Using the Cluster Autoscaler
This section applies only to worker Machines. Cluster Autoscaler is a tool that automatically adjusts the size of the Kubernetes cluster based on the utilization of Pods and Nodes in your cluster. For more general information about the Cluster Autoscaler, please see the project documentation.
The following instructions are a reproduction of the Cluster API provider specific documentation from the Autoscaler project documentation.
Cluster Autoscaler on Cluster API
The cluster autoscaler on Cluster API uses the cluster-api project to manage the provisioning and de-provisioning of nodes within a Kubernetes cluster.
Table of Contents:
- Kubernetes Version
- Starting the Autoscaler
- Configuring node group auto discovery
- Connecting cluster-autoscaler to Cluster API management and workload Clusters
- Autoscaler running in a joined cluster using service account credentials
- Autoscaler running in workload cluster using service account credentials, with separate management cluster
- Autoscaler running in management cluster using service account credentials, with separate workload cluster
- Autoscaler running anywhere, with separate kubeconfigs for management and workload clusters
- Autoscaler running anywhere, with a common kubeconfig for management and workload clusters
- Enabling Autoscaling
- Specifying a Custom Resource Group
- Specifying a Custom Resource Version
- Sample manifest
- Autoscaling with ClusterClass and Managed Topologies
- Special note on GPU instances
- Special note on balancing similar node groups
The cluster-api provider requires Kubernetes v1.16 or greater to run the v1alpha3 version of the API.
Starting the Autoscaler
To enable the Cluster API provider, you must first specify it in the command line arguments to the cluster autoscaler binary. For example:
Please note, this example only shows the cloud provider options, you will
most likely need other command line flags. For more information you can invoke
cluster-autoscaler --help to see a full list of options.
Configuring node group auto discovery
If you do not configure node group auto discovery, cluster autoscaler will attempt to match nodes against any scalable resources found in any namespace and belonging to any Cluster.
Limiting cluster autoscaler to only match against resources in the blue namespace
Limiting cluster autoscaler to only match against resources belonging to Cluster test1
Limiting cluster autoscaler to only match against resources matching the provided labels
These can be mixed and matched in any combination, for example to only match resources in the staging namespace, belonging to the purple cluster, with the label owner=jim:
Connecting cluster-autoscaler to Cluster API management and workload Clusters
You will also need to provide the path to the kubeconfig(s) for the management
and workload cluster you wish cluster-autoscaler to run against. To specify the
kubeconfig path for the workload cluster to monitor, use the
option and supply the path to the kubeconfig. If the
--kubeconfig option is
not specified, cluster-autoscaler will attempt to use an in-cluster configuration.
To specify the kubeconfig path for the management cluster to monitor, use the
--cloud-config option and supply the path to the kubeconfig. If the
--cloud-config option is not specified it will fall back to using the kubeconfig
that was provided with the
Autoscaler running in a joined cluster using service account credentials
+-----------------+ | mgmt / workload | | --------------- | | autoscaler | +-----------------+
Use in-cluster config for both management and workload cluster:
Autoscaler running in workload cluster using service account credentials, with separate management cluster
+--------+ +------------+ | mgmt | | workload | | | cloud-config | ---------- | | |<-------------+ autoscaler | +--------+ +------------+
Use in-cluster config for workload cluster, specify kubeconfig for management cluster:
cluster-autoscaler --cloud-provider=clusterapi \ --cloud-config=/mnt/kubeconfig
Autoscaler running in management cluster using service account credentials, with separate workload cluster
+------------+ +----------+ | mgmt | | workload | | ---------- | kubeconfig | | | autoscaler +------------>| | +------------+ +----------+
Use in-cluster config for management cluster, specify kubeconfig for workload cluster:
cluster-autoscaler --cloud-provider=clusterapi \ --kubeconfig=/mnt/kubeconfig \ --clusterapi-cloud-config-authoritative
Autoscaler running anywhere, with separate kubeconfigs for management and workload clusters
+--------+ +------------+ +----------+ | mgmt | | ? | | workload | | | cloud-config | ---------- | kubeconfig | | | |<--------------+ autoscaler +------------>| | +--------+ +------------+ +----------+
Use separate kubeconfigs for both management and workload cluster:
cluster-autoscaler --cloud-provider=clusterapi \ --kubeconfig=/mnt/workload.kubeconfig \ --cloud-config=/mnt/management.kubeconfig
Autoscaler running anywhere, with a common kubeconfig for management and workload clusters
+---------------+ +------------+ | mgmt/workload | | ? | | | kubeconfig | ---------- | | |<------------+ autoscaler | +---------------+ +------------+
Use a single provided kubeconfig for both management and workload cluster:
cluster-autoscaler --cloud-provider=clusterapi \ --kubeconfig=/mnt/workload.kubeconfig
To enable the automatic scaling of components in your cluster-api managed cloud there are a few annotations you need to provide. These annotations must be applied to either MachineSet or MachineDeployment resources depending on the type of cluster-api mechanism that you are using.
