Intel ARCHITECTURE IA-32 User Manual download pdf (Page 419)

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101

Power Optimization for Mobile Usages 9

9-5

The index of a C-state type designates the depth of sleep. Higher

numbers indicate a deeper sleep state and lower power consumption.

They also require more time to wake up (higher exit latency).

C-state types are described below:

• C0: The processor is active and performing computations and

executing instructions.

• C1: This is the lowest-latency idle state, which has very low exit

latency. In the C1 power state, the processor is able to maintain the

context of the system caches.

• C2: This level has improved power savings over the C1 state. The

main improvements are provided at the platform level.

• C3: This level provides greater power savings than C1 or C2. In C3,

the processor stops clock generating and snooping activity. It also

allows system memory to enter self-refresh mode.

The basic technique to implement OS power management policy to

reduce static power consumption is by evaluating processor idle

durations and initiating transitions to higher-numbered C-state types.

This is similar to the technique of reducing active power consumption

by evaluating processor utilization and initiating P-state transitions. The

OS looks at history within a time window and then sets a target C-state

type for the next time window, as illustrated in Figure 9-3:

1 2 ... 414 415 416 417 418 419 420 421 422 423 424 ... 567 568

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