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Wednesday, 26 March 2014

Processor States (P-State and C-State)

Processor performance states (P-states) and processor operating states (C-states) are the capability of a processor to switch between different supported operating frequencies and voltages to modulate power consumption. The Advanced Configuration and Power Interface (ACPI) specification ( defines the CPU P-states power management states. The number of P-states is processor specific. If configured properly according to system workload, this feature provides power savings. Higher P-state numbers represent slower processor speeds. Power consumption is lower at higher P-states. For example, a P3 state is higher than a P1 state. A processor in P3 state will run more slowly and use less power than a processor running at P1 state. To operate at any P-state, the processor must be in the C0 operational state where the processor is working and not idling.
The ACPI specification also defines the CPU C-states power management states. CPU operating states (C-states) are the capability of an idle processor to turn off unused components to save power. When a processor runs in the C0 state it is working. A processor running in any other C-state is idle. Higher C-state numbers represent deeper CPU sleep states. At higher C-states, more components shut down to save power. Some components that are shut down include stopping the processor clock and stopping interrupts. A disadvantage is that deeper sleep states have slower wake up times.
  • P-States are responsible for lowering the CPU multiplier and CPU voltage when there is no work load. These are configured in the BIOS as PPM or Speed Step. They are passed onto the OS and configured in Power Options control panel (in Windows).
  • S-States are sleep states. These are set in the BIOS and then configured in the power options control panel (timeout in minutes). There are various sleep technologies such as S1 sleep, S3 sleep, Hybrid sleep and Hibernation (which is a laptop sleep technology).
  • G-States are global operating states and are not configurable by the user. These are just used in documentation to specify certain system states such as on, off, sleeping.
  • C-States are advanced CPU current lowering technologies. These are configured in the BIOS and are automatically used by the OS.