There are two annotations that control how a cluster resource should be scaled:
cluster.x-k8s.io/cluster-api-autoscaler-node-group-min-size- This specifies the minimum number of nodes for the associated resource group. The autoscaler will not scale the group below this number. Please note that the cluster-api provider will not scale down to, or from, zero unless that capability is enabled (see Scale from zero support).
cluster.x-k8s.io/cluster-api-autoscaler-node-group-max-size- This specifies the maximum number of nodes for the associated resource group. The autoscaler will not scale the group above this number.
The autoscaler will monitor any
both of these annotations.
Scale from zero support
The Cluster API community has defined an opt-in method for infrastructure providers to enable scaling from zero-sized node groups in the Opt-in Autoscaling from Zero enhancement. As defined in the enhancement, each provider may add support for scaling from zero to their provider, but they are not required to do so. If you are expecting built-in support for scaling from zero, please check with the Cluster API infrastructure providers that you are using.
If your Cluster API provider does not have support for scaling from zero, you may still use this feature through the capacity annotations. You may add these annotations to your MachineDeployments, or MachineSets if you are not using MachineDeployments (it is not needed on both), to instruct the cluster autoscaler about the sizing of the nodes in the node group. At the minimum, you must specify the CPU and memory annotations, these annotations should match the expected capacity of the nodes created from the infrastructure.
For example, if my MachineDeployment will create nodes that have “16000m” CPU, “128G” memory, “100Gi” ephemeral disk storage, 2 NVidia GPUs, and can support 200 max pods, the following annotations will instruct the autoscaler how to expand the node group from zero replicas:
apiVersion: cluster.x-k8s.io/v1alpha4 kind: MachineDeployment metadata: annotations: cluster.x-k8s.io/cluster-api-autoscaler-node-group-max-size: "5" cluster.x-k8s.io/cluster-api-autoscaler-node-group-min-size: "0" capacity.cluster-autoscaler.kubernetes.io/memory: "128G" capacity.cluster-autoscaler.kubernetes.io/cpu: "16" capacity.cluster-autoscaler.kubernetes.io/ephemeral-disk: "100Gi" capacity.cluster-autoscaler.kubernetes.io/gpu-type: "nvidia.com/gpu" capacity.cluster-autoscaler.kubernetes.io/gpu-count: "2" capacity.cluster-autoscaler.kubernetes.io/maxPods: "200"
maxPods annotation will default to
110 if it is not supplied.
This value is inspired by the Kubernetes best practices
Considerations for large clusters.
RBAC changes for scaling from zero
If you are using the opt-in support for scaling from zero as defined by the
Cluster API infrastructure provider, you will need to add the infrastructure
machine template types to your role permissions for the service account
associated with the cluster autoscaler deployment. The service account will
need permission to
list the infrastructure machine templates for
your infrastructure provider.
