About cluster autoscaling

This page explains how Google Kubernetes Engine (GKE) automatically resizes your Standard cluster's node pools based on the demands of your workloads. When demand is high, the cluster autoscaler adds nodes to the node pool. To learn how to configure the cluster autoscaler, see Autoscaling a cluster.

This page is for Admins, Architects, and Operators who plan capacity and infrastructure needs and optimize systems architecture and resources to ensure the lowest total cost of ownership for their company or business unit. To learn more about common roles and example tasks that we reference in Google Cloud content, see Common GKE Enterprise user roles and tasks.

With Autopilot clusters, you don't need to worry about provisioning nodes or managing node pools because node pools are automatically provisioned through node auto-provisioning, and are automatically scaled to meet the requirements of your workloads.

Best practice:

Plan and design your cluster configuration with your organization's Admins and architects, Developers, or other team who is responsible for the implementation and maintenance of your application.

Why use cluster autoscaler

GKE's cluster autoscaler automatically resizes the number of nodes in a given node pool, based on the demands of your workloads. When demand is low, the cluster autoscaler scales back down to a minimum size that you designate. This can increase the availability of your workloads when you need it, while controlling costs. You don't need to manually add or remove nodes or over-provision your node pools. Instead, you specify a minimum and maximum size for the node pool, and the rest is automatic.

If resources are deleted or moved when autoscaling your cluster, your workloads might experience transient disruption. For example, if your workload consists of a controller with a single replica, that replica's Pod might be rescheduled onto a different node if its current node is deleted. Before enabling cluster autoscaler, design your workloads to tolerate potential disruption or ensure that critical Pods are not interrupted.

Best practice:

To increase your workload's tolerance to interruption, deploy your workload using a controller with multiple replicas, such as a Deployment.

You can increase the cluster autoscaler performance with Image streaming, which remotely streams required image data from eligible container images while simultaneously caching the image locally to allow workloads on new nodes to start faster.

How cluster autoscaler works

Cluster autoscaler works per node pool. When you configure a node pool with cluster autoscaler, you specify a minimum and maximum size for the node pool.

Cluster autoscaler increases or decreases the size of the node pool automatically by adding or removing virtual machine (VM) instances in the underlying Compute Engine Managed Instance Group (MIG) for the node pool. Cluster autoscaler makes these scaling decisions based on the resource requests (rather than actual resource utilization) of Pods running on that node pool's nodes. It periodically checks the status of Pods and nodes, and takes action:

  • If Pods fail to be scheduled on any of the current nodes, the cluster autoscaler adds nodes, up to the maximum size of the node pool. For more information about when cluster autoscaler change the size of a cluster, see When does Cluster Autoscaler change the size of a cluster?
  • If GKE decides to add new nodes into the node pool, cluster autoscaler adds as many nodes as needed, up to per-nodepool or per-cluster limits.
  • Cluster autoscaler doesn't wait for one node to come up before creating the next one. Once GKE decides how many nodes to create, node creation happens in parallel. The objective is to minimize the time needed for unschedulable Pods to become Active.
  • If some nodes fail to be created due to quota exhaustion, Cluster autoscaler wait while until resources can be successfully scheduled.
  • If nodes are underutilized, and all Pods could be scheduled even with fewer nodes in the node pool, Cluster autoscaler removes nodes, down to the minimum size of the node pool. If there are Pods on a node that cannot move to other nodes in the cluster, cluster autoscaler does not attempt to scale down that node. If Pods can be moved to other nodes, but the node cannot be drained gracefully after a timeout period (currently 10 minutes), the node is forcibly terminated. The grace period is not configurable for GKE clusters. For more information about how scale down works, see the cluster autoscaler documentation.

The frequency at which cluster autoscaler inspects a cluster for unschedulable Pods largely depends on the cluster's size. In small clusters, the inspection might happen every few seconds. It is not possible to define an exact timeframe required for this inspection.

If your nodes are experiencing shortages because your Pods have requested or defaulted to insufficient resources, the cluster autoscaler does not correct the situation. You can help ensure cluster autoscaler works as accurately as possible by making explicit resource requests for all of your workloads.

