×

You can run multiple custom schedulers alongside the default scheduler and configure which scheduler to use for each pod.

It is supported to use a custom scheduler with OKD, but Red Hat does not directly support the functionality of the custom scheduler.

For information on how to configure the default scheduler, see Configuring the default scheduler to control pod placement.

To schedule a given pod using a specific scheduler, specify the name of the scheduler in that Pod specification.

Deploying a custom scheduler

To include a custom scheduler in your cluster, include the image for a custom scheduler in a deployment.

Prerequisites
  • You have access to the cluster as a user with the cluster-admin role.

  • You have a scheduler binary.

    Information on how to create a scheduler binary is outside the scope of this document. For an example, see Configure Multiple Schedulers in the Kubernetes documentation. Note that the actual functionality of your custom scheduler is not supported by Red Hat.

  • You have created an image containing the scheduler binary and pushed it to a registry.

Procedure
  1. Create a file that contains the deployment resources for the custom scheduler:

    Example custom-scheduler.yaml file
    apiVersion: v1
    kind: ServiceAccount
    metadata:
      name: custom-scheduler
      namespace: kube-system (1)
    ---
    apiVersion: rbac.authorization.k8s.io/v1
    kind: ClusterRoleBinding
    metadata:
      name: custom-scheduler-as-kube-scheduler
    subjects:
    - kind: ServiceAccount
      name: custom-scheduler
      namespace: kube-system (1)
    roleRef:
      kind: ClusterRole
      name: system:kube-scheduler
      apiGroup: rbac.authorization.k8s.io
    ---
    apiVersion: apps/v1
    kind: Deployment
    metadata:
      labels:
        component: scheduler
        tier: control-plane
      name: custom-scheduler
      namespace: kube-system (1)
    spec:
      selector:
        matchLabels:
          component: scheduler
          tier: control-plane
      replicas: 1
      template:
        metadata:
          labels:
            component: scheduler
            tier: control-plane
            version: second
        spec:
          serviceAccountName: custom-scheduler
          containers:
          - command:
            - /usr/local/bin/kube-scheduler
            - --address=0.0.0.0
            - --leader-elect=false
            - --scheduler-name=custom-scheduler (2)
            image: "<namespace>/<image_name>:<tag>" (3)
            livenessProbe:
              httpGet:
                path: /healthz
                port: 10251
              initialDelaySeconds: 15
            name: kube-second-scheduler
            readinessProbe:
              httpGet:
                path: /healthz
                port: 10251
            resources:
              requests:
                cpu: '0.1'
            securityContext:
              privileged: false
            volumeMounts: []
          hostNetwork: false
          hostPID: false
          volumes: []
    1 This procedure uses the kube-system namespace, but you can use the namespace of your choosing.
    2 The command for your custom scheduler might require different arguments. For example, you can pass configuration as a mounted volume using the --config argument.
    3 Specify the container image that you created for the custom scheduler.
  2. Create the deployment resources in the cluster:

    $ oc create -f custom-scheduler.yaml
Verification
  • Verify that the scheduler pod is running:

    $ oc get pods -n kube-system

    The custom scheduler pod is listed as Running:

    NAME                                                       READY   STATUS    RESTARTS   AGE
    custom-scheduler-6cd7c4b8bc-854zb                          1/1     Running   0          2m

Deploying pods using a custom scheduler

After the custom scheduler is deployed in your cluster, you can configure pods to use that scheduler instead of the default scheduler.

Each scheduler has a separate view of resources in a cluster. For that reason, each scheduler should operate over its own set of nodes.

If two or more schedulers operate on the same node, they might intervene with each other and schedule more pods on the same node than there are available resources for. Pods might get rejected due to insufficient resources in this case.

Prerequisites
  • You have access to the cluster as a user with the cluster-admin role.

  • The custom scheduler has been deployed in the cluster.

Procedure
  1. If your cluster uses role-based access control (RBAC), add the custom scheduler name to the system:kube-scheduler cluster role.

    1. Edit the system:kube-scheduler cluster role:

      $ oc edit clusterrole system:kube-scheduler
    2. Add the name of the custom scheduler to the resourceNames lists for the leases and endpoints resources:

      apiVersion: rbac.authorization.k8s.io/v1
      kind: ClusterRole
      metadata:
        annotations:
          rbac.authorization.kubernetes.io/autoupdate: "true"
        creationTimestamp: "2021-07-07T10:19:14Z"
        labels:
          kubernetes.io/bootstrapping: rbac-defaults
        name: system:kube-scheduler
        resourceVersion: "125"
        uid: 53896c70-b332-420a-b2a4-f72c822313f2
      rules:
      ...
      - apiGroups:
        - coordination.k8s.io
        resources:
        - leases
        verbs:
        - create
      - apiGroups:
        - coordination.k8s.io
        resourceNames:
        - kube-scheduler
        - custom-scheduler (1)
        resources:
        - leases
        verbs:
        - get
        - update
      - apiGroups:
        - ""
        resources:
        - endpoints
        verbs:
        - create
      - apiGroups:
        - ""
        resourceNames:
        - kube-scheduler
        - custom-scheduler (1)
        resources:
        - endpoints
        verbs:
        - get
        - update
      ...
      1 This example uses custom-scheduler as the custom scheduler name.
  2. Create a Pod configuration and specify the name of the custom scheduler in the schedulerName parameter:

    Example custom-scheduler-example.yaml file
    apiVersion: v1
    kind: Pod
    metadata:
      name: custom-scheduler-example
      labels:
        name: custom-scheduler-example
    spec:
      schedulerName: custom-scheduler (1)
      containers:
      - name: pod-with-second-annotation-container
        image: docker.io/ocpqe/hello-pod
    1 The name of the custom scheduler to use, which is custom-scheduler in this example. When no scheduler name is supplied, the pod is automatically scheduled using the default scheduler.
  3. Create the pod:

    $ oc create -f custom-scheduler-example.yaml
Verification
  1. Enter the following command to check that the pod was created:

    $ oc get pod custom-scheduler-example

    The custom-scheduler-example pod is listed in the output:

    NAME                       READY     STATUS    RESTARTS   AGE
    custom-scheduler-example   1/1       Running   0          4m
  2. Enter the following command to check that the custom scheduler has scheduled the pod:

    $ oc describe pod custom-scheduler-example

    The scheduler, custom-scheduler, is listed as shown in the following truncated output:

    Events:
      Type    Reason          Age        From                                               Message
      ----    ------          ----       ----                                               -------
      Normal  Scheduled       <unknown>  custom-scheduler                                   Successfully assigned default/custom-scheduler-example to <node_name>

Additional resources