CPU requests and limits in Kubernetes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a CPU request in Kubernetes?

A CPU request is the amount of CPU guaranteed to a container. Kubernetes uses this to decide which node can run the container.

beginner

What happens if a container exceeds its CPU limit in Kubernetes?

If a container uses more CPU than its limit, Kubernetes throttles it to prevent it from using more CPU than allowed.

intermediate

Why should you set both CPU requests and limits?

Setting both ensures your container gets enough CPU to run well (requests) and prevents it from using too much CPU (limits), helping cluster stability.

intermediate

How does Kubernetes use CPU requests during scheduling?

Kubernetes sums CPU requests of all containers on a node and only schedules new containers if the node has enough free CPU to meet the request.

beginner

What unit is used to specify CPU requests and limits in Kubernetes?

CPU is specified in cores or millicores (m). For example, 500m means half a CPU core.

What does a CPU limit do in Kubernetes?

A Prevents a container from using more CPU than the limit by throttling it

B Guarantees the container will always get this CPU amount

C Sets the maximum memory a container can use

D Schedules the container on the node with the most CPU

CPU limits stop containers from using more CPU than allowed by throttling their CPU usage.

What unit is used to specify CPU requests in Kubernetes?

A Bytes

B Cores or millicores (m)

C Milliseconds

D Percentage

CPU is specified in cores or millicores (m), where 1000m equals 1 full CPU core.

If a container has a CPU request of 200m and a limit of 500m, what does this mean?

A It is guaranteed 200m CPU and can use up to 500m CPU

B It can only use 200m CPU and no more

C It must use exactly 500m CPU

D It has no CPU guarantees

The container is guaranteed 200m CPU but can burst up to 500m CPU if available.

What happens if you set a CPU limit but no CPU request?

A The container is guaranteed CPU equal to the limit

B The container can use unlimited CPU

C The container has no guaranteed CPU but cannot exceed the limit

D The container will not start

Kubernetes automatically assigns a request that matches the limit, guaranteeing the container CPU equal to the limit.

Why is it important to set CPU requests in Kubernetes?

A To speed up container startup

B To limit the container's CPU usage

C To increase container memory

D To help Kubernetes schedule containers on nodes with enough CPU

CPU requests tell Kubernetes how much CPU a container needs, helping it schedule containers properly.

Explain the difference between CPU requests and CPU limits in Kubernetes.

Think about what Kubernetes guarantees versus what it restricts.

Describe why setting CPU requests and limits helps maintain cluster stability.

Consider how resources are shared in a busy kitchen.

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