CPU Hotplug

The CPU hotplug feature was introduced in KubeVirt v1.0, making it possible to configure the VM workload to allow for adding or removing virtual CPUs while the VM is running.

Abstract

A virtual CPU (vCPU) is the CPU that is seen to the Guest VM OS. A VM owner can manage the amount of vCPUs from the VM spec template using the CPU topology fields (spec.template.spec.domain.cpu). The cpu object has the integers cores,sockets,threads so that the virtual CPU is calculated by the following formula: cores * sockets * threads.

Before CPU hotplug was introduced, the VM owner could change these integers in the VM template while the VM is running, and they were staged until the next boot cycle. With CPU hotplug, it is possible to patch the sockets integer in the VM template and the change will take effect right away.

Per each new socket that is hot-plugged, the amount of new vCPUs that would be seen by the guest is cores * threads, since the overall calculation of vCPUs is cores * sockets * threads.

Configuration

Enable feature-gate

In order to enable CPU hotplug we need to add the VMLiveUpdateFeatures feature gate in Kubevirt CR:

apiVersion: kubevirt.io/v1
kind: KubeVirt
spec:
  configuration:
    developerConfiguration:
      featureGates:
        - VMLiveUpdateFeatures

Configure the workload update strategy

Current implementation of the hotplug process requires the VM to live-migrate. The migration will be triggered automatically by the workload updater. The workload update strategy in the KubeVirt CR must be configured with LiveMigrate, as follows:

apiVersion: kubevirt.io/v1
kind: KubeVirt
spec:
  workloadUpdateStrategy:
    workloadUpdateMethods:
    - LiveMigrate

Enable in VM spec

The VM object should look like this:

apiVersion: kubevirt.io/v1
kind: VirtualMachine
spec:
  liveUpdateFeatures:
    cpu: {}

Specifying the cpu.maxSockets value is optional. If you leave it unset, it will default to 4 times the sockets value.

NOTE: In order for these changes to take effect the VM needs to be rebooted.

[OPTIONAL] Set maximum sockets

You can explicitly set the maximum amount of sockets in two ways - VM level or Cluster level

VM level	Cluster level (Kubevirt CR)
apiVersion: kubevirt.io/v1 kind: VirtualMachine spec: liveUpdateFeatures: cpu: maxSockets: 8	apiVersion: kubevirt.io/v1 kind: KubeVirt spec: configuration: liveUpdateConfiguration: maxCpuSockets: 8

The VM-level configuration will take precendence over the cluster-wide configuration.

Hotplug process

Let's assume we have a running VM with the 4 vCPUs, which were configured with sockets:4 cores:1 threads:1 In the VMI status we can observe the current CPU topology the VM is running with:

apiVersion: kubevirt.io/v1
kind: VirtualMachineInstance
...
status:
  currentCPUTopology:
    cores: 1
    sockets: 4
    threads: 1

Now we want to hotplug another socket, by patching the VM object:

kubectl patch vm vm-cirros --type='json' \
-p='[{"op": "replace", "path": "/spec/template/spec/domain/cpu/sockets", "value": 5}]'

We can observe the CPU hotplug process in the VMI status:

status:
  conditions:
  - lastProbeTime: null
    lastTransitionTime: null
    status: "True"
    type: LiveMigratable
  - lastProbeTime: null
    lastTransitionTime: null
    status: "True"
    type: HotVCPUChange
  currentCPUTopology:
    cores: 1
    sockets: 4
    threads: 1

Please note the condition HotVCPUChange that indicates the hotplug process is taking place. Also you can notice the VirtualMachineInstanceMigration object that was created for the VM in subject:

NAME                             PHASE     VMI
kubevirt-workload-update-kflnl   Running   vm-cirros

When the hotplug process has completed, the currentCPUTopology will be updated with the new number of sockets and the migration is marked as successful.

#kubectl get vmi vm-cirros -oyaml

apiVersion: kubevirt.io/v1
kind: VirtualMachineInstance
metadata:
  name: vm-cirros
spec:
  domain:
    cpu:
      cores: 1
      sockets: 5
      threads: 1
...
...
status:
  currentCPUTopology:
    cores: 1
    sockets: 5
    threads: 1


#kubectl get vmim -l kubevirt.io/vmi-name=vm-cirros
NAME                             PHASE       VMI
kubevirt-workload-update-cgdgd   Succeeded   vm-cirros

Limitations

• VPCU hotplug is currently not supported by ARM64 architecture.
• Current hotplug implementation involves live-migration of the VM workload.