Modern Operating System --- Power Management

The first general-purpose electronic comuter, 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 dramatiscally and the computer

industry lost interest in power requirements. However, 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 supply (which is

typically 85% efficient, that is, loses 15% of the incoming energy to heat). If 100 million

of these machines are turned out at once worldwide, together they use 20,000 megawatts

of electricity. This is the total output of 20 average-sized nuclear power plants. From an

environment point of view, getting rid of 10 nuclear power plant (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, incluing notebooks,

handhelds, and Webpads, among others. The heart of the problem is that the batteries can

not hold enough charge to last very long, a few hours at most. Furthermore, despite massive

research efforts by battery companies, computer companies, and consumer electronics 

companies, progress is glacial. To an industry used to a doubling of performance every 18 

months (Moore's Law), having no progress at all seems like a violation of the laws of physics,

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

rule here, as we will see below.

At the lowest level, hardware vendors are trying to make their electronics more energy efficient.

Techniques used include reducing transistor size, employing dynamic voltage scaling, using low-swing

and adiabatic buses, and similar technique. 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, possibily degrading the quility 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.