Memory limits and reservations in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Understanding Docker Memory Limit Output

What is the output of the following Docker command when inspecting a container's memory limits?

Docker

docker inspect --format='{{.HostConfig.Memory}}' my_container

A 1073741824

B 0

C 536870912

D Memory limit not set

💡 Hint

If no memory limit is set, Docker returns 0 for the Memory field.

The .HostConfig.Memory field shows the memory limit in bytes. If no limit is set, it returns 0.

🧠 Conceptual

intermediate

Difference Between Memory Limit and Memory Reservation

Which statement correctly describes the difference between Docker's memory limit and memory reservation?

A Memory limit is a hard cap; memory reservation is a soft guarantee.

B Memory limit applies only to CPU, memory reservation applies only to disk.

C Both memory limit and reservation are hard caps on memory usage.

D Memory reservation is a hard cap; memory limit is a soft guarantee.

💡 Hint

Think about which setting restricts usage strictly and which one reserves resources.

Memory limit sets the maximum memory a container can use (hard cap). Memory reservation reserves memory for the container but allows it to use more if available (soft guarantee).

❓ Configuration

advanced

Correct Docker Compose Memory Settings

Which Docker Compose service configuration correctly sets a memory limit of 512MB and a memory reservation of 256MB?

Docker

services:
  app:
    image: myapp
    deploy:
      resources:
        limits:
          memory: 512M
        reservations:
          memory: 256M

A

services:
  app:
    image: myapp
    deploy:
      resources:
        limits:
          memory: 512mb
        reservations:
          memory: 256mb

B

services:
  app:
    image: myapp
    deploy:
      resources:
        limits:
          memory: 512MB
        reservations:
          memory: 256MB

C

services:
  app:
    image: myapp
    deploy:
      resources:
        limits:
          memory: 512
        reservations:
          memory: 256

D

services:
  app:
    image: myapp
    deploy:
      resources:
        limits:
          memory: 512M
        reservations:
          memory: 256M

💡 Hint

Memory values in Docker Compose use uppercase 'M' for megabytes without 'B'.

Docker Compose expects memory values with units like '512M' or '256M'. Adding 'B' or lowercase letters causes errors or misinterpretation.

❓ Troubleshoot

advanced

Diagnosing Memory Limit Enforcement Failure

A container is set with a memory limit of 256MB but uses more memory without being killed. What is the most likely cause?

A The container is running with the --memory-swap flag set to unlimited.

B The container was started without the --memory flag.

C The host machine has swap disabled.

D The container is using the memory reservation setting instead of limit.

💡 Hint

Check if swap settings affect memory limit enforcement.

If --memory-swap is unlimited, the container can use swap beyond the memory limit, so it may not be killed when exceeding the limit.

🔀 Workflow

expert

Order of Steps to Set Memory Limits on a Running Container

What is the correct order of steps to apply a memory limit of 1GB to an already running Docker container named 'webapp'?

A 1,4,2,3

B 2,1,3,4

C 1,2,3,4

D 4,1,2,3

💡 Hint

You cannot change memory limits on a running container without restarting it.

You must stop the container first, then restart it with the new memory limit flag. Afterwards, verify the setting and update configuration files if needed.