ESXi can take advantage of several power management features that the host hardware provides to adjust the trade-off between performance and power use. You can control how ESXi uses these features by selecting a power management policy.

In general, selecting a high-performance policy provides more absolute performance, but at lower efficiency (performance per watt). Lower-power policies provide less absolute performance, but at higher efficiency.

ESXi provides five power management policies. If the host does not support power management, or if the BIOS settings specify that the host operating system is not allowed to manage power, only the Not Supported policy is available.

You select a policy for a host using the vSphere Web Client. If you do not select a policy, ESXi uses Balanced by default.

CPU Power Management Policies

Power Management Policy

Description

Not supported

The host does not support any power management features or power management is not enabled in the BIOS.

High Performance

The VMkernel detects certain power management features, but will not use them unless the BIOS requests them for power capping or thermal events.

Balanced (Default)

The VMkernel uses the available power management features conservatively to reduce host energy consumption with minimal compromise to performance.

Low Power

The VMkernel aggressively uses available power management features to reduce host energy consumption at the risk of lower performance.

Custom

The VMkernel bases its power management policy on the values of several advanced configuration parameters. You can set these parameters in the vSphere Web Client Advanced Settings dialog box.

When a CPU runs at lower frequency, it can also run at lower voltage, which saves power. This type of power management is typically called Dynamic Voltage and Frequency Scaling (DVFS). ESXi attempts to adjust CPU frequencies so that virtual machine performance is not affected.

When a CPU is idle, ESXi can take advantage of deep halt states (known as C-states). The deeper the C-state, the less power the CPU uses, but the longer it takes for the CPU to resume running. When a CPU becomes idle, ESXi applies an algorithm to predict how long it will be in an idle state and chooses an appropriate C-state to enter. In power management policies that do not use deep C-states, ESXi uses only the shallowest halt state (C1) for idle CPUs.