System information
134 System Analysis and Tuning Guide
11.1.1 C-States (Processor Operating
States)
Modern processors have several power saving modes called C-states. They re-
flect the capability of an idle processor to turn off unused components in order to save
power. Whereas C-states have been available for laptops for some time, they are a
rather recent trend in the server market (for example, with Intel* processors, C-modes
are only available since Nehalem).
When a processor runs in the C0 state, it is executing instructions. A processor run-
ning in any other C-state is idle. The higher the C number, the deeper the CPU sleep
mode: more components are shut down to save power. Deeper sleep states are very
efficient concerning power consumption in an idle system. But the downside is that
they introduce higher latency (the time the CPU needs to go back to C0). Depending
on the workload (threads waking up, triggering some CPU utilization and then going
back to sleep again for a short period of time) or hardware (for example, interrupt ac-
tivity of a network device), disabling the deepest sleep states can significantly increase
overall performance. For details on how to do so, refer to Section11.3.2.2, “Viewing
and Modifying Kernel Idle Statistics with cpupower” (page143).
Some states also have submodes with different power saving latency levels. Which C-
states and submodes are supported depends on the respective processor. However, C1
is always available.
Table11.1, “C-States” (page134) gives an overview of the most common C-states.
Table11.1: C-States
Mode Definition
C0 Operational state. CPU fully turned
on.
C1 First idle state. Stops CPU main inter-
nal clocks via software. Bus interface
unit and APIC are kept running at full
speed.
C2 Stops CPU main internal clocks via
hardware. State where the proces-