How it works…

The VMM determines a set of possible monitor modes to use and then picks one to use as the default monitor mode unless something other than Automatic has been specified. The decision is based on:

• The physical CPU's features and guest operating system type
• Configuration file settings

There are three valid combinations for the monitor mode, as follows:

• BT: Binary translation and shadow page tables
• HV: AMD-V or Intel VT-x and shadow page tables
• HWMMU: AMD-V with RVI or Intel VT-x with EPT (RVI is inseparable from AMD-V, and EPT is inseparable from Intel VT-x)

BT, HV, and HWMMU are abbreviations used by ESXi to identify each combination.

When a VM is powering on, the VMM inspects the physical CPU's features and the guest operating system type to determine the set of possible execution modes. It first finds the set of modes allowed. Then it restricts the allowed modes by configuring file settings. Finally, among the remaining candidates, it chooses the preferred mode, which is the default monitor mode. This default mode is then used if you have Automatic selected.

For the majority of workloads, the default monitor mode chosen by the VMM works best. The default monitor mode for each guest operating system on each CPU has been carefully selected after a performance evaluation of the available choices. However, some applications have special characteristics that can result in better performance when using a non-default monitor mode. These should be treated as exceptions, not rules.

The chosen settings are honored by the VMM only if the settings are supported on the intended hardware. For example, if you select Software CPU and MMU for a 64-bit guest operating system running on a 64-bit Intel processor, the VMM will choose Intel VT-x for CPU virtualization instead of BT. This is because BT is not supported by the 64-bit guest operating system on this processor.