Using the Kubernetes Cluster Autoscaler

Recommendations when using the Kubernetes Cluster Autoscaler in Production Environments

Consider the following recommendations:

• Always have at least one node pool in a cluster that is not managed by the Kubernetes Cluster Autoscaler. This node pool is required to run critcal cluster add-ons. Also note that it is your responsibility to manually scale any node pools that are not managed by the Kubernetes Cluster Autoscaler.
• Always define multiple replicas for the Kubernetes Cluster Autoscaler deployment in the configuration file. If there is only one replica and it is evicted and cannot be rescheduled, the Kubernetes Cluster Autoscaler is unable to create more worker nodes.
• In the configuration file, set max-node-provision-time to 25 minutes. Typically, new worker nodes are provisioned and move to the Ready condition in significantly less time than this. However, when many nodes are added to a node pool at the same time, it can take longer to provision them. Note that the Console displays a warning beside worker nodes that take more than 20 minutes to initialize. Assuming worker nodes move to the Ready condition within the time you specify for max-node-provision-time, you can ignore the warning.
• In the configuration file, you specify the maximum number of nodes allowed in the node pool. Make sure the maximum number of nodes you specify does not exceed the tenancy limit for the worker node shape defined for the node pool. The Kubernetes Cluster Autoscaler will never create more nodes than the tenancy limit. If you specify a number greater than the tenancy limit, the Kubernetes Cluster Autoscaler will periodically attempt to create additional nodes, but will not do so until the tenancy limit has been increased.
• When defining clusters that you want the Kubernetes Cluster Autoscaler to manage, we recommend creating multiple node pools with one availability domain specified per node pool.
• Always design applications to tolerate the disruptions that can occur when the Kubernetes Cluster Autoscaler removes worker nodes, or moves pods to a different worker node prior to removing an under-utilized worker node. For example, design applications that leverage pod disruption budgets (see Specifying a Disruption Budget for your Application in the Kubernetes documentation).
• Do not manually change node pools that are managed by the Kubernetes Cluster Autoscaler. For example, do not add or remove nodes using kubectl, or using the Console (or the Oracle Cloud Infrastructure CLI or API). The Kubernetes Cluster Autoscaler might override such changes, or they might modify its behavior. For example, if you want to remove all the nodes in a node pool managed by the Kubernetes Cluster Autoscaler, always use the Kubernetes Cluster Autoscaler to scale the node pool to zero.

Supported Kubernetes Cluster Autoscaler Parameters

You control how the Kubernetes Cluster Autoscaler resizes a cluster's node pools using configuration parameters.

The configuration parameters you can use to manage clusters created by Container Engine for Kubernetes, and how to specify them, depend on whether you are using the Kubernetes Cluster Autoscaler as a standalone program or as a cluster add-on.

• Using the Kubernetes Cluster Autoscaler as a standalone program

  If you are using the Kubernetes Cluster Autoscaler as a standalone program, you specify the configuration parameters in a configuration file.

  When using the Kubernetes Cluster Autoscaler as a standalone program, you cannot use the following configuration parameters to manage clusters created by Container Engine for Kubernetes:

  • --node-group-auto-discovery : Not supported (cloud provider specific).
  • --node-autoprovisioning-enabled=true : Not supported.
  • --gpu-total : Not supported (cloud provider specific).
  • --expander=price : Not supported (cloud provider specific).

  Except for the above parameters, you can use any of the configuration parameters when using the Kubernetes Cluster Autoscaler as a standalone program.

  For a full list of Kubernetes Cluster Autoscaler configuration parameters, see What are the parameters to CA? in the Cluster Autoscaler FAQ on GitHub.

• Using the Kubernetes Cluster Autoscaler as a cluster add-on

  If you are using the Kubernetes Cluster Autoscaler as a cluster add-on, you specify the configuration parameters in a configuration file (when using the CLI or API), or in the Console.

  When using the Kubernetes Cluster Autoscaler as a cluster add-on, you cannot use the following configuration parameters to manage clusters created by Container Engine for Kubernetes:

  • --node-group-auto-discovery : Not supported (cloud provider specific).
  • --node-autoprovisioning-enabled=true : Not supported.
  • --gpu-total : Not supported (cloud provider specific).
  • --expander=price : Not supported (cloud provider specific).

  Note that a number of other Kubernetes Cluster Autoscaler configuration parameters are also not supported when using the Kubernetes Cluster Autoscaler as a cluster add-on.

  For a list of the configuration parameters that are supported when using the Kubernetes Cluster Autoscaler as a cluster add-on, along with the names to use when specifying them, see Cluster Autoscaler add-on add-on configuration arguments.

Notes about the Kubernetes Cluster Autoscaler

Note the following:

• The Kubernetes Cluster Autoscaler adds and removes nodes to and from existing node pools in existing clusters. More specifically, note that the Kubernetes Cluster Autoscaler:

  • Does not create additional clusters.
  • Does not create additional node pools.
  • Does not create additional pods. Adding pods is supported by the Horizontal Pod Autoscaler (see Using the Kubernetes Horizontal Pod Autoscaler).
  • Does not adjust CPU and memory requests and limits for containers. Adjusting limits is supported by the Vertical Pod Autoscaler (see Using the Kubernetes Vertical Pod Autoscaler).

• When removing worker nodes from a node pool, the Kubernetes Cluster Autoscaler respects pod scheduling and eviction rules. These rules can prevent the Kubernetes Cluster Autoscaler from removing a worker node.

  For example, by default, the Kubernetes Cluster Autoscaler will not remove nodes running kube-system pods (such as coredns pods), with the exception of DaemonSet or mirror pods. You can control this behavior using a configuration parameter (skip-nodes-with-system-pods when deployed as a standalone program, skipNodesWithSystemPods when deployed as a cluster add-on). The parameter is set to true by default.

  For more information, see What types of pods can prevent CA from removing a node? in the Cluster Autoscaler FAQ on GitHub.

• If you are using both Terraform and the Kubernetes Cluster Autoscaler to manage node pools, note that Terraform might revert changes to the size of a node pool made by the Kubernetes Cluster Autoscaler. To prevent such behavior, use the Terraform ignore_changes feature, which is specifically intended for situations in which Terraform shares responsibility for managing a single object with another process. For example, when a resource is created with reference to data that might change in the future, but you don't want those potential data changes to affect the resource after its creation. The ignore_changes meta-argument specifies resource attributes that Terraform is to ignore when planning updates to the associated remote object. For more information, see The Lifecycle Meta-Argument in the Terraform documentation.

  For example:

  resource "node_pool_1" "example" {
    # ...
  
    lifecycle {
      ignore_changes = [
        node_config_details.size
      ]
    }
  }

• When you upgrade a cluster to a new version of Kubernetes, it is your responsibility to manually upgrade the Kubernetes Cluster Autoscaler to make it compatible with the cluster's new Kubernetes version. The Kubernetes Cluster Autoscaler is not upgraded automatically.
• To use the Kubernetes Cluster Autoscaler to manage a cluster you have created with Container Engine for Kubernetes, the cluster must be running certain versions of Kubernetes as follows:
  • When using the Kubernetes Cluster Autoscaler as a standalone program, clusters must be running Kubernetes version 1.17 or later.
  • When using the Kubernetes Cluster Autoscaler as a cluster add-on, clusters must be running Kubernetes version 1.25 or later.
• You can use the Kubernetes Cluster Autoscaler with managed node pools, but not with virtual node pools.