Don't enable Compute Engine autoscaling for managed instance groups for your cluster nodes. GKE's cluster autoscaler is separate from Compute Engine autoscaling. This can lead to node pools failing to scale up or scale down because the Compute Engine autoscaler will be in conflict with GKE's cluster autoscaler.

Operating criteria

When resizing a node pool, the cluster autoscaler makes the following assumptions:

  • All replicated Pods can be restarted on some other node, possibly causing a brief disruption.
  • Users or administrators are not manually managing nodes. Cluster autoscaler can override any manual node management operations you perform.
  • All nodes in a single node pool have the same set of labels.
  • Cluster autoscaler considers the relative cost of the instance types in the various pools, and attempts to expand the least expensive possible node pool. The cluster autoscaler takes into account the reduced cost of node pools containing Spot VMs, which are preemptible.
  • Cluster autoscaler considers the init container requests before scheduling Pods. Init container requests can use any unallocated resources available on the nodes, which might prevent Pods from being scheduled. Cluster autoscaler follows the same request calculation rules that Kubernetes uses. To learn more, see the Kubernetes documentation for using init containers.
  • Labels that are manually added after initial cluster or node pool creation are not tracked. Nodes that are created by the cluster autoscaler are assigned labels specified with --node-labels at the time of node pool creation.
  • In GKE version 1.21 or earlier, cluster autoscaler considers the taint information of the existing nodes from a node pool to represent the whole node pool. Starting in GKE version 1.22, cluster autoscaler combines information from existing nodes in the cluster and the node pool. Cluster autoscaler also detects the manual changes you make to the node and node pool.
Best practice:

Don't enable the cluster autoscaler if your applications are not disruption-tolerant.

Balancing across zones

If your node pool contains multiple managed instance groups with the same instance type, the cluster autoscaler attempts to keep these managed instance group sizes balanced when scaling up. This helps prevent an uneven distribution of nodes among managed instance groups in multiple zones of a node pool. GKE does not consider the autoscaling policy when scaling down.

Cluster autoscaler only balances across zones during a scale-up event. Cluster autoscaler scales down underutilized nodes regardless of the relative sizes of underlying managed instance groups in a node pool, which can cause the nodes to be distributed unevenly across zones.

Location policy

Starting in GKE version 1.24.1-gke.800, you can change the location policy of the cluster autoscaler. You can control the cluster autoscaler distribution policy by specifying the location_policy flag with any of the following values:

  • BALANCED: The cluster autoscaler considers Pod requirements and the availability of resources in each zone. This does not guarantee similar node groups will have exactly the same sizes, because the cluster autoscaler considers many factors, including available capacity in a given zone and zone affinities of Pods that triggered the scale-up.
  • ANY: The cluster autoscaler prioritizes utilization of unused reservations and accounts for current constraints of available resources.
Best practice:

Use the ANY policy if you are using Spot VMs or if you want to use VM reservations that are not equal between zones.

Reservations

Starting in GKE version 1.27, the cluster autoscaler always considers reservations when making the scale-up decisions. The node pools with matching unused reservations are prioritized when choosing the node pool to scale up, even when the node pool is not the most efficient one. Additionally, unused reservations are always prioritized when balancing multi-zonal scale-ups.

Default values

For Spot VMs node pools, the default cluster autoscaler distribution policy is ANY. In this policy, Spot VMs have a lower risk of being preempted.

For non-preemptible node pools, the default cluster autoscaler distribution policy is BALANCED.

Minimum and maximum node pool size

When creating a new node pool, you can specify the minimum and maximum size for each node pool in your cluster, and the cluster autoscaler makes rescaling decisions within these scaling constraints. To update the minimum size, manually resize the cluster to a size within the new constraints after specifying the new minimum value. The cluster autoscaler then makes rescaling decisions based on the new constraints.

