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Configuring the cluster autoscaler
2022-05-17

Objective

OVHcloud Managed Kubernetes service provides you Kubernetes clusters without the hassle of installing or operating them.

During the day-to-day life of your cluster, you may want to dynamically adjust the size of your cluster to accommodate to your workloads. The cluster autoscaler simplifies the task by scaling up or down your OVHcloud Managed Kubernetes cluster to meet the demand of your workloads.

This guide follows up the Using the cluster autoscaler guide, and it will cover a description of the cluster autoscaler configuration.

Requirements

Instructions

Cluster autoscaler configuration

When the autoscaler is enabled on a node pool, it uses a default configuration.

Here you have a description of the parameters used in the autoscaler configuration, and their default value:

Setting Description Default value
scan-interval How often cluster is reevaluated for scale up or down 10 seconds
scale-down-delay-after-add How long after scale up that scale down evaluation resumes 10 minutes
scale-down-delay-after-delete How long after node deletion that scale down evaluation resumes scan-interval
scale-down-delay-after-failure How long after scale down failure that scale down evaluation resumes 3 minutes
scale-down-unneeded-time How long a node should be unneeded before it is eligible for scale down 10 minutes
scale-down-unready-time How long an unready node should be unneeded before it is eligible for scale down 20 minutes
scale-down-utilization-threshold Node utilization level, defined as sum of requested resources divided by capacity, below which a node can be considered for scale down 0.5
max-graceful-termination-sec Maximum number of seconds the cluster autoscaler waits for pod termination when trying to scale down a node 600 seconds
balance-similar-node-groups Detects node pools with the same instance type and the same set of labels and balances the number of nodes between them false
expander Type of node pool expander to be used in scale up. Possible values: most-pods, random, least-waste, priority random
skip-nodes-with-local-storage If true cluster autoscaler will never delete nodes with pods with local storage, for example, EmptyDir or HostPath false
skip-nodes-with-system-pods If true cluster autoscaler will never delete nodes with pods from kube-system (except for DaemonSet or mirror pods) false
max-empty-bulk-delete Maximum number of empty nodes that can be deleted at the same time 10 nodes
new-pod-scale-up-delay For scenarios like burst/batch scale where you don't want the cluster autoscaler to act before the kubernetes scheduler could schedule all the pods, you can tell the CA to ignore unscheduled pods before they're a certain age. 0 seconds
max-total-unready-percentage Maximum percentage of unready nodes in the cluster. After this percentage is exceeded, the CA halts operations 45%
max-node-provision-time Maximum time the autoscaler waits for a node to be provisioned 15 minutes
ok-total-unready-count Number of allowed unready nodes, irrespective of max-total-unready-percentage 3 node

You can get more information on those parameters on the Kubernetes autoscaler documentation.

If you consider we should reevaluate the default value and/or prioritize the possible customization of one of those parameters, you are welcome to create an issue on our Public roadmap.

Configuring the autoscaler

The easiest way to enable the autoscaler is using the Kubernetes API, for example using kubectl.

Using Kubernetes API

When the autoscaler is enabled on a node pool, it uses a default configuration.

To list node pools, you can use:

kubectl get nodepools

You can change several parameters values through kubectl command:

kubectl patch nodepool <your_nodepool_name> --type="merge" --patch='{"spec": {"autoscaling": {"scaleDownUnneededTimeSeconds": <a_value>, "scaleDownUnreadyTimeSeconds": <another_value>, "scaleDownUtilizationThreshold": "<and_another_one>"}}}'

In my example cluster:

$ kubectl get nodepool nodepool-b2-7 -o json | jq .spec
{
  "antiAffinity": false,
  "autoscale": true,
  "autoscaling": {
    "scaleDownUnneededTimeSeconds": 600,
    "scaleDownUnreadyTimeSeconds": 1200,
    "scaleDownUtilizationThreshold": "0.5"
  },
  "desiredNodes": 3,
  "flavor": "b2-7",
  "maxNodes": 100,
  "minNodes": 0,
  "monthlyBilled": false
}
$ kubectl patch nodepool nodepool-b2-7 --type="merge" --patch='{"spec": {"autoscaling": {"scaleDownUnneededTimeSeconds": 900, "scaleDownUnreadyTimeSeconds": 1500, "scaleDownUtilizationThreshold": "0.7"}}}'
nodepool.kube.cloud.ovh.com/nodepool-b2-7 patched
$ kubectl get nodepool nodepool-b2-7 -o json | jq .spec
{
  "antiAffinity": false,
  "autoscale": true,
  "autoscaling": {
    "scaleDownUnneededTimeSeconds": 900,
    "scaleDownUnreadyTimeSeconds": 1500,
    "scaleDownUtilizationThreshold": "0.7"
  },
  "desiredNodes": 3,
  "flavor": "b2-7",
  "maxNodes": 100,
  "minNodes": 0,
  "monthlyBilled": false
}

For the moment, only these following parameters are editable:

  • autoscale
  • autoscaling
  • desiredNodes
  • minNodes
  • maxNodes

If you consider that we should prioritize the possible customization of other autoscaling parameters, you are welcome to create an issue on our Public roadmap.

Go further

To have an overview of OVHcloud Managed Kubernetes service, you can go to the OVHcloud Managed Kubernetes page.

Otherwise to skip it and learn more about using your Kubernetes cluster the practical way, we invite you to look at our tutorials .

  • If you need training or technical assistance to implement our solutions, contact your sales representative or click on this link to get a quote and ask our Professional Services experts for assisting you on your specific use case of your project.

  • Join our community of users.