CPU requests and limits in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding CPU Requests vs Limits

In Kubernetes, what happens when a container exceeds its CPU request but stays below its CPU limit?

A The container is allowed to use unlimited CPU resources.

B The container is throttled to stay within the CPU limit but can use more CPU than requested.

C The container is paused until CPU usage drops below the request.

D The container is immediately terminated for exceeding the CPU request.

💡 Hint

Think about how Kubernetes manages guaranteed CPU resources versus maximum allowed CPU.

CPU requests define the guaranteed CPU for a container. If the container uses more CPU than requested but less than the limit, Kubernetes allows it but may throttle it to not exceed the limit.

💻 Command Output

intermediate

CPU Limit Enforcement Output

Given this pod spec snippet with CPU requests and limits, what will be the CPU usage behavior if the container tries to use 2 CPUs but the limit is set to 1 CPU?

resources:
  requests:
    cpu: "500m"
  limits:
    cpu: "1"

A The container will be throttled and cannot use more than 1 CPU.

B The container will use 2 CPUs without restriction.

C The container will be killed for exceeding the CPU limit.

D The container will only use 500m CPU and no more.

💡 Hint

CPU limits set the maximum CPU a container can use.

The CPU limit of 1 CPU means the container cannot use more than 1 CPU. If it tries to use 2 CPUs, Kubernetes throttles it to 1 CPU.

❓ Configuration

advanced

Correct CPU Resource Configuration

Which of the following Kubernetes container resource configurations correctly sets a CPU request of 250m and a CPU limit of 500m?

A

resources:
  requests:
    cpu: "250m"
  limits:
    cpu: "500m"

B

resources:
  requests:
    cpu: 250
  limits:
    cpu: 500

C

resources:
  requests:
    cpu: "0.25"
  limits:
    cpu: "0.5"

D

resources:
  requests:
    cpu: "250"
  limits:
    cpu: "500"

💡 Hint

CPU values in Kubernetes are usually specified in millicores with 'm' suffix.

CPU requests and limits should be specified as strings with units. "250m" means 250 millicores (0.25 CPU). Options A and D lack units and are invalid. Option A uses decimal notation which is valid but less common; however, Kubernetes expects strings for resource quantities.

❓ Troubleshoot

advanced

Diagnosing CPU Throttling Issues

A developer complains their pod is running slower than expected. The pod has CPU requests and limits set. Which kubectl command helps check if the pod is being CPU throttled?

A kubectl exec <pod-name> -- top

B kubectl get pods --all-namespaces

C kubectl logs <pod-name>

D kubectl describe pod <pod-name>

💡 Hint

Look for events and resource usage details in pod descriptions.

The kubectl describe pod command shows detailed pod info including events and resource usage, which can indicate CPU throttling. Other commands do not provide this info directly.

🔀 Workflow

expert

Optimizing CPU Requests and Limits for a Production Pod

You want to optimize CPU resource settings for a production pod that sometimes spikes in CPU usage but should not be throttled during spikes. Which approach is best?

A Set CPU request to maximum spike and no CPU limit.

B Set CPU request and limit both to the maximum expected spike.

C Set CPU request to average usage and CPU limit to maximum expected spike.

D Set CPU request to zero and CPU limit to maximum spike.

💡 Hint

Think about balancing guaranteed resources and allowing burst capacity.

Setting CPU request to average usage guarantees baseline CPU. Setting limit to max spike allows bursting without throttling. Setting request and limit equal wastes resources. No limit risks node stability. Zero request risks eviction.

Practice

1. What does setting a cpu request in a Kubernetes pod specification do?

easy

A. It guarantees a minimum amount of CPU for the container.

B. It limits the maximum CPU the container can use.

C. It sets the CPU priority for the container.

D. It reserves memory for the container.

Solution

1. Step 1: Understand CPU request meaning

   CPU request is the minimum CPU resource Kubernetes guarantees to a container.

2. Step 2: Differentiate from CPU limit

   CPU limit sets the maximum CPU usage, not the minimum guaranteed.

3. Final Answer:

   It guarantees a minimum amount of CPU for the container. -> Option A

4. Quick Check:

   CPU request = minimum CPU guaranteed [OK]

Hint: Requests guarantee minimum CPU, limits cap maximum CPU [OK]

Common Mistakes:

• Confusing CPU request with CPU limit
• Thinking requests set maximum CPU
• Mixing CPU with memory resource

2. Which of the following is the correct way to specify CPU requests and limits in a pod YAML?

easy

A. resources:
cpu:
request: 500m
limit: 1

B. resources:
requests:
cpu: 500
limits:
cpu: 1000

C. resources:
requests:
cpu: "500m"
limits:
cpu: "1"

D. resources:
requests:
cpu: "1.0"
limits:
cpu: "0.5"

Solution

1. Step 1: Check correct YAML structure for resources

   The correct keys are requests and limits under resources, with CPU values as strings with units like "500m" or "1".

