Pod priority and preemption in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Pod Priority in Kubernetes

What happens when a new pod with a higher priority is scheduled but the cluster lacks free resources?

A The new pod waits indefinitely until resources free up without affecting other pods.

B The new pod shares resources with existing pods causing resource contention.

C The new pod is rejected and never scheduled if resources are insufficient.

D The new pod preempts lower priority pods to free resources and gets scheduled immediately.

💡 Hint

Think about how Kubernetes manages resources when priorities differ.

Kubernetes uses pod priority and preemption to ensure higher priority pods can run by evicting lower priority pods if needed.

💻 Command Output

intermediate

Output of Pod Preemption Event

What is the output of the following command when a pod is preempted due to priority?

kubectl describe pod low-priority-pod

A

Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  5m    default-scheduler  Successfully assigned default/low-priority-pod
  Warning Preempted  1m    default-scheduler  Preempted by pod high-priority-pod

B

Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  5m    default-scheduler  Successfully assigned default/low-priority-pod

C Error from server (NotFound): pods "low-priority-pod" not found

D

Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Warning Failed  1m    kubelet  Pod failed to start

💡 Hint

Look for event messages related to preemption in pod description.

The pod description shows an event with reason 'Preempted' indicating it was evicted to make room for a higher priority pod.

❓ Configuration

advanced

Correct Pod Priority Class YAML

Which YAML snippet correctly defines a PodPriorityClass named high-priority with value 1000 and global default set to false?

A

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: "1000"
globalDefault: false
description: "High priority pods"

B

apiVersion: v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000
globalDefault: false
description: "High priority pods"

C

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000
globalDefault: false
description: "High priority pods"

D

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000
globalDefault: true
description: "High priority pods"

💡 Hint

Check the correct apiVersion and data types for fields.

The correct apiVersion is scheduling.k8s.io/v1. The value must be an integer, not a string. globalDefault is a boolean. Option C matches all requirements.

❓ Troubleshoot

advanced

Troubleshooting Pod Not Preempting

A high priority pod is stuck in Pending state even though lower priority pods exist. What could be a reason?

A The lower priority pods have <code>podDisruptionBudget</code> preventing eviction.

B The high priority pod's priority class is not defined or missing in the cluster.

C The cluster has no nodes available at all, so preemption cannot occur.

D The high priority pod has <code>tolerations</code> that do not match any node taints.

💡 Hint

Think about what can block eviction of pods even if they have lower priority.

PodDisruptionBudgets can prevent eviction of pods even if preemption is needed, causing higher priority pods to remain pending.

🔀 Workflow

expert

Pod Scheduling and Preemption Order

Given these events, what is the correct order of actions Kubernetes takes when scheduling a high priority pod that requires preemption?

A 1,2,4,3

B 1,2,3,4

C 2,1,3,4

D 1,3,2,4

💡 Hint

Think about the logical sequence from detecting resource shortage to scheduling.

The scheduler first detects resource shortage, then selects pods to preempt, evicts them, and finally schedules the new pod.

Practice

1. What is the main purpose of pod priority in Kubernetes?

easy

A. To decide which pods get scheduled first when resources are limited

B. To assign network policies to pods

C. To control pod security settings

D. To manage pod storage volumes

Solution

1. Step 1: Understand pod priority concept

   Pod priority is used to rank pods by importance for scheduling decisions.

2. Step 2: Identify scheduling impact

   When resources are scarce, higher priority pods are scheduled before lower priority ones.

3. Final Answer:

   To decide which pods get scheduled first when resources are limited -> Option A

4. Quick Check:

   Pod priority controls scheduling order = A [OK]

Hint: Pod priority ranks pods for scheduling under resource limits [OK]

Common Mistakes:

• Confusing priority with security or network policies
• Thinking priority controls storage or volumes
• Assuming priority affects pod runtime behavior

2. Which YAML field correctly assigns a priority class to a pod?

easy

A. spec.priority: high-priority

B. metadata.priorityClass: high-priority

C. metadata.priorityClassName: high-priority

D. spec.priorityClassName: high-priority

Solution

1. Step 1: Recall pod spec fields

   The priority class is set under the pod's spec using priorityClassName.

2. Step 2: Verify correct YAML syntax

   Correct field is spec.priorityClassName, not metadata or spec.priority.

3. Final Answer:

   spec.priorityClassName: high-priority -> Option D

4. Quick Check:

   Priority class set in spec.priorityClassName = C [OK]

Hint: Use spec.priorityClassName to assign priority class in pod YAML [OK]

Common Mistakes:

• Placing priorityClassName under metadata instead of spec
• Using spec.priority instead of priorityClassName
• Misspelling the field name

3. Given these pods with priorities: Pod A (priority 1000), Pod B (priority 500), Pod C (priority 1000). If resources are limited and only two pods can run, which pods will Kubernetes schedule?

medium

A. Pod B and Pod C

B. Pod A and Pod C

C. Pod A and Pod B

D. Pod B only

Solution

1. Step 1: Compare pod priorities

   Pod A and Pod C have priority 1000, Pod B has 500.

2. Step 2: Select highest priority pods

   Kubernetes schedules pods with highest priority first, so Pod A and Pod C run.

3. Final Answer:

   Pod A and Pod C -> Option B

4. Quick Check:

   Highest priority pods scheduled = A [OK]

Hint: Schedule pods with highest priority first when resources are limited [OK]

Common Mistakes:

• Choosing pods with lower priority over higher
• Assuming all pods run regardless of resources
• Ignoring equal priority pods

4. You created a pod with priorityClassName: "high-priority" but it is not preempting lower priority pods when scheduled. What could be the reason?

medium

A. The priority class "high-priority" is not defined in the cluster

B. Pod preemption is disabled by default in Kubernetes

C. The pod spec is missing resource requests

D. The pod is running on a node with no other pods

Solution

1. Step 1: Check priority class existence

   If the priority class "high-priority" is not defined, the pod cannot use it for preemption.

2. Step 2: Understand preemption behavior

   Preemption requires a valid priority class; missing it disables preemption.

3. Final Answer:

   The priority class "high-priority" is not defined in the cluster -> Option A

4. Quick Check:

   Undefined priority class blocks preemption = D [OK]

Hint: Ensure priority class exists before expecting preemption [OK]

Common Mistakes:

• Assuming preemption is disabled by default
• Ignoring missing resource requests impact
• Not checking node pod status

5. You have a cluster with pods of different priorities. A new pod with priority 2000 is pending due to lack of resources. How does Kubernetes handle this situation?

hard

A. It waits until resources are manually freed without preemption

B. It schedules the new pod alongside others, causing resource overcommit

C. It preempts lower priority pods to free resources for the new pod

D. It deletes the new pod automatically due to insufficient resources

Solution

1. Step 1: Understand preemption role

   When a high priority pod is pending, Kubernetes can preempt lower priority pods to free resources.

2. Step 2: Apply preemption behavior

   Kubernetes stops lower priority pods to schedule the higher priority pod if possible.

3. Final Answer:

   It preempts lower priority pods to free resources for the new pod -> Option C

4. Quick Check:

   High priority pod triggers preemption = B [OK]

Hint: High priority pods preempt lower ones to get scheduled [OK]

Common Mistakes:

• Thinking Kubernetes waits without preemption
• Assuming resource overcommit happens automatically
• Believing new pods get deleted if pending