6.5.1

Potential Issues with CPU Affinity
Before you use CPU anity, you might need to consider certain issues.
Potential issues with CPU anity include:
n
For multiprocessor systems, ESXi systems perform automatic load balancing. Avoid manual
specication of virtual machine anity to improve the scheduler’s ability to balance load across
processors.
n
Anity can interfere with the ESXi host’s ability to meet the reservation and shares specied for a
virtual machine.
n
Because CPU admission control does not consider anity, a virtual machine with manual anity
seings might not always receive its full reservation.
Virtual machines that do not have manual anity seings are not adversely aected by virtual
machines with manual anity seings.
n
When you move a virtual machine from one host to another, anity might no longer apply because the
new host might have a dierent number of processors.
n
The NUMA scheduler might not be able to manage a virtual machine that is already assigned to certain
processors using anity.
n
Anity can aect the host's ability to schedule virtual machines on multicore or hyperthreaded
processors to take full advantage of resources shared on such processors.
Host Power Management Policies
You can apply several power management features in ESXi that the host hardware provides to adjust the
balance between performance and power. You can control how ESXi uses these features by selecting a power
management policy.
Selecting a high-performance policy provides more absolute performance, but at lower eciency and
performance per wa. Low-power policies provide less absolute performance, but at higher eciency.
You can select a policy for the host that you manage by using the VMware Host Client. If you do not select a
policy, ESXi uses Balanced by default.
Table 4‑1. CPU Power Management Policies
Power Management Policy Description
High Performance Do not use any power management features.
Balanced (Default) Reduce energy consumption with minimal performance
compromise
Low Power Reduce energy consumption at the risk of lower
performance
Custom User-dened power management policy. Advanced
conguration becomes available.
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 aempts to
adjust CPU frequencies so that virtual machine performance is not aected.
When a CPU is idle, ESXi can apply deep halt states, also known as C-states. The deeper the C-state, the less
power the CPU uses, but it also takes longer for the CPU to start running again. When a CPU becomes idle,
ESXi applies an algorithm to predict the idle state duration 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 for idle
CPUs, C1.
Chapter 4 Administering CPU Resources
VMware, Inc. 23