Pod priority and preemption in Kubernetes - Step-by-Step Execution

Process Flow - Pod priority and preemption

Pod submitted

↓

Check node resources

↓

Enough resources?

Yes→Schedule pod

No↓

Check pod priorities

↓

Preempt lower priority pods?

Yes→Evict lower priority pods

No↓

Pod remains pending

When a pod is submitted, Kubernetes checks if nodes have enough resources. If not, it tries to preempt lower priority pods to make room for higher priority pods.

Execution Sample

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: high-priority-pod
spec:
  priorityClassName: high-priority
  containers:
  - name: app
    image: nginx

This pod manifest defines a pod with a high priority class to influence scheduling and preemption.

Process Table

Step	Action	Node Resources Available	Pod Priority	Lower Priority Pods Found	Preemption Action	Pod Scheduling Result
1	Pod submitted	CPU: 2 cores, Memory: 4Gi free	High	N/A	N/A	Check resources
2	Check node resources	CPU: 2 cores, Memory: 4Gi free	High	N/A	N/A	Not enough resources for pod (needs 3 cores)
3	Check pod priorities	CPU: 2 cores, Memory: 4Gi free	High	Yes, low-priority pods using 2 cores	Evict low-priority pods	Evicted pods to free resources
4	Schedule pod	CPU: 3 cores free after eviction	High	N/A	N/A	Pod scheduled successfully
5	End	CPU: 1 core free after pod scheduled	High	N/A	N/A	Scheduling complete

💡 Pod scheduled after evicting lower priority pods to free resources

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Node CPU Free	2 cores	2 cores	3 cores	1 core	1 core
Pod Priority	N/A	High	High	High	High
Lower Priority Pods Present	N/A	N/A	Yes	No	No
Pod Scheduled	No	No	No	Yes	Yes

Key Moments - 3 Insights

Why does Kubernetes evict pods before scheduling a new pod?

Kubernetes evicts lower priority pods to free resources for higher priority pods, as shown in step 3 where eviction happens to make room.

What happens if no lower priority pods are found during scheduling?

If no lower priority pods exist to evict, the new pod remains pending, as the scheduler cannot free resources (not shown in this trace but implied after step 2).

Does pod priority affect scheduling if resources are already sufficient?

No, if resources are enough, the pod schedules directly without preemption, as in step 1 when resources are checked.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Kubernetes evict lower priority pods?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Preemption Action' column in the execution table.

According to the variable tracker, what is the node CPU free after step 4?

A 2 cores

B 3 cores

C 1 core

D 4 cores

💡 Hint

Look at the 'Node CPU Free' row under 'After Step 4' in the variable tracker.

If the pod priority was low instead of high, what would happen in the execution table?

A Pod would remain pending without eviction

B Lower priority pods would be evicted

C Pod would be scheduled immediately

D Scheduler would increase node resources

💡 Hint

Refer to the 'Pod Priority' and 'Preemption Action' columns in the execution table.

Concept Snapshot

Pod priority and preemption in Kubernetes:
- Pods have priority classes (high, low, etc.)
- Scheduler checks node resources before placing pods
- If resources insufficient, scheduler evicts lower priority pods
- Eviction frees resources for higher priority pods
- If no eviction possible, pod stays pending

Full Transcript

When a pod is submitted, Kubernetes checks if nodes have enough resources to run it. If resources are enough, the pod is scheduled immediately. If not, Kubernetes looks for lower priority pods running on nodes. It evicts those lower priority pods to free resources for the higher priority pod. After eviction, the pod is scheduled. If no lower priority pods can be evicted, the pod remains pending until resources free up. This process ensures important pods get resources first.