VirtualMachineInstance with dedicated CPU resources

Certain workloads, requiring a predictable latency and enhanced performance during its execution would benefit from obtaining dedicated CPU resources. KubeVirt, relying on the Kubernetes CPU manager, is able to pin guest’s vCPUs to the host’s pCPUs.

Kubernetes CPU manager ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Kubernetes CPU manager is a mechanism that affects the scheduling of workloads, placing it on a host which can allocate Guaranteed resources and pin certain POD’s containers to host pCPUs, if the following requirement are met:

  • POD’s QOS is Guaranteed

    • resources requests and limits are equal

    • all containers in the POD express CPU and memory requirements

  • Requested number of CPUs is an Integer

Requesting dedicated CPU resources

Setting spec.domain.cpu.dedicatedCpuPlacement to true in a VMI spec will indicate the desire to allocate dedicated CPU resource to the VMI

Kubevirt will verify that all the necessary conditions are met, for the Kubernetes CPU manager to pin the virt-launcher container to dedicated host CPUs. Once, virt-launcher is running, the VMI’s vCPUs will be pinned to the pCPUS that has been dedicated for the virt-launcher container.

Expressing the desired amount of VMI’s vCPUs can be done by either setting the guest topology in spec.domain.cpu (sockets, cores, threads) or spec.domain.resources.[requests/limits].cpu to a whole number, integer (e.g. 1, 2, etc) indicating the number of vCPUs requested for the VMI. Number of vCPUs is counted as sockets * cores * threads or if spec.domain.cpu is empty then it takes value from spec.domain.resources.requests.cpu or spec.domain.resources.limits.cpu.

Note: Users should not specify both spec.domain.cpu and spec.domain.resources.[requests/limits].cpu

Note: spec.domain.resources.requests.cpu must be equal to spec.domain.resources.limits.cpu

Note: Multiple cpu-bound microbenchmarks show a significant performance advantage when using spec.domain.cpu.sockets instead of spec.domain.cpu.cores.

All inconsistent requirements will be rejected.

apiVersion: kubevirt.io/v1alpha3
kind: VirtualMachineInstance
spec:
  domain:
    cpu:
      sockets: 2
      cores: 1
      threads: 1
      dedicatedCpuPlacement: true
    resources:
      limits:
        memory: 2Gi
[...]

OR

apiVersion: kubevirt.io/v1alpha3
kind: VirtualMachineInstance
spec:
  domain:
    cpu:
      dedicatedCpuPlacement: true
    resources:
      limits:
        cpu: 2
        memory: 2Gi
[...]

Identifying nodes with a running CPU manager

At this time, Kubernetes doesn’t label the nodes that has CPU manager running on it.

KubeVirt has a mechansim to identify which nodes has the CPU manager running and manually add a cpumanager=true label. This label will be removed when KubeVirt will identify that CPU manager is no longer running on the node. This automatic identification should be viewed as a temporary workaround until Kubernetes will provide the required functionality. Therefore, this feature should be manually enabled by adding CPUManager to the kube-config feature-gate field.

When automatic identification is disabled, cluster administrator may manually add the above label to all the nodes when CPU Manager is running.

  • Nodes’ labels are view-able: kubectl describe nodes

  • Administrators may manually label a missing node: kubectl label node [node_name] cpumanager=true

Enabling the CPU Manager automatic identification feature gate

To enable the automatic idetification, user may expand the feature-gates field in the kubevirt-config config map by adding the CPUManager to it.

apiVersion: v1
kind: ConfigMap
metadata:
  name: kubevirt-config
  namespace: kubevirt
  labels:
    kubevirt.io: ""
data:
  feature-gates: "CPUManager"

Alternatively, users can edit an existing kubevirt-config:

kubectl edit configmap kubevirt-config -n kubevirt

...
data:
  feature-gates: "DataVolumes,CPUManager"

Sidecar containers and CPU allocation overhead

Note: In order to run sidecar containers, KubeVirt requires the Sidecar feature gate to be enabled by adding Sidecar to the kubevirt-config ConfigMap’s feature-gates field.

According to the Kubernetes CPU manager model, in order the POD would reach the required QOS level Guaranteed, all containers in the POD must express CPU and memory requirements. At this time, Kubevirt often uses a sidecar container to mount VMI’s registry disk. It also uses a sidecar container of it’s hooking mechanism. These additional resources can be viewed as an overhead and should be taken into account when calculating a node capacity. > Note: The current defaults for sidecar’s resources: > * CPU: 200m > * Memory: 64M > As the CPU resource is not expressed as a whole number, CPU manager will not > attempt to pin the sidecar container to a host CPU.