Power Management in Operating Systems
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 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 companies, 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 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 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.
Lithium ion batteries are even better, and may be recharged without first being fully drained, but their capacities are also severely limited.
The general approach most computer vendors take to battery conservation is to design the CPU, memory, and I/O devices to have multiple states: on, sleeping, hibernating, and off. To use the device, it must be on. When the device will not be needed for a short time, it can be put to sleep, which reduces energy consumption. When it is not expected to be needed for a longer interval, it can be made to hibernate, which reduces energy consumption even more. The trade-off here is that getting a device out of hibernation often takes more time and energy than getting it out of sleep state. Finally, when a device is off, it does nothing and consumes no power. Not all devices have all these states, but when they do, it is up to the operating system to manage the state transitions at the right moments.
Some computers have two or even three power buttons. One of these may put the whole computer in sleep state, from which it can be awakened quickly by typing a character or moving the mouse. Another may put the computer into hibernation, from which wakeup takes much longer. In both cases, these buttons typically do nothing except send a signal to the operating system, which does the rest in software. In some countries, electrical devices must, by law, have a mechanical power switch that breaks a circuit and removes power from the device, for safety reasons. To comply with this law, another switch may be needed.
Power management brings up a number of questions that the operating system must deal with. They include the following. Which devices can be controlled? Are they on/off, or do they have intermediate states? How much power is saved in the low-power states? Is energy expended to restart the device? Must some context be saved when going to a low-power state? How long does it take to go back to full power? Of course, the answers to these questions vary from device to device, so the operating system must be able to deal with a range of possibilities.
Various researchers have examined laptop computers to see where the power goes. Li et al. (1994) measured various workloads and came to the conclusions shown in Fig. 5-1. Lorch and Smith (1998) made measurements on other machines and came to the conclusions shown in Fig. 5-1. Weiser et al. (1994) also made measurements but did not publish the numerical values. They simply stated that the top three energy sinks were the display, hard disk, and CPU, in that order. While these numbers do not agree closely, possibly because the different brands of computers measured indeed have different energy requirements, it seems clear that the display, hard disk, and CPU are obvious targets for saving energy.
Figure 5-1 Power consumption of various parts of a laptop computer.