vSphere processor allocation

Reserving CPU resources in VMware vSphere ensures that a virtual machine (VM) always has a guaranteed amount of CPU resources, even during resource contention.

In VMware vSphere, reservation is set by CPU frequency rather than by the percentage of allocated CPU cores.

Steps to reserve CPU for a VM in vSphere

1. Log in to vSphere Client.

2. Navigate to the VM and right-click the VM you want to configure, then select Edit Settings.

   1. CPU: This is the number of virtual processors you are assigning to the VM. Selecting the number of virtual processors is the first step.

3. Under the CPU section, edit CPU Reservation:

   1. Reservation (MHz or GHz): This sets the amount of processing power reserved for the VM. Enter this number after calculating the value as shown below.

   2. Limit: This sets the maximum CPU the VM can use. Make sure to configure the Limit as “Unlimited.”

   3. Shares: Determines the priority during contention. As a minimum, make sure Shares is set to "Normal".

4. Save the settings and power on the VM (if it was off).

How to calculate CPU reservation

Important! Refer to Configured vs effective processor allocation for recommended CPU reservation values before you calculate the reservation.

CPU reservation is specified in MHz, so you need to understand the host CPU capacity:

Step 1: Identify host CPU specs

• Use vSphere Client to check:

  • Number of cores per CPU

  • CPU clock speed (GHz)

Example:

• 2 sockets × 8 cores = 16 cores

• Each core = 2.6 GHz = 2600 MHz

• Total CPU capacity = 16 × 2600 = 41,600 MHz

Step 2: Calculate based on VM vCPU and need

If your VM has 4 vCPUs and you want to reserve 50% CPU for each core:

• 4 vCPUs × 2600 MHz × 50% = 5200 MHz reservation

If you want to guarantee 100% CPU to each vCPU:

• 4 vCPUs × 2600 MHz = 10,400 MHz reservation