Hyper-V processor allocation

While allocating processors for a VM on Hyper-V, you must ensure that processor resource control is configured correctly.

Hyper-V uses relative weight and CPU % caps/reservations per virtual processor.

You can reserve CPU for a VM in Hyper-V Manager.

Steps to reserve CPU for a VM in Hyper-V

1. Open Hyper-V Manager.

2. Right-click the VM and select Settings.

3. Expand Processor:

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

   • Resource Control (under Processor):

     • Virtual machine reserve (%): This sets the minimum percentage of CPU resources per virtual processor guaranteed to the VM.

     • Virtual machine limit (%): This sets the maximum CPU the VM can use. Always keep this at the default setting of 100%.

     • Relative weight: Determines the priority during contention (higher number = higher priority).

4. Click OK to save changes.

   

How to calculate CPU reservation in Hyper-V

CPU reservation is per vCPU, and the percentage is based on one logical processor (core/thread).

Example scenario:

• Host has 128 logical processors (64 cores × 2 threads per core).

• VM has 8 vCPUs.

• If you want each vCPU to have at least 50% guaranteed of a logical processor, set reservation to 50%.

Recommendation: For all mission-critical System Platform VMs, it is recommended that you reserve processors for each VM. For details, see Configured vs effective processor allocation.

Important! Make sure that you leave enough resources for the hypervisor to run (refer to your hypervisor guidelines). If you reserve more than 100% of available processors, a node may fail to start.