Current node pool size Cluster autoscaler action Scaling constraints
Lower than the minimum you specified Cluster autoscaler scales up to provision pending pods. Scaling down is disabled. The node pool does not scale down below the value you specified.
Within the minimum and maximum size you specified Cluster autoscaler scales up or down according to demand. The node pool stays within the size limits you specified.
Greater than the maximum you specified Cluster autoscaler scales down only the nodes that can be safely removed. Scaling up is disabled. The node pool does not scale above the value you specified.

On Standard clusters, the cluster autoscaler never automatically scales down a cluster to zero nodes. One or more nodes must always be available in the cluster to run system Pods. Additionally, if the current number of nodes is zero due to manual removal of nodes, cluster autoscaler and node auto-provisioning can scale up from zero node clusters.

To learn more about autoscaler decisions, see cluster autoscaler limitations.

Autoscaling limits

You can set the minimum and maximum number of nodes for the cluster autoscaler to use when scaling a node pool. Use the --min-nodes and --max-nodes flags to set the minimum and maximum number of nodes per zone

Starting in GKE version 1.24, you can use the --total-min-nodes and --total-max-nodes flags for new clusters. These flags set the minimum and maximum number of the total number of nodes in the node pool across all zones.

Min and max nodes example

The following command creates an autoscaling multi-zonal cluster with six nodes across three zones initially, with a minimum of one node per zone and a maximum of four nodes per zone:

gcloud container clusters create example-cluster \
    --num-nodes=2 \
    --zone=us-central1-a \
    --node-locations=us-central1-a,us-central1-b,us-central1-f \
    --enable-autoscaling --min-nodes=1 --max-nodes=4

In this example, the total size of the cluster can be between three and twelve nodes, spread across the three zones. If one of the zones fails, the total size of the cluster can be between two and eight nodes.

Total nodes example

The following command, available in GKE version 1.24 or later, creates an autoscaling multi-zonal cluster with six nodes across three zones initially, with a minimum of three nodes and a maximum of twelve nodes in the node pool across all zones:

gcloud container clusters create example-cluster \
    --num-nodes=2 \
    --zone=us-central1-a \
    --node-locations=us-central1-a,us-central1-b,us-central1-f \
    --enable-autoscaling --total-min-nodes=3 --total-max-nodes=12

In this example, the total size of the cluster can be between three and twelve nodes, regardless of spreading between zones.

Autoscaling profiles

The decision of when to remove a node is a trade-off between optimizing for utilization or the availability of resources. Removing underutilized nodes improves cluster utilization, but new workloads might have to wait for resources to be provisioned again before they can run.

You can specify which autoscaling profile to use when making such decisions. The available profiles are:

  • balanced: The default profile that prioritizes keeping more resources readily available for incoming pods and thus reducing the time needed for having them active for Standard clusters. The balanced profile isn't available for Autopilot clusters.
  • optimize-utilization: Prioritize optimizing utilization over keeping spare resources in the cluster. When you enable this profile, the cluster autoscaler scales down the cluster more aggressively. GKE can remove more nodes, and remove nodes faster. GKE prefers to schedule Pods in nodes that already have high allocation of CPU, memory, or GPUs. However, other factors influence scheduling, such as spread of Pods belonging to the same Deployment, StatefulSet or Service, across nodes.

The optimize-utilization autoscaling profile helps the cluster autoscaler to identify and remove underutilized nodes. To achieve this optimization, GKE sets the scheduler name in the Pod spec to gke.io/optimize-utilization-scheduler. Pods that specify a custom scheduler are not affected.

The following command enables optimize-utilization autoscaling profile in an existing cluster:

gcloud container clusters update CLUSTER_NAME \
    --autoscaling-profile optimize-utilization

Considering Pod scheduling and disruption

When scaling down, the cluster autoscaler respects scheduling and eviction rules set on Pods. These restrictions can prevent a node from being deleted by the autoscaler. A node's deletion could be prevented if it contains a Pod with any of these conditions:

  • The Pod's affinity or anti-affinity rules prevent rescheduling.
  • The Pod is not managed by a Controller such as a Deployment, StatefulSet, Job or ReplicaSet.
  • The Pod has local storage and the GKE control plane version is lower than 1.22. In GKE clusters with control plane version 1.22 or later, Pods with local storage no longer block scaling down.
  • The Pod has the "cluster-autoscaler.kubernetes.io/safe-to-evict": "false" annotation.
  • The node's deletion would exceed the configured PodDisruptionBudget.

