Managing the maximum number of Pods per Node

Configuring the maximum number of pods per Node

In OpenShift Container Platform, you can configure the number of pods that can run on a node based on the number of processor cores on the node, a hard limit or both. If you use both options, the lower of the two limits the number of pods on a node.

Exceeding these values can result in:

Increased CPU utilization by OpenShift Container Platform.
Slow pod scheduling.
Potential out-of-memory scenarios, depending on the amount of memory in the node.
Exhausting the IP address pool.
Resource overcommitting, leading to poor user application performance.

A pod that is holding a single container actually uses two containers. The second container sets up networking prior to the actual container starting. As a result, a node running 10 pods actually has 20 containers running.

The podsPerCore parameter limits the number of pods the node can run based on the number of processor cores on the node. For example, if podsPerCore is set to 10 on a node with 4 processor cores, the maximum number of pods allowed on the node is 40.

The maxPods parameter limits the number of pods the node can run to a fixed value, regardless of the properties of the node.

Configuring the maximum number of pods per Node

Two parameters control the maximum number of pods that can be scheduled to a node: podsPerCore and maxPods. If you use both options, the lower of the two limits the number of pods on a node.

For example, if podsPerCore is set to 10 on a node with 4 processor cores, the maximum number of pods allowed on the node will be 40.

Prerequisite

Obtain the label associated with the static Machine Config Pool CRD for the type of node you want to configure. Perform one of the following steps:

View the machine config pool:

$ oc describe machineconfigpool <name>

For example:

$ oc describe machineconfigpool worker

apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfigPool
metadata:
  creationTimestamp: 2019-02-08T14:52:39Z
  generation: 1
  labels:
    custom-kubelet: small-pods (1)

1	If a label has been added it appears under `labels`.

If the label is not present, add a key/value pair:

$ oc label machineconfigpool worker custom-kubelet=small-pods

Procedure

Create a custom resource (CR) for your configuration change.

Sample configuration for a max-pods CR

apiVersion: machineconfiguration.openshift.io/v1
kind: KubeletConfig
metadata:
  name: set-max-pods (1)
spec:
  machineConfigPoolSelector:
    matchLabels:
      custom-kubelet: small-pods (2)
  kubeletConfig:
    podsPerCore: 10 (3)
    maxPods: 250 (4)

1	Assign a name to CR.
2	Specify the label to apply the configuration change.
3	Specify the number of pods the node can run based on the number of processor cores on the node.
4	Specify the number of pods the node can run to a fixed value, regardless of the properties of the node.

Setting podsPerCore to 0 disables this limit.

In the above example, the default value for podsPerCore is 10 and the default value for maxPods is 250. This means that unless the node has 25 cores or more, by default, podsPerCore will be the limiting factor.

List the machine config pool CRDs to see if the change is applied. The UPDATING column reports True if the change is picked up by the Machine Config Controller:

$ oc get machineconfigpools
NAME     CONFIG                        UPDATED   UPDATING   DEGRADED
master   master-9cc2c72f205e103bb534   False     False      False
worker   worker-8cecd1236b33ee3f8a5e   False     True       False

Once the change is complete, the UPDATED column reports True.

$ oc get machineconfigpools
NAME     CONFIG                        UPDATED   UPDATING   DEGRADED
master   master-9cc2c72f205e103bb534   False     True       False
worker   worker-8cecd1236b33ee3f8a5e   True      False      False

Configuring the maximum number of pods per Node