Understanding Kubernetes: Part 26 Preemption and Priority

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What is a Pod Priority?

A Pod Priority defines the importance of a Pod relative to others. Higher-priority Pods get scheduled before lower-priority ones. If resources are insufficient, Kubernetes may preempt (evict) lower-priority Pods to make room for higher-priority ones.

Defining a Pod Priority Class

Pod priorities are defined using PriorityClasses, which assign numerical values to indicate importance.

Example: Creating a Priority Class

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000
preemptionPolicy: PreemptLowerPriority
globalDefault: false

• value: Determines the priority level; higher values indicate higher priority.

• preemptionPolicy: Defines whether the Pod can preempt lower-priority Pods (PreemptLowerPriority).

• globalDefault: If true, this priority applies to all Pods by default.

Assigning a Priority to a Pod

After defining a PriorityClass, assign it to a Pod:

apiVersion: v1
kind: Pod
metadata:
  name: critical-app
spec:
  priorityClassName: high-priority
  containers:
    - name: app
      image: my-app

This Pod is now considered high-priority and may preempt lower-priority Pods if needed.

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What is Preemption?

Preemption occurs when a high-priority Pod cannot be scheduled due to resource constraints. Kubernetes then evicts lower-priority Pods to free up resources for the high-priority Pod.

Preemption Process

1. A high-priority Pod cannot be scheduled due to lack of resources.

2. Kubernetes identifies lower-priority Pods that can be evicted.

3. The lower-priority Pods are terminated.

4. The high-priority Pod is scheduled.

Controlling Preemption

By setting preemptionPolicy: Never, you can prevent a Pod from evicting other workloads:

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: non-preempting-priority
value: 500
preemptionPolicy: Never

This ensures that Pods with this priority will not preempt lower-priority Pods.

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Use Cases

1. Ensuring Critical Workloads Run First

• Assign higher priority to critical applications to guarantee their availability.

2. Protecting Low-Priority Batch Jobs

• Set low priority for batch processing jobs to avoid interference with real-time applications.

3. Controlled Eviction of Non-Essential Workloads

• Preempt lower-priority workloads during resource shortages, ensuring vital services remain operational.

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Removing a Priority Class

If a PriorityClass is no longer needed, remove it using:

kubectl delete priorityclass high-priority

This prevents future Pods from using this priority but does not affect running Pods.

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Conclusion

Kubernetes Preemption and Priority allow fine-grained control over workload scheduling, ensuring that essential applications always have the resources they need. By assigning priorities wisely, you can optimize cluster stability and resource allocation efficiently.

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In My Previous Role

As a Senior DevOps Engineer, I leveraged Kubernetes Priority and Preemption to ensure efficient workload scheduling:

• Critical Services: Assigned high-priority classes to essential applications to prevent downtime.

• Batch Jobs: Set lower priorities for non-essential batch jobs to minimize their impact on critical workloads.

• Resource Optimization: Used preemption to dynamically allocate resources based on workload demand, ensuring efficient cluster utilization.

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