Cstates

Cstates, often written as C-states, are a set of idle power states used by modern CPUs to reduce energy consumption when the processor is not executing instructions. The active state is C0; all other states are progressively deeper sleep modes (for example, C1, C2, C3, C6, C7 in many architectures). Each deeper C-state allows more components to be powered down or clock-gated, yielding greater power savings at the cost of longer wake-up latencies.

The availability and behavior of C-states vary by processor family and firmware. The exact boundaries and capabilities of each state are defined by the processor's silicon, firmware, and operating system. Transitions into and out of C-states are managed automatically by the OS, with the goal of balancing power savings and responsiveness. BIOS or firmware settings may enable, disable, or tune C-state behavior, and some systems offer selectable power-performance modes.

C-states are distinct from P-states, which govern processor performance through frequency and voltage scaling. A processor may simultaneously be in a low P-state (reduced frequency) while in a particular C-state, depending on workload and policy.

Practical considerations include wake latency, workload sensitivity, and thermal design. Deep C-states save energy during idle but can add latency when new work arrives. Tools and monitoring can help assess C-state residency and power impact.