C-state and P-state

    The two ways to decrease the power consumption of a processor:

    • powering down subsystems
    • voltage/frequency reduction

    C-states describe the first case, so they are the idle (power saving) states. In order to power down a subsystem, that subsystem should not be running anything, so it should be at idle, doing nothing, executing nothing. So C-state x, Cx, means one or more subsystems of the CPU is at idle, powered down.

    On the other hand, P-states describe the second case, so they are the executing (power saving) states. The subsystem is actually running but it does not require full performance so the voltage and/or frequency it operates is decreased. So P-state x, Px, means the subsystem it refers to (e.g. a CPU core) is operating at a specific (frequency, voltage) pair.