Power management

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Power management is a feature of some electrical appliances, especially copiers, computers and computer peripherals such as monitors and printers, that turns off the power or switches the system to a low-power state when inactive. One power management standard for computers is ACPI, which supersedes APM. All recent (consumer) computers have ACPI support.

1 Motivations
2 Processor level techniques
3 Operating system level: Hibernation
4 See also
5 References
6 External links

Motivations

Power management for computer systems are desired for many reasons, particularly:

Prolong battery life for portable and embedded systems.
Reduce cooling requirements.
Reduce noise.
Reduce operating costs for energy and cooling.

Lower power consumption also means lower heat dissipation, which increases system stability, and less energy use, which saves money and reduces the impact on the environment.

Processor level techniques

The power management for microprocessors can be done over the whole processor, or in specific areas.

With dynamic voltage scaling and dynamic frequency scaling, the CPU core voltage, clock rate, or both, can be altered to decrease power consumption at the price of potentially lower performance. This is sometimes done in real time to optimize the power-performance tradeoff.

Examples:

AMD Cool'n'Quiet
AMD PowerNow! ^[1]
IBM EnergyScale ^[2]
Intel SpeedStep
Transmeta LongRun and LongRun2
VIA LongHaul (PowerSaver)

Additionally, processors can selectively power off internal circuitry (power gating). For example:

Newer Intel Core processors support ultra-fine power control over the function units within the processors.
AMD CoolCore technology get more efficient performance by dynamically activating or turning off parts of the processor.^[3]

Intel VRT technology split the chip into a 3.3V I/O section and a 2.9V core section. The lower core voltage reduces power consumption.

Operating system level: Hibernation

Main article: Hibernate (OS feature)

When a computer system hibernates it saves the contents of the RAM to disk and powers down the machine. On startup it reloads the data. This allows the system to be completely powered off while in hibernate mode. This requires a file the size of the installed RAM to be placed on the hard disk, potentially using up space even when not in hibernate mode. Hibernate mode is enabled by default in some versions of Windows and can be disabled in order to recover this disk space.