For example, when using the Kubemark provider you will need to set the following permissions:
rules: - apiGroups: - infrastructure.cluster.x-k8s.io resources: - kubemarkmachinetemplates verbs: - get - list
Pre-defined labels and taints on nodes scaled from zero
To provide labels or taint information for scale from zero, the optional capacity annotations may be supplied as a comma separated list, as demonstrated in the example below:
apiVersion: cluster.x-k8s.io/v1alpha4 kind: MachineDeployment metadata: annotations: cluster.x-k8s.io/cluster-api-autoscaler-node-group-max-size: "5" cluster.x-k8s.io/cluster-api-autoscaler-node-group-min-size: "0" capacity.cluster-autoscaler.kubernetes.io/memory: "128G" capacity.cluster-autoscaler.kubernetes.io/cpu: "16" capacity.cluster-autoscaler.kubernetes.io/labels: "key1=value1,key2=value2" capacity.cluster-autoscaler.kubernetes.io/taints: "key1=value1:NoSchedule,key2=value2:NoExecute"
Specifying a Custom Resource Group
By default all Kubernetes resources consumed by the Cluster API provider will
use the group
cluster.x-k8s.io, with a dynamically acquired version. In
some situations, such as testing or prototyping, you may wish to change this
group variable. For these situations you may use the environment variable
CAPI_GROUP to change the group that the provider will use.
Please note that setting the
CAPI_GROUP environment variable will also cause the
annotations for minimum and maximum size to change.
This behavior will also affect the machine annotation on nodes, the machine deletion annotation,
and the cluster name label. For example, if
then the minimum size annotation key will be
the machine annotation on nodes will be
test.k8s.io/machine, the machine deletion
annotation will be
test.k8s.io/delete-machine, and the cluster name label will be
Specifying a Custom Resource Version
When determining the group version for the Cluster API types, by default the autoscaler
will look for the latest version of the group. For example, if
exist in the
cluster.x-k8s.io group at versions
autoscaler will choose
In some cases it may be desirable to specify which version of the API the cluster autoscaler should use. This can be useful in debugging scenarios, or in situations where you have deployed multiple API versions and wish to ensure that the autoscaler uses a specific version.
CAPI_VERSION environment variable will instruct the autoscaler to use
the version specified. This works in a similar fashion as the API group environment
variable with the exception that there is no default value. When this variable is not
set, the autoscaler will use the behavior described above.
A sample manifest that will create a deployment running the autoscaler is
available. It can be deployed by passing it through
these environment variables to set the namespace to deploy into as well as the image and tag to use:
export AUTOSCALER_NS=kube-system export AUTOSCALER_IMAGE=us.gcr.io/k8s-artifacts-prod/autoscaling/cluster-autoscaler:v1.20.0 envsubst < examples/deployment.yaml | kubectl apply -f-
A note on permissions
cluster-autoscaler-management role for accessing cluster api scalable resources is scoped to
This may not be ideal for all environments (eg. Multi tenant environments).
In such cases, it is recommended to scope it to a
Role mapped to a specific namespace.
Autoscaling with ClusterClass and Managed Topologies
For users using ClusterClass and Managed Topologies the Cluster Topology controller attempts to set MachineDeployment replicas based on the
spec.topology.workers.machineDeployments.replicas field. In order to use the Cluster Autoscaler this field can be left unset in the Cluster definition.
The below Cluster definition shows which field to leave unset:
apiVersion: cluster.x-k8s.io/v1beta1 kind: Cluster metadata: name: "my-cluster" namespace: default spec: clusterNetwork: services: cidrBlocks: ["10.128.0.0/12"] pods: cidrBlocks: ["192.168.0.0/16"] serviceDomain: "cluster.local" topology: class: "quick-start" version: v1.24.0 controlPlane: replicas: 1 workers: machineDeployments: - class: default-worker name: linux ## replicas field is not set. ## replicas: 1
Warning: If the Autoscaler is enabled and the replicas field is set for a
MachineSet the Cluster may enter a broken state where replicas become unpredictable.
