In this short article, I’ll go through the prioritizing pods in Kubernetes.

  • By default, the kube-scheduler doesn’t have any priorities.

  • Scheduling priority is managed using PriorityClass resource.

  • The higher value of the PriorityClass gives higher priority

  • When resources are scarce, lower-priority pods are evicted to make room for higher-priority ones.

  • By using preemptionPolicy with the never value will prevent eviction for pods, even for high-priority ones.

  • Using a certain priority PriorityClass is managed through priorityClassName.

  • The defined PriorityClass can be set as globalDefault, which means that all pods without specified class will be scheduled with this class.

Example commands

  • Creating priorityClass named high-priority, with priority value 1000 and preemtion-policy Never
[admin@workstation ~]$ kubectl create priorityclass high-priority --value=1000 --description="High priority class" --preemption-policy="Never"
priorityclass.scheduling.k8s.io/high-priority created
[admin@workstation ~]$
[admin@workstation ~]$ kubectl get priorityclasses.scheduling.k8s.io
NAME                      VALUE        GLOBAL-DEFAULT   AGE   PREEMPTIONPOLICY
high-priority             1000         false            10s   Never
system-cluster-critical   2000000000   false            13d   PreemptLowerPriority
system-node-critical      2000001000   false            13d   PreemptLowerPriority
[admin@workstation ~]$
  • Creating another priorityClass named mid-priority, with priority value 125. This class is predefined as default global class
[admin@workstation ~]$ kubectl create priorityclass mid-priority --value=125 --description="mid priority" --global-default=true
priorityclass.scheduling.k8s.io/mid-priority created
[admin@workstation ~]$
[admin@workstation ~]$ kubectl get priorityclasses.scheduling.k8s.io
NAME                      VALUE        GLOBAL-DEFAULT   AGE   PREEMPTIONPOLICY
high-priority             1000         false            13m   Never
mid-priority              125          true             6s    PreemptLowerPriority
system-cluster-critical   2000000000   false            13d   PreemptLowerPriority
system-node-critical      2000001000   false            13d   PreemptLowerPriority
[admin@workstation ~]$
  • Running testpod
[admin@workstation ~]$ kubectl run testpod --image=nginx
pod/testpod created
[admin@workstation ~]$
  • Check which priorityclass is applied by default
[admin@workstation ~]$ kubectl get pod testpod -o yaml | grep -B 2 -i priorityclass
  preemptionPolicy: PreemptLowerPriority
  priority: 125
  priorityClassName: mid-priority
[admin@workstation ~]$
  • Deploying new image
[admin@workstation ~]$ kubectl create deploy testpod-highprio --image nginx
deployment.apps/testpod-highprio created
[admin@workstation ~]$
  • By default the pod of this deployment has the default priorityClass:
[admin@workstation ~]$ kubectl get pod testpod-highprio-598fcf4467-zj49m -o yaml | grep -B 2 -i priorityClass
  preemptionPolicy: PreemptLowerPriority
  priority: 125
  priorityClassName: mid-priority
[admin@workstation ~]$
  • Changing priorityClass in deployment in spec.template.spec.priorityClassName.

[admin@workstation ~]$ kubectl edit deployments.apps testpod-highprio

spec:
  template:
    spec:
      priorityClassName: high-priority
  • Add check the priorityClass of the pod
[admin@workstation ~]$ kubectl get pod testpod-highprio-c8cb74968-wjh86 -o yaml | grep -B 2 -i priorityClass
  preemptionPolicy: Never
  priority: 1000
  priorityClassName: high-priority
[admin@workstation ~]$

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