2. Step 2: Validate resources: requests: cpu: "500m" limits: cpu: "1" format

   resources: requests: cpu: "500m" limits: cpu: "1" uses correct keys and units: "500m" means 0.5 CPU, "1" means 1 CPU core.

3. Final Answer:

   resources: requests: cpu: "500m" limits: cpu: "1" -> Option C

4. Quick Check:

   Correct YAML keys and CPU units = resources: requests: cpu: "500m" limits: cpu: "1" [OK]

Hint: Use 'requests' and 'limits' keys with CPU as string with units [OK]

Common Mistakes:

• Using 'request' instead of 'requests'
• Specifying CPU as numbers without quotes or units
• Mixing up 'cpu' and 'memory' keys

3. Given this pod spec snippet:

resources:
  requests:
    cpu: "200m"
  limits:
    cpu: "500m"

What happens if the container tries to use 600m CPU?

medium

A. The container will be killed immediately.

B. The container will be throttled to 500m CPU.

C. The container will be allowed to use 600m CPU.

D. The container will be throttled to 200m CPU.

Solution

1. Step 1: Understand CPU limit effect

   The CPU limit is the maximum CPU the container can use. Here it is 500m (0.5 CPU).

2. Step 2: Predict container behavior when exceeding limit

   If the container tries to use 600m CPU, Kubernetes throttles it to the limit, 500m CPU.

3. Final Answer:

   The container will be throttled to 500m CPU. -> Option B

4. Quick Check:

   CPU limit caps usage = throttled to 500m [OK]

Hint: CPU usage above limit gets throttled to limit [OK]

Common Mistakes:

• Thinking container can exceed CPU limit
• Confusing throttling with killing container
• Assuming CPU request is the max limit

4. You wrote this pod spec:

resources:
  requests:
    cpu: "100m"
  limits:
    cpu: "50m"

What is wrong with this configuration?

medium

A. CPU requests and limits cannot be set together.

B. CPU request is less than CPU limit, which is invalid.

C. CPU values must be integers, not strings.

D. CPU limit is less than CPU request, which is invalid.

Solution

1. Step 1: Compare CPU request and limit values

   CPU request is 100m (0.1 CPU), CPU limit is 50m (0.05 CPU).

2. Step 2: Validate Kubernetes rules for requests and limits

   CPU limit must be equal or greater than CPU request. Here limit < request, which is invalid.

3. Final Answer:

   CPU limit is less than CPU request, which is invalid. -> Option D

4. Quick Check:

   Limit >= Request rule violated = CPU limit is less than CPU request, which is invalid. [OK]

Hint: Limit must be >= request, else config is invalid [OK]

Common Mistakes:

• Setting limit less than request
• Thinking CPU values must be integers
• Believing requests and limits can't coexist

5. You want to ensure a container always gets at least 250m CPU but never uses more than 750m CPU. Which YAML snippet correctly sets this with CPU requests and limits?

hard

A. resources:
requests:
cpu: "250m"
limits:
cpu: "750m"

B. resources:
requests:
cpu: "750m"
limits:
cpu: "250m"

C. resources:
requests:
cpu: 0.25
limits:
cpu: 0.75

D. resources:
requests:
cpu: "0.75"
limits:
cpu: "0.25"

Solution

1. Step 1: Identify correct CPU units and values

   CPU values can be specified as millicores with 'm' suffix, e.g., "250m" for 0.25 CPU.

2. Step 2: Check request and limit order

   Request must be less than or equal to limit. resources: requests: cpu: "250m" limits: cpu: "750m" sets request to 250m and limit to 750m correctly.

3. Final Answer:

   resources: requests: cpu: "250m" limits: cpu: "750m" -> Option A

4. Quick Check:

   Request 250m ≤ Limit 750m with correct units = resources: requests: cpu: "250m" limits: cpu: "750m" [OK]

Hint: Use millicores with 'm' and keep request ≤ limit [OK]

Common Mistakes:

• Swapping request and limit values
• Using decimal numbers without quotes
• Not using millicores for fractional CPU