If the replica field is unset in the Cluster definition Autoscaling can be enabled as described above
Special note on GPU instances
As with other providers, if the device plugin on nodes that provides GPU resources takes some time to advertise the GPU resource to the cluster, this may cause Cluster Autoscaler to unnecessarily scale out multiple times.
To avoid this, you can configure
kubelet on your GPU nodes to label the node
before it joins the cluster by passing it the
--node-labels flag. For the
CAPI cloudprovider, the label format is as follows:
<gpu-type> is arbitrary.
It is important to note that if you are using the
--gpu-total flag to limit the number
of GPU resources in your cluster that the
<gpu-type> value must match
between the command line flag and the node labels. Setting these values incorrectly
can lead to the autoscaler creating too many GPU resources.
For example, if you are using the autoscaler command line flag
--gpu-total=gfx-hardware:1:2 to limit the number of
to a minimum of 1 and maximum of 2, then you should use the kubelet node label flag
Special note on balancing similar node groups
The Cluster Autoscaler feature to enable balancing similar node groups
(activated with the
--balance-similar-node-groups flag) is a powerful and
popular feature. When enabled, the Cluster Autoscaler will attempt to create
new nodes by adding them in a manner that balances the creation between
similar node groups. With Cluster API, these node groups correspond directly
to the scalable resources associated (usually MachineDeployments and MachineSets)
with the nodes in question. In order for the nodes of these scalable resources
to be considered similar by the Cluster Autoscaler, they must have the same
capacity, labels, and taints for the nodes which will be created from them.
To help assist the Cluster Autoscaler in determining which node groups are
similar, the command line flags
--balancing-label are provided. For an expanded discussion about balancing
similar node groups and the options which are available, please see the
Cluster Autoscaler FAQ.
Because Cluster API can address many different cloud providers, it is important
to configure the balancing labels to ignore provider-specific labels which
are used for carrying zonal information on Kubernetes nodes. The Cluster
Autoscaler implementation for Cluster API does not assume any labels (aside from
the well-known Kubernetes labels)
to be ignored when running. Users must configure their Cluster Autoscaler deployment
to ignore labels which might be different between nodes, but which do not
otherwise affect node behavior or size (for example when two MachineDeployments
are the same except for their deployment zones). The Cluster API community has
decided not to carry cloud provider specific labels in the Cluster Autoscaler
to reduce the possibility for labels to clash between providers. Additionally,
the community has agreed to promote documentation and the use of the
flag as the preferred method of deployment to reduce the extended need for
maintenance on the Cluster Autoscaler when new providers are added or updated.
For further context around this decision, please see the
Cluster API Deep Dive into Cluster Autoscaler Node Group Balancing discussion from 2022-09-12.
The following table shows some of the most common labels used by cloud providers to designate regional or zonal information on Kubernetes nodes. It is shared here as a reference for users who might be deploying on these infrastructures.
|Cloud Provider||Label to ignore||Notes|
|Alibaba Cloud||Used by the Alibaba Cloud CSI driver as a target for persistent volume node affinity|
|AWS||Used by |
|AWS||Used by |
|AWS||Used by EKS to identify node groups|
|AWS||Used by the AWS CNI for custom networking|
|AWS||Used by AWS as a label for spot instances|
|AWS||Used by the AWS EBS CSI driver as a target for persistent volume node affinity|
|Azure||Used as the topology key by the Azure Disk CSI driver|
|Azure||Legacy label used to specify to which Azure node pool a particular node belongs|
|Azure||Used by AKS to identify to which node pool a particular node belongs|
|GCE||Used to specify the zone of the node|
|IBM Cloud||Used by the IBM Cloud Cloud Controller Manager to identify the node|
|IBM Cloud||Used by the IBM Cloud CSI driver as a target for persistent volume node affinity|
|IBM Cloud||Used when a VPC is in use on IBM Cloud|