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 Power management

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* ACPI power states.

  http://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface

** Global states.

 * G0 (S0) Working

 * G1 Sleeping (subdivides into the four states S1 through S4)

   * S1: All processor caches are flushed, and the CPU(s) stop executing

     instructions. Power to the CPU(s) and RAM is maintained; devices that do

     not indicate they must remain on may be powered down.

   * S2: CPU powered off.

   * S3: Commonly referred to as Standby, Sleep, or Suspend to RAM. RAM

     remains powered.

   * S4: Hibernation or Suspend to Disk. All content of main memory is saved

     to non-volatile memory such as a hard drive, and is powered down.

 * G2 (S5) Soft Off. G2, S5, and Soft Off are synonyms. G2 is almost the same

   as G3 Mechanical Off, but some components remain powered so the computer

   can "wake" from input from the keyboard, clock, modem, LAN, or USB device.

 * G3 Mechanical Off: The computer's power consumption approaches close to

   zero, to the point that the power cord can be removed and the system is

   safe for dis-assembly (typically, only the real-time clock is running off

   its own small battery).

** System states.

 * S0 Working (G0), Processor in C0-C3, full context save RAM maintained

 * S1 Sleeping with processor context maintained, RAM maintained

 * S2 Sleeping with processor content not necessarily maintained, RAM

   maintained, most devices in D3

 * S3 Sleeping, lower than S2, RAM maintained, most devices in D3

 * S4 Sleeping, lower than S3, RAM not maintained, most devices in D3

 * S5 Sleeping, lower than S4, no context saved, reboot necessary

** Device states.

 * D0 Fully-On is the operating state.

 * D1 and D2 are intermediate power-states whose definition varies by device.

 * D3 Off has the device powered off and unresponsive to its bus.

** Processor states.

 * C0 is the operating state.

 * C1 (often known as Halt) is a state where the processor is not executing

   instructions, but can return to an executing state essentially

   instantaneously. All ACPI-conformant processors must support this power state.

 * C2 (often known as Stop-Clock) is a state where the processor maintains all

   software-visible state, but may take longer to wake up. This processor

   state is optional.

 * C3 (often known as Sleep) is a state where the processor does not need to

   keep its cache coherent, but maintains other state. Some processors have

   variations on the C3 state (Deep Sleep, Deeper Sleep, etc.) that differ in

   how long it takes to wake the processor. This processor state is optional.

** Performance states.

 * P0 max power and frequency.

 * P1 less than P0, voltage/frequency scaled.

 * Pn less than P(n-1), voltage/frequency scaled.

* Control power schema under Windows.

Use powercfg utility. To se current state:

  cmd> powercfg /QUERY

To see all available states:

  cmd> powercfg /LIST

To restore power configurtion:

  cmd> powercfg /RestoreDefaultPolicies

* Intel.

** Intel Speedstep® Technology.

  http://www.intel.com/support/processors/sb/CS-028855.htm

                Frequently asked questions for Intel Speedstep® Technology

  http://www.intel.com/p/en_US/support/highlights/processors/frequencyid

                Intel® Processor Frequency ID Utility

  http://software.intel.com/en-us/articles/enhanced-intel-speedstepr-technology-and-demand-based-switching-on-linux

                Enhanced Intel SpeedStep® Technology and Demand-Based Switching on Linux