Pod priority and preemption in Kubernetes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is pod priority in Kubernetes?

Pod priority is a value assigned to pods that indicates their importance. Higher priority pods are scheduled before lower priority ones when resources are limited.

beginner

What does preemption mean in the context of Kubernetes pods?

Preemption means Kubernetes can evict (remove) lower priority pods to free resources for higher priority pods that need to be scheduled.

intermediate

How do you assign a priority to a pod?

You assign a priority by specifying a priorityClassName in the pod's spec, which refers to a PriorityClass resource defining the priority value.

intermediate

What happens if a high priority pod cannot be scheduled due to lack of resources?

Kubernetes tries to preempt lower priority pods by evicting them to free resources so the high priority pod can be scheduled.

advanced

Can preemption cause disruption to running applications?

Yes, preemption evicts pods which can cause temporary downtime or disruption for those lower priority applications.

What does the priorityClassName field in a pod spec do?

A Specifies the container image version

B Defines the namespace of the pod

C Assigns a priority to the pod for scheduling decisions

D Sets the pod's resource limits

priorityClassName links the pod to a PriorityClass resource that sets its scheduling priority.

What triggers preemption in Kubernetes?

A A node goes offline

B A pod exceeds its CPU limits

C A pod crashes repeatedly

D A high priority pod cannot be scheduled due to insufficient resources

Preemption happens when a high priority pod needs resources but none are free, so lower priority pods are evicted.

Which of these is true about pod priority values?

A Higher values mean higher priority

B Lower values mean higher priority

C Priority values must be between 0 and 10

D Priority values are assigned randomly

In Kubernetes, higher numeric priority values indicate higher importance.

What happens to a pod that is preempted?

A It is evicted and terminated to free resources

B It is paused and resumed later

C It is moved to another node automatically

D It continues running without changes

Preempted pods are evicted and terminated to make room for higher priority pods.

Can a pod with no priorityClassName be preempted?

A No, it cannot be preempted

B Yes, it has the lowest priority by default

C Only if it requests more than 1 CPU

D Only if it is in a special namespace

Pods without a priorityClassName get the lowest priority and can be preempted by any pod with a priority.

Explain how pod priority and preemption work together in Kubernetes scheduling.

Think about how Kubernetes decides which pods run when resources are tight.

Describe the impact of preemption on running applications and how to manage it.

Consider what happens when a pod is removed to make space for another.

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