C-State residencies are collected from hardware and/or the operating system (OS).

For systems that collect OS C-State residencies, CPU C-states are core power states requested by the Operating System Directed Power Management (OSPM) infrastructure that define the degree to which the processor is "idle".

For systems that collect hardware C-State residencies, CPU C-States are obtained by reading the processor’s MSRs which count the actual time spent in each C-State.

C-States range from C0 to Cn. C0 indicates an active state. All other C-states (C1-Cn) represent idle sleep states where the processor clock is inactive (cannot execute instructions) and different parts of the processor are powered down. As the C-States get deeper, the exit latency duration becomes longer (the time to transition to C0) and the power savings becomes greater.

For more complete information about compiler optimizations, see our Optimization Notice.