Power down peripherals

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Devices attached to a computer system that are not necessary for basic operation are called peripherals. Peripherals are often storage, input or output devices.

When peripherals are not in use, often they can be set to a low power state or shut down. Powering down a device gives room for energy savings.

A lot of these mechanisms are taken care of by the operating system, but where possible a software developer should allow for this by optimizing for underlying layers

Contents

Drawbacks

When a powered down device receives a request, it first has to turn back on before it can respond introducing latency. During the time it is powering on, a device usually consumes more energy.

Both of these conditions make powering down peripherals unsuitable for short idle times. Forecasting whether an idle period will be long or short is often difficult, making it hard to decide when to power down peripherals.

Implementations

On systems running a Windows operating system, the hard disk is powered down if it goes unused for an X amount of minutes.

On mobile platforms dimming the background light of a screen and keyboard after just a few seconds and turning off the screen again a few seconds later, is already common practice and saves significant amounts of energy.

Cases

On an unnamed project for a handheld device energy where was very scarce, a bug was found disallowing proper powering down of an RFID reader when it was not in use. Instead of having to save energy by disabling functions, suddenly there was plenty of energy after this single bug was fixed.

On the same project, all peripherals (including the screen, RFID reader, barcode scanner, wireless antenna) are constantly turned off when they are not directly in use to save energy. This is a necessity to comply with the non-functional requirements regarding battery life.

At FOSDEM 2009 in Brussels Matthew Garret had held an interesting talk on aggressive power management in Linux. He pointed out that reclocking and clock gating can save a lot of power. Reclocking a GPU from 600 Mhz to 100 Mhz combined with clock gating, can save in 5 W on a Radeon graphics card.

See Also

In a lower level, where parts of a chip are powered off, this technique is called clock gating.

Sources

This best practices was recognized as such by IT professionals, described in Energy efficient software.
This best practices is mentioned in:
Nick Jones. 2007. Eight Software Approaches Can Enable Energy-Efficient Computing. Gartner research publication.

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