new task for config pod density (#1237)

* new task for config pod density

* fixup

* use env var instead of of arg

* revise

website/content/en/preview/tasks/pod-density.md

@@ -0,0 +1,59 @@
+---
+title: "Control Pod Density"
+linkTitle: "Control Pod Density"
+weight: 20
+---
+
+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](https://github.com/awslabs/amazon-eks-ami/blob/master/files/eni-max-pods.txt) in the default configuration. 
+
+## Max Pods
+
+Do not use the `max-pods` argument to kubelet. Karpenter is not aware of this value. For example, Karpenter may provision an instance expecting it to accommodate more pods than this static limit. 
+
+## Increase Pod Density
+
+### Networking Limitations 
+
+*☁️ AWS Specific*
+
+The number of pods on a node is limited by the number of networking interfaces (ENIs) that may be attached to a node. 
+
+[AWS VPC CNI v1.9 introduced prefix assignment.](https://aws.amazon.com/blogs/containers/amazon-vpc-cni-increases-pods-per-node-limits/) In short, a single ENI can provide IP addresses for multiple pods. Much higher pod densities are now supported. 
+
+Run the Karpenter controller with the enviornment variable `AWS_ENI_LIMITED_POD_DENSITY` (or the argument  `--aws-eni-limited-pod-density=true`) to enable nodes with more than 110 pods. 
+
+Environment variables for the Karpenter controller may be specified as [helm chart values](https://github.com/aws/karpenter/blob/c73f425e924bb64c3f898f30ca5035a1d8591183/charts/karpenter/values.yaml#L15). 
+
+## 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'
+```
+
+
+
+