Memory requests and limits in Kubernetes - Time & Space Complexity

Time Complexity: Memory requests and limits

O(n)

Understanding Time Complexity

We want to understand how the time to schedule and run pods changes as we increase memory requests and limits in Kubernetes.

How does setting memory requests and limits affect the system's work as more pods are added?

Scenario Under Consideration

Analyze the time complexity of the following Kubernetes pod spec snippet.

apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  containers:
  - name: example-container
    image: nginx
    resources:
      requests:
        memory: "256Mi"
      limits:
        memory: "512Mi"

This snippet sets memory requests and limits for a container inside a pod, guiding how the scheduler allocates resources.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The Kubernetes scheduler checks each pod's memory requests against available nodes.
• How many times: For each pod, the scheduler compares memory requests with nodes, repeating this for all pods and nodes.

How Execution Grows With Input

As the number of pods increases, the scheduler must check more memory requests against nodes.

Input Size (n pods)	Approx. Operations (pod-node checks)
10	~10 x number_of_nodes
100	~100 x number_of_nodes
1000	~1000 x number_of_nodes

Pattern observation: The work grows linearly with the number of pods, as each pod's memory request is checked against nodes.

Final Time Complexity

Time Complexity: O(n)

This means the scheduling time grows roughly in direct proportion to the number of pods requesting memory.

Common Mistake

[X] Wrong: "Setting higher memory limits makes scheduling time grow exponentially."

[OK] Correct: The scheduler checks requests linearly per pod; limits affect runtime but not the scheduling checks count.

Interview Connect

Understanding how resource requests affect scheduling helps you explain system behavior clearly and shows you grasp how Kubernetes manages resources efficiently.

Self-Check

"What if we added node selectors to pods? How would that change the time complexity of scheduling with memory requests?"

Practice

1. What does the memory request specify in a Kubernetes pod configuration?

easy

A. The total memory available on the node

B. The maximum amount of memory the container can use

C. The amount of memory the container has currently used

D. The minimum amount of memory guaranteed for the container

Solution

1. Step 1: Understand memory request meaning

   Memory request is the minimum memory Kubernetes guarantees to a container so it can run without being killed due to lack of memory.

2. Step 2: Differentiate from memory limit

   Memory limit is the max memory allowed, not the minimum guaranteed.

3. Final Answer:

   The minimum amount of memory guaranteed for the container -> Option D

4. Quick Check:

   Memory request = minimum guaranteed memory [OK]

Hint: Requests = minimum guaranteed memory, limits = max allowed [OK]

Common Mistakes:

• Confusing memory request with memory limit
• Thinking request is the max memory
• Assuming request is current usage

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

easy

A. resources:
requests:
memory: "256Mi"
limits:
memory: "512Mi"

B. resources:
memory:
request: 256
limit: 512

C. memory:
requests: 256Mi
limits: 512Mi

D. resources:
requests:
mem: 256Mi
limits:
mem: 512Mi

Solution

1. Step 1: Recall correct YAML structure for resources

   In Kubernetes, memory requests and limits are under resources.requests.memory and resources.limits.memory with values as strings including units like "Mi".

2. Step 2: Check each option for syntax correctness

   resources: requests: memory: "256Mi" limits: memory: "512Mi" matches the correct syntax. Options A, B, and C use wrong keys or structure.

3. Final Answer:

   resources: requests: memory: "256Mi" limits: memory: "512Mi" -> Option A

4. Quick Check:

   Correct YAML keys and units = resources: requests: memory: "256Mi" limits: memory: "512Mi" [OK]

Hint: Use resources.requests.memory and resources.limits.memory keys [OK]

Common Mistakes:

• Using wrong keys like 'mem' or 'memory' outside resources
• Omitting quotes or units for memory values
• Swapping 'request' and 'requests'

3. Given this pod spec snippet:

resources:
  requests:
    memory: "128Mi"
  limits:
    memory: "256Mi"

What happens if the container tries to use 300Mi of memory?

medium

A. The container is allowed to use 300Mi without issues

B. The container is throttled but not killed

C. The container is killed by Kubernetes for exceeding the memory limit

D. The container's memory usage is capped at 256Mi but continues running

Solution

1. Step 1: Understand memory limit enforcement

   Memory limit is the maximum memory a container can use. If it exceeds this, Kubernetes kills the container.

2. Step 2: Apply to given values

   The limit is 256Mi. Using 300Mi exceeds this limit, so the container will be terminated.

3. Final Answer:

   The container is killed by Kubernetes for exceeding the memory limit -> Option C

4. Quick Check:

   Memory usage > limit = container killed [OK]

Hint: Memory over limit kills container, not throttled [OK]

Common Mistakes:

• Thinking container is throttled instead of killed
• Confusing requests with limits
• Assuming container can burst beyond limit safely

4. You set memory requests to 512Mi and limits to 256Mi in your pod YAML. What is the problem?

medium

A. Memory limits cannot be lower than 256Mi

B. Memory requests cannot be higher than memory limits

C. This configuration is valid and will work fine

D. Requests and limits must be equal

Solution

1. Step 1: Recall relationship between requests and limits

   Memory requests must be less than or equal to memory limits. Requests are minimum guaranteed, limits are max allowed.

2. Step 2: Analyze given values

   Requests = 512Mi, Limits = 256Mi. Requests are higher than limits, which is invalid and causes an error.

3. Final Answer:

   Memory requests cannot be higher than memory limits -> Option B

4. Quick Check:

   Requests ≤ Limits rule violated [OK]

Hint: Requests must be ≤ limits, else config error [OK]

Common Mistakes:

• Setting requests higher than limits
• Assuming limits can be lower than requests
• Thinking requests and limits can be any values

5. You want to ensure your pod always has at least 200Mi memory but never uses more than 400Mi. How should you set memory requests and limits in the pod spec?

hard

A. requests.memory: "200Mi" and limits.memory: "400Mi"

B. requests.memory: "400Mi" and limits.memory: "200Mi"

C. requests.memory: "200Mi" only, no limits

D. limits.memory: "400Mi" only, no requests

Solution

1. Step 1: Identify minimum guaranteed memory

   Minimum memory guaranteed is set by requests.memory, so set it to 200Mi.

2. Step 2: Identify maximum allowed memory

   Maximum memory allowed is set by limits.memory, so set it to 400Mi.

3. Step 3: Verify correct order

   Requests must be ≤ limits, so 200Mi ≤ 400Mi is valid.

4. Final Answer:

   requests.memory: "200Mi" and limits.memory: "400Mi" -> Option A

5. Quick Check:

   Requests = 200Mi, Limits = 400Mi [OK]

Hint: Requests = min guaranteed, limits = max allowed [OK]

Common Mistakes:

• Swapping requests and limits values
• Setting only requests or only limits
• Setting requests higher than limits