CPU performance states (P-states) and CPU operating states (C-states)

CPUFreq is the interface and infrastructure provided by the Linux® kernel to control processor performance state capability.

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 (http://www.acpi.info/spec.htm) 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.