CPU / Device Performance States (P-States)

P-states define the power management state while the CPU / device is within its executing state, C0 for CPU and D0 for device.

P0 : Minimum p-state, highest power consumption.
P1, P2, P3…. : P1 > P2 > P3 and so on, in terms of power consumption.
Pn : Maximum p-state with the lowest power consumption.

P-state power management can be seen in modern CPUs and GPUs. It allows them to control their active power according to the load at any particular moment. The number of P-states is implementation-specific.

For instance, low-end GPUs like the NVIDIA GeForce 7300 GS has only one P-state, where the 2D and 3D clocks remain constant over time. On the other hand, high-end GPUs like the NVIDIA GeForce 7900 GT has at least two P-states - P0 state for running at maximum clock and voltage while playing 3D games, and a P1 state for running at the minimum clock and voltage in 2D mode.

However, P-state power management in CPU is far more complex. Different CPU models have their own unique P-state lookup tables. Take for example, the P-state lookup tables for these CPUs.

As you can see in the examples above, a CPU with higher multiplier has more P-states than a CPU with a lower multiplier. The smallest clock ratio supported by Intel is 6x, while AMD has a minimum multiplier of 4x. However, their Cool n Quiet feature uses 5x as the minimum clock ratio.

Note :

Different processor stepping may implement a different set of clock ratio / voltage lookup table. The clock ratio / voltage tables shown above are just examples to describe the differences in P-states used in different CPU models, even of the same family.

The CPU load incorporated with various P-states is one of the ways to utilize the P-state power management. Windows XP may transit a processor to P1 but Linux, on the other hand, may transit a processor into P2 at the same CPU load. It therefore depends on the operating system implementation.