For more information about cluster autoscaler and preventing disruptions, see the following questions in the Cluster autoscaler FAQ:

Autoscaling TPUs in GKE

GKE supports Tensor Processing Units (TPUs) to accelerate machine learning workloads. Both single-host TPU slice node pool and multi-host TPU slice node pool support autoscaling and auto-provisioning.

With the --enable-autoprovisioning flag on a GKE cluster, GKE creates or deletes single-host or multi-host TPU slice node pools with a TPU version and topology that meets the requirements of pending workloads.

When you use --enable-autoscaling, GKE scales the node pool based on its type, as follows:

  • Single-host TPU slice node pool: GKE adds or removes TPU nodes in the existing node pool. The node pool may contain any number of TPU nodes between zero and the maximum size of the node pool as determined by the --max-nodes and the --total-max-nodes flags. When the node pool scales, all the TPU nodes in the node pool have the same machine type and topology. To learn more how to create a single-host TPU slice node pool, see Create a node pool.

  • Multi-host TPU slice node pool: GKE atomically scales up the node pool from zero to the number of nodes required to satisfy the TPU topology. For example, with a TPU node pool with a machine type ct5lp-hightpu-4t and a topology of 16x16, the node pool contains 64 nodes. The GKE autoscaler ensures that this node pool has exactly 0 or 64 nodes. When scaling back down, GKE evicts all scheduled pods, and drains the entire node pool to zero. To learn more how to create a multi-host TPU slice node pool, see Create a node pool.

Additional information

You can find more information about cluster autoscaler in the Autoscaling FAQ in the open-source Kubernetes project.

Limitations

Cluster autoscaler has the following limitations:

  • Local PersistentVolumes are not supported by the cluster autoscaler.
  • In GKE control plane version earlier than 1.24.5-gke.600, when Pods request ephemeral storage, the cluster autoscaler does not support scaling up a node pool with zero nodes that uses Local SSDs as ephemeral storage.
  • Cluster size limitations: up to 15,000 nodes. Account for other cluster limits and our best practices when running clusters of this size.
  • When scaling down, the cluster autoscaler honors a graceful termination period of 10 minutes for rescheduling the node's Pods onto a different node before forcibly terminating the node.
  • Occasionally, the cluster autoscaler cannot scale down completely and an extra node exists after scaling down. This can occur when required system Pods are scheduled onto different nodes, because there is no trigger for any of those Pods to be moved to a different node. See I have a couple of nodes with low utilization, but they are not scaled down. Why?. To work around this limitation, you can configure a Pod disruption budget.
  • Custom scheduling with altered Filters is not supported.
  • Nodes won't scale up if Pods have a PriorityClass value below -10. Learn more in How does Cluster Autoscaler work with Pod Priority and Preemption?
  • Cluster autoscaler might not have enough unallocated IP address space to use to add new nodes or Pods, resulting in scale-up failures, which are indicated by eventResult events with the reason scale.up.error.ip.space.exhausted. You can add more IP addresses for nodes by expanding the primary subnet, or add new IP addresses for Pods using discontiguous multi-Pod CIDR. For more information, see Not enough free IP space for Pods.
  • GKE cluster autoscaler is different from Cluster autoscaler of the open source Kubernetes project. The parameters of the GKE Cluster autoscaler depend on the cluster configuration and are subject to change. If you need more control over the autoscaling behavior, disable GKE Cluster autoscaler and run Cluster autoscaler of the open source Kubernetes. However, the open source Kubernetes has no Google Cloud support.

Known issues

  • In GKE control plane version prior to 1.22, GKE cluster autoscaler stops scaling up all node pools on empty (zero node) clusters. This behavior doesn't occur in GKE version 1.22 and later.

Troubleshooting

For troubleshooting advice, see the following pages:

What's next