Chapter 5. Applying autoscaling to an OpenShift Container Platform cluster

The ClusterAutoscaler adjusts the size of an OpenShift Container Platform cluster to meet its current deployment needs. It uses declarative, Kubernetes-style arguments to provide infrastructure management that does not rely on objects of a specific cloud provider. The ClusterAutoscaler has a cluster scope, and is not associated with a particular namespace.

The ClusterAutoscaler increases the size of the cluster when there are Pods that failed to schedule on any of the current nodes due to insufficient resources or when another node is necessary to meet deployment needs. The ClusterAutoscaler does not increase the cluster resources beyond the limits that you specify.

The ClusterAutoscaler decreases the size of the cluster when some nodes are consistently not needed for a significant period, such as when it has low resource use and all of its important Pods can fit on other nodes.

If the following types of Pods are present on a node, the ClusterAutoscaler will not remove the node:

If you configure the ClusterAutoscaler, additional usage restrictions apply:

The Horizontal Pod Autoscaler (HPA) and the ClusterAutoscaler modify cluster resources in different ways. The HPA changes the deployment’s or ReplicaSet’s number of replicas based on the current CPU load. If the load increases, the HPA creates new replicas, regardless of the amount of resources available to the cluster. If there are not enough resources, the ClusterAutoscaler adds resources so that the HPA-created Pods can run. If the load decreases, the HPA stops some replicas. If this action causes some nodes to be underutilized or completely empty, the ClusterAutoscaler deletes the unnecessary nodes.

The ClusterAutoscaler takes Pod priorities into account. The Pod Priority and Preemption feature enables scheduling Pods based on priorities if the cluster does not have enough resources, but the ClusterAutoscaler ensures that the cluster has resources to run all Pods. To honor the intention of both features, the ClusterAutoscaler inclues a priority cutoff function. You can use this cutoff to schedule "best-effort" Pods, which do not cause the ClusterAutoscaler to increase resources but instead run only when spare resources are available.

Pods with priority lower than the cutoff value do not cause the cluster to scale up or prevent the cluster from scaling down. No new nodes are added to run the Pods, and nodes running these Pods might be deleted to free resources.

The MachineAutoscaler adjusts the number of Machines in the MachineSets that you deploy in an OpenShift Container Platform cluster. You can scale both the default worker MachineSet and any other MachineSets that you create. The MachineAutoscaler makes more Machines when the cluster runs out of resources to support more deployments. Any changes to the values in MachineAutoscaler resources, such as the minimum or maximum number of instances, are immediately applied to the MachineSet they target.

First, deploy the ClusterAutoscaler to manage automatic resource scaling in your OpenShift Container Platform cluster.

apiVersion: "autoscaling.openshift.io/v1"
kind: "ClusterAutoscaler"
metadata:
  name: "default"
spec:
  podPriorityThreshold: -10 1
  resourceLimits:
    maxNodesTotal: 24 2
    cores:
      min: 8 3
      max: 128 4
    memory:
      min: 4 5
      max: 256 6
    gpus:
      - type: nvidia.com/gpu 7
        min: 0 8
        max: 16 9
      - type: amd.com/gpu 10
        min: 0 11
        max: 4 12
  scaleDown: 13
    enabled: true 14
    delayAfterAdd: 10m 15
    delayAfterDelete: 5m 16
    delayAfterFailure: 30s 17
    unneededTime: 60s 18

After you deploy the ClusterAutoscaler, deploy MachineAutoscaler resources that reference the MachineSets that are used to scale the cluster.

apiVersion: "autoscaling.openshift.io/v1beta1"
kind: "MachineAutoscaler"
metadata:
  name: "worker-us-east-1a" 1
  namespace: "openshift-machine-api"
spec:
  minReplicas: 1 2
  maxReplicas: 12 3
  scaleTargetRef: 4
    apiVersion: machine.openshift.io/v1beta1
    kind: MachineSet 5
    name: worker-us-east-1a 6