Why CPU requests and limits in Kubernetes? - Purpose & Use Cases

The Big Idea

What if one app could silently steal all your CPU and crash your whole system?

The Scenario

Imagine you have a busy kitchen where multiple chefs are cooking different dishes at the same time. Without any rules, some chefs might take too many pots and pans, leaving others with nothing to work with.

The Problem

Manually managing how much CPU each application gets is like trying to tell chefs how many pots to use without any system. It's slow, confusing, and often leads to some apps hogging all the CPU while others starve, causing crashes or slowdowns.

The Solution

CPU requests and limits in Kubernetes act like a smart kitchen manager. They set clear rules on how much CPU each app can expect and the maximum it can use, so every app gets a fair share and the system stays stable.

Before vs After

✗ Before

No CPU settings, apps compete freely for CPU

✓ After

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

What It Enables

It enables reliable and efficient use of CPU resources, preventing any app from slowing down others or crashing the whole system.

Real Life Example

In a busy online store, CPU requests and limits ensure the checkout service always has enough CPU to process orders quickly, even when the product catalog service is using a lot of CPU for updates.

Key Takeaways

Manual CPU management is confusing and risky.

CPU requests and limits set fair and clear CPU usage rules.

This keeps apps stable and the system efficient.

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