at once worldwide, together
they use 20,000 megawatts of electricity. This is the total output of 20 average-
sized nuclear power plants. If power requirements could be cut in half, we could
get rid of 10 nuclear power plants. From an environmental point of view, getting
rid of 10 nuclear power plants (or an equivalent number of fossil fuel plants) is a
big win and well worth pursuing.
The other place where power is a big issue is on battery-powered computers,
including notebooks, laptops, palmtops, and Webpads, among others. The heart
of the problem is that the batteries cannot hold enough charge to last very long, a
few hours at most. Furthermore, despite massive research efforts by battery com-
panies, computer companies, and consumer electronics companies, progress is
glacial. To an industry used to a doubling of the performance every 18 months
(Moore’s law), having no progress at all seems like a violation of the laws of phy-
sics, but that is the current situation. As a consequence, making computers use
less energy so existing batteries last longer is high on everyone’s agenda. The
operating system plays a major role here, as we will see below.
There are two general approaches to reducing energy consumption. The first
one is for the operating system to turn off parts of the computer (mostly I/O
devices) when they are not in use because a device that is off uses little or no
energy. The second one is for the application program to use less energy, possibly
degrading the quality of the user experience, in order to stretch out battery time.
We will look at each of these approaches in turn, but first we will say a little bit
about hardware design with respect to power usage.
5.9.1 Hardware Issues
Batteries come in two general types: disposable and rechargeable. Disposable
batteries (most commonly AAA, AA, and D cells) can be used to run handheld
devices, but do not have enough energy to power laptop computers with large
bright screens. A rechargeable battery, in contrast, can store enough energy to
power a laptop for a few hours. Nickel cadmium batteries used to dominate here,
but they gave way to nickel metal hydride batteries, which last longer and do not
pollute the environment quite as badly when they are eventually discarded.