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pod-density.md

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Control Pod Density
Control Pod Density
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Learn ways to specify pod density with Karpenter

Pod density is the number of pods per node.

Kubernetes has a default limit of 110 pods per node. If you are using the EKS Optimized AMI on AWS, the number of pods is limited by instance type in the default configuration.

Increase Pod Density

Networking Limitations

☁️ AWS Specific

By default, the number of pods on a node is limited by both the number of networking interfaces (ENIs) that may be attached to an instance type and the number of IP addresses that can be assigned to each ENI. See IP addresses per network interface per instance type for a more detailed information on these instance types' limits.

Karpenter can be configured to disable nodes' ENI-based pod density. This is especially useful for small to medium instance types which have a lower ENI-based pod density.

{{% alert title="Note" color="primary" %}} When using small instance types, it may be necessary to enable prefix assignment mode in the AWS VPC CNI plugin to more pods per node. Prefix assignment mode was introduced in AWS VPC CNI v1.9 and allows ENIs to manage a broader set of IP addresses. Much higher pod densities are supported as a result. {{% /alert %}}

Provisioner-Specific Pod Density

Static Pod Density

Static pod density can be configured at the provisioner level by specifying maxPods within the .spec.kubeletConfiguration. All nodes spawned by this provisioner will set this maxPods value on their kubelet and will account for this value during scheduling.

See Provisioner API Kubelet Configuration for more details.

Dynamic Pod Density

Dynamic pod density (density that scales with the instance size) can be configured at the provisioner level by specifying podsPerCore within the .spec.kubeletConfiguration. Karpenter will calculate the expected pod density for each instance based on the instance's number of logical cores (vCPUs) and will account for this during scheduling.

See Provisioner API Kubelet Configuration for more details.

Controller-Wide Pod Density

{{% alert title="Deprecation Warning" color="warning" %}} AWS_ENI_LIMITED_POD_DENSITY is deprecated in favor of the .spec.kubeletConfiguration.maxPods set at the Provisioner-level {{% /alert %}}

Set the environment variable AWS_ENI_LIMITED_POD_DENSITY: "false" (or the argument --aws-eni-limited-pod-density=false) in the Karpenter controller to allow nodes to host up to 110 pods by default.

Environment variables for the Karpenter controller may be specified as helm chart values.

VPC CNI Custom Networking

By default, the VPC CNI allocates IPs for a node and pods from the same subnet. With VPC CNI Custom Networking, the pods will receive IP addresses from another subnet dedicated to pod IPs. This approach makes it easier to manage IP addresses and allows for separate Network Access Control Lists (NACLs) applied to your pods. VPC CNI Custom Networking reduces the pod density of a node since one of the ENI attachments will be used for the node and cannot share the allocated IPs on the interface to pods. Karpenter supports VPC CNI Custom Networking and similar CNI setups where the primary node interface is separated from the pods interfaces through a global setting within the karpenter-global-settings configmap: aws.reservedENIs. In the common case, aws.reservedENIs should be set to "1" if using Custom Networking.

Limit Pod Density

Generally, increasing pod density is more efficient. However, some use cases exist for limiting pod density.

Topology Spread

You can use [topology spread]({{< relref "scheduling.md#topology-spread" >}}) features to reduce blast radius. For example, spreading workloads across EC2 Availability Zones.

Restrict Instance Types

Exclude large instance sizes to reduce the blast radius of an EC2 instance failure.

Consider setting up upper or lower boundaries on target instance sizes with the node.kubernetes.io/instance-type key.

The following example shows how to avoid provisioning large Graviton instances in order to reduce the impact of individual instance failures:

-key: node.kubernetes.io/instance-type
    operator: NotIn
    values:
      'm6g.16xlarge'
      'm6gd.16xlarge'
      'r6g.16xlarge'
      'r6gd.16xlarge'
      'c6g.16xlarge'