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The cpuidle subsystem

May 12, 2010, 04:34 (0 Talkback[s])
(Other stories by Jonathan Corbet)

"Your editor recently had cause to dig around in the cpuidle subsystem. It never makes sense to let such work go to only a single purpose when it could be applied toward the creation of a kernel-page article. So, what follows is a multi-level discussion of cpuidle, what it's for, and how it works. Doing nothing, it turns out, is more complicated than one might think.

"On most systems, the processor is idle much of the time. We can't always be running CPU-intensive work like kernel builds, video transcoding, weather modeling, or yum. When there is nothing left to do, the processor will go into the idle state to wait until it is needed again. Once upon a time, on many systems, the "idle state" was literally a thread running at the lowest possible priority which would execute an infinite loop until the system found something better to do. Killing the idle process was a good way to panic a VAX/VMS machine, which had no clue of how to do nothing without a task dedicated to that purpose.

"Running a busy-wait loop requires power; contemporary concerns have led us to the conclusion that expending large amounts of power toward the accomplishment of nothing is rarely a good idea. So CPU designers have developed ways for the processor to go into a lower-power state when there is nothing for it to do. Typically, when put into this state, the CPU will stop clocks and power down part or all of its circuitry until the next interrupt arrives. That results in the production of far more nothing per watt than busy-waiting."

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