One of the notable features of Xen 3.3 is to incorporate full support for processor power management features, C-states and P-states. Power management is getting more crucial not only for clients, but also for servers. C/P is from ACPI nomenclature which stand for different set of power/thermal technologies. Combining both, Xen 3.3 is now expected to achieve far better performance per watt.
The Xen 3.3 idle governor can support full line of C-states with great power reduction, especially for idle power consumption. The idle governor is triggered when the CPU is fully idle, and then the governor chooses the appropriate low power state based on the power budget and latency tolerance accordingly. The deeper C-state is, less power is consumed with longer entry/exit latency. The previous Xen releases only supported C1, as triggered by the HLT instruction.
The Xen 3.3 on-demand governor monitors CPU utilization in small intervals and dynamically chooses a suitable operation point with lower power and negligible performance impacts. P-states are set of operation points about CPU frequency/voltage, which can help reach optimal performance at the lowest power even when the system is in load line. Unlike C-states, P-state transitions can be triggered at fine-grained level, upon fast-changing workload characteristics. CPU vendors provide smart hardware level enhancements to ensure fast on-demand frequency/voltage change, like Intel’s Enhanced Intel SpeedStep@ and AMD’s PowerNow!@.