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nodes-cluster-overcommit-node-enforcing.adoc

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Disabling or enforcing CPU limits using CPU CFS quotas

Nodes by default enforce specified CPU limits using the Completely Fair Scheduler (CFS) quota support in the Linux kernel.

Prerequisites

  1. Obtain the label associated with the static Machine Config Pool CRD for the type of node you want to configure. Perform one of the following steps:

    1. View the Machine Config Pool:

      $ oc describe machineconfigpool <name>

      For example:

      $ oc describe machineconfigpool worker

apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfigPool
metadata:
  creationTimestamp: 2019-02-08T14:52:39Z
  generation: 1
  labels:
    custom-kubelet: small-pods (1)

      1. If a label has been added it appears under labels.

    2. If the label is not present, add a key/value pair:

      $ oc label machineconfigpool worker custom-kubelet=small-pods
Procedure

  1. Create a Custom Resource (CR) for your configuration change.

    Sample configuration for a disabling CPU limits
    apiVersion: machineconfiguration.openshift.io/v1
kind: KubeletConfig
metadata:
  name: disable-cpu-units (1)
spec:
  machineConfigPoolSelector:
    matchLabels:
      custom-kubelet: small-pods (2)
  kubeletConfig:
    cpu-cfs-quota: (3)
      - "false"

    1. Assign a name to CR.

    2. Specify the label to apply the configuration change.

    3. Set the cpu-cfs-quota parameter to false.

If CPU limit enforcement is disabled, it is important to understand the impact that will have on your node:

  • If a container makes a request for CPU, it will continue to be enforced by CFS shares in the Linux kernel.

  • If a container makes no explicit request for CPU, but it does specify a limit, the request will default to the specified limit, and be enforced by CFS shares in the Linux kernel.

  • If a container specifies both a request and a limit for CPU, the request will be enforced by CFS shares in the Linux kernel, and the limit will have no impact on the node.