Container resource usage stats in Docker - Commands & Configuration

Introduction

Sometimes containers use too much memory or CPU and slow down your system. Checking resource usage helps you find which container is causing the problem and fix it quickly.

When your server feels slow and you want to see if a container is using too much CPU.

When you want to monitor memory use of a container running a database to avoid crashes.

When you need to check network traffic of a container to troubleshoot slow responses.

When you want to track resource use over time to plan for scaling your app.

When you want to confirm that resource limits you set on containers are working.

Commands

This command lists all running containers so you can see their names and IDs before checking their stats.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE          COMMAND                  CREATED       STATUS       PORTS                    NAMES

b1c2d3e4f5g6   nginx:1.23     "/docker-entrypoint.…"   2 hours ago   Up 2 hours   0.0.0.0:8080->80/tcp     my-nginx

7h8i9j0k1l2m   redis:7.0      "docker-entrypoint.s…"   3 hours ago   Up 3 hours   6379/tcp                 my-redis

This command shows the current CPU, memory, and network usage of the container named 'my-nginx'. The --no-stream flag makes it show the stats once and then stop.

Terminal

docker stats my-nginx --no-stream

Expected Output

CONTAINER ID   NAME      CPU %     MEM USAGE / LIMIT     MEM %     NET I/O           BLOCK I/O         PIDS

b1c2d3e4f5g6   my-nginx  0.15%     12.3MiB / 1GiB        1.20%     1.2MB / 1.1MB     0B / 0B           5

→

--no-stream - Show stats only once instead of continuously updating

This command shows live resource usage stats for all running containers, updating every second until you stop it with Ctrl+C.

Terminal

docker stats

Expected Output

CONTAINER ID   NAME      CPU %     MEM USAGE / LIMIT     MEM %     NET I/O           BLOCK I/O         PIDS

b1c2d3e4f5g6   my-nginx  0.20%     12.5MiB / 1GiB        1.22%     1.3MB / 1.2MB     0B / 0B           5

7h8i9j0k1l2m   my-redis  0.05%     8.7MiB / 512MiB       1.70%     0.5MB / 0.4MB     0B / 0B           3

Key Concept

If you remember nothing else from this pattern, remember: docker stats shows real-time resource use of containers to help you monitor and troubleshoot performance.

Common Mistakes

Running 'docker stats' without specifying --no-stream when you only want a single snapshot.

The command keeps running and updating, which can be confusing if you expect just one output.

Add the --no-stream flag to get a one-time snapshot of container stats.

Trying to check stats of a container that is not running.

Docker stats only works for running containers and will show an error or no output.

Use 'docker ps' first to confirm the container is running before checking stats.

Summary

Use 'docker ps' to list running containers and find their names or IDs.

Use 'docker stats container-name --no-stream' to get a one-time snapshot of resource usage.

Use 'docker stats' to watch live resource usage of all running containers until you stop it.

Practice

1. What does the docker stats command show you?

easy

A. Docker network settings

B. Real-time resource usage like CPU and memory of containers

C. Logs of a running container

D. List of all Docker images on the system

Solution

1. Step 1: Understand the purpose of docker stats

   This command is designed to show resource usage statistics of running containers.

2. Step 2: Identify what resource stats include

   It shows CPU, memory, network IO, and block IO usage in real time.

3. Final Answer:

   Real-time resource usage like CPU and memory of containers -> Option B

4. Quick Check:

   docker stats = resource usage [OK]

Hint: Remember: stats = resource usage snapshot [OK]

Common Mistakes:

• Confusing stats with logs
• Thinking it lists images
• Assuming it shows network config

2. Which of these is the correct syntax to get a one-time snapshot of container stats for container named webapp?

easy

A. docker stats --no-stream webapp

B. docker stats webapp

C. docker stats --once webapp

D. docker stats webapp --snapshot

Solution

1. Step 1: Recall the correct option for one-time snapshot

   The flag --no-stream disables continuous updates and shows a single snapshot.

2. Step 2: Check correct command order

   The flag --no-stream should come before the container name webapp.

3. Final Answer:

   docker stats --no-stream webapp -> Option A

4. Quick Check:

   One-time stats = --no-stream before container [OK]

Hint: Use --no-stream before container name for snapshot [OK]

Common Mistakes:

• Using wrong flag like --once or --snapshot
• Omitting the --no-stream flag
• Omitting container name

3. What output will you get when running docker stats --no-stream with two running containers named db and cache?

medium

A. A single snapshot table showing CPU, memory, and IO stats for db and cache

B. An error saying container names are missing

C. A live updating table of resource usage for db and cache containers

D. Only stats for the first container found

Solution

1. Step 1: Understand the effect of --no-stream flag

   This flag disables live updates and shows only one snapshot of stats.

2. Step 2: Check default behavior without container names

   Without specifying containers, stats shows all running containers, here db and cache.

3. Final Answer:

   A single snapshot table showing CPU, memory, and IO stats for db and cache -> Option A

4. Quick Check:

   --no-stream + multiple containers = single snapshot for all [OK]

Hint: No container names + --no-stream = snapshot for all running [OK]

Common Mistakes:

• Expecting live updates with --no-stream
• Thinking container names are required
• Assuming only one container stats shown

4. You run docker stats --no-stream mycontainer but get an error: no such container: mycontainer. What is the likely cause?

medium

A. Docker daemon is not running

B. The --no-stream flag is not supported

C. The container name is misspelled or does not exist

D. You need to add --all flag to see stopped containers

Solution

1. Step 1: Analyze the error message

   The error says container not found, so the name is likely wrong or container is not running.

2. Step 2: Check other options

   --no-stream is valid, Docker daemon error would be different, and --all is not for stats command.

3. Final Answer:

   The container name is misspelled or does not exist -> Option C

4. Quick Check:

   Container not found error = wrong name [OK]

Hint: Check container name spelling if 'no such container' error [OK]

Common Mistakes:

• Assuming --no-stream causes error
• Ignoring Docker daemon status
• Thinking --all shows stats for stopped containers

5. You want to monitor CPU and memory usage of a container named api-server but only want a single snapshot, not continuous updates. Which command should you use and why?

hard

A. docker stats api-server --interval=1 to update every second

B. docker stats api-server because it shows live updates by default

C. docker stats --all api-server to include stopped containers

D. docker stats --no-stream api-server because it shows one-time resource usage snapshot

Solution

1. Step 1: Identify the need for a single snapshot

   Using --no-stream disables continuous updates and shows one snapshot.

2. Step 2: Confirm container name usage

   Specify the container name api-server to get stats only for it.

3. Step 3: Evaluate other options

   Default docker stats streams live updates, --all is invalid here, and --interval is not a valid flag.

4. Final Answer:

   docker stats --no-stream api-server because it shows one-time resource usage snapshot -> Option D

5. Quick Check:

   Single snapshot = --no-stream with container name [OK]

Hint: Use --no-stream with container name for single snapshot [OK]

Common Mistakes:

• Omitting --no-stream for single snapshot
• Using invalid flags like --all or --interval
• Expecting live updates when not wanted