Resource monitoring per container in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Check CPU usage of a running container

You want to see the current CPU usage of a container named webapp. Which command will show this information?

A docker stats webapp --no-stream

B docker inspect webapp --format '{{.State.CpuUsage}}'

C docker top webapp

D docker logs webapp --cpu

💡 Hint

Use a command that shows live resource usage statistics for containers.

The command 'docker stats webapp --no-stream' shows a snapshot of resource usage including CPU, memory, and network for the container named 'webapp'. Other options either show process info, logs, or incorrect fields.

💻 Command Output

intermediate

Memory usage output format

You run docker stats --no-stream and see the memory usage column showing 50MiB / 500MiB. What does this mean?

A The container is using 500 MiB and has 50 MiB available

B The container has 50 MiB free and 500 MiB used

C The container is using 50 MiB of memory out of a 500 MiB limit

D The container has 50 MiB total memory and 500 MiB swap

💡 Hint

The format is usually 'used / limit'.

The memory usage column shows how much memory the container is currently using and the maximum allowed memory. '50MiB / 500MiB' means 50 MiB used out of 500 MiB limit.

❓ Configuration

advanced

Limit CPU usage of a container

You want to limit a container named db to use at most 0.5 CPU cores. Which command correctly starts the container with this limit?

A docker run --cpu-shares=0.5 --name db mydbimage

B docker run --cpus=0.5 --name db mydbimage

C docker run --cpu-quota=50000 --name db mydbimage

D docker run --cpulimit=0.5 --name db mydbimage

💡 Hint

Use the flag that sets the number of CPUs directly.

The '--cpus=0.5' flag limits the container to use half a CPU core. '--cpu-shares' is a relative weight, not a hard limit. '--cpu-quota' requires specific values and units. '--cpulimit' is not a valid Docker flag.

❓ Troubleshoot

advanced

Diagnose why container memory usage is high

A container api is using more memory than expected. You want to check its memory limit and usage. Which sequence of commands helps you find this info?

A docker stats api --no-stream; docker inspect api --format '{{.HostConfig.Memory}}'

B docker exec api free -m; docker inspect api --format '{{.State.MemoryUsage}}'

C docker logs api; docker top api

D docker stats api; docker inspect api --format '{{.Config.Memory}}'

💡 Hint

One command shows current usage, the other shows configured limits.

The 'docker stats' command shows current memory usage. 'docker inspect' with '{{.HostConfig.Memory}}' shows the memory limit set on the container. Other options either show logs, processes, or invalid fields.

✅ Best Practice

expert

Efficient resource monitoring setup for multiple containers

You manage many containers and want to monitor CPU and memory usage continuously with minimal overhead. Which approach is best?

A Use 'docker stats' with --no-stream repeatedly in a script every second

B Check container logs for resource usage messages periodically

C Run 'docker top' on each container every minute and parse output

D Install a monitoring agent like cAdvisor or Prometheus node exporter on the host

💡 Hint

Look for a scalable and low-overhead monitoring solution.

Using dedicated monitoring tools like cAdvisor or Prometheus exporters is efficient and scalable for many containers. 'docker stats' in a loop causes overhead. 'docker top' shows processes, not resource usage. Logs usually don't contain resource metrics.

Practice

1. What does the docker stats command show for each running container?

easy

A. The container's file system contents

B. The container's log output

C. Live resource usage like CPU, memory, network, and disk I/O

D. The container's environment variables

Solution

1. Step 1: Understand the purpose of docker stats

   The docker stats command is designed to show live resource usage statistics for running containers.

2. Step 2: Identify the resources monitored

   It monitors CPU, memory, network I/O, and disk I/O per container to help track performance.

3. Final Answer:

   Live resource usage like CPU, memory, network, and disk I/O -> Option C

4. Quick Check:

   docker stats = Live resource usage [OK]

Hint: Remember: docker stats shows live resource use per container [OK]

Common Mistakes:

• Confusing docker stats with docker logs
• Thinking it shows file system contents
• Assuming it shows environment variables

2. Which of the following is the correct command to show live resource usage for all running Docker containers?

easy

A. docker stats

B. docker monitor

C. docker inspect stats

D. docker top

Solution

1. Step 1: Recall the command for resource monitoring

   The command docker stats is the official Docker command to show live resource usage for containers.

2. Step 2: Verify other options

   docker monitor and docker inspect stats are invalid commands, and docker top shows running processes, not resource stats.

3. Final Answer:

   docker stats -> Option A

4. Quick Check:

   Correct command = docker stats [OK]

Hint: Use 'docker stats' to see live container resource use [OK]

Common Mistakes:

• Using docker top to check resource usage
• Trying docker monitor which doesn't exist
• Confusing docker inspect with stats

3. You run docker stats --no-stream on a container. What will you see?

medium

A. Continuous live updates of resource usage

B. A single snapshot of current resource usage

C. Only CPU usage without memory info

D. Error because --no-stream is invalid

Solution

1. Step 1: Understand the --no-stream option

   The --no-stream option tells docker stats to show resource usage only once, not continuously.

2. Step 2: Identify the output behavior

   It outputs a single snapshot of CPU, memory, network, and disk I/O usage at the moment the command runs.

3. Final Answer:

   A single snapshot of current resource usage -> Option B

4. Quick Check:

   --no-stream = single snapshot [OK]

Hint: Use --no-stream for one-time resource snapshot [OK]

Common Mistakes:

• Expecting continuous updates with --no-stream
• Thinking it shows only CPU usage
• Assuming --no-stream is invalid

4. You try to run docker stats --format '{{.CPUPerc}} but get no output. What is the likely problem?

medium

A. The format string is missing a closing single quote

B. The container is stopped, so no stats show

C. The --format option is not supported in docker stats

D. You need to add --no-stream to see output

Solution

1. Step 1: Check the format string syntax

   The command uses --format '{{.CPUPerc}} which is missing the closing single quote after the curly braces.

2. Step 2: Understand impact of missing quote

   Without the closing quote, the shell or Docker cannot parse the format string, so no output appears.

3. Final Answer:

   The format string is missing a closing single quote -> Option A

4. Quick Check:

   Format string syntax error = missing quote [OK]

Hint: Always close quotes in --format strings [OK]

Common Mistakes:

• Assuming stopped containers show no stats (they do not appear in stats)
• Thinking --format is unsupported
• Believing --no-stream is required for output

5. You want to monitor CPU and memory usage of a specific container named webapp and save the output once to a file. Which command should you use?

hard

A. docker stats --stream=false webapp > usage.txt

B. docker stats webapp --output usage.txt

C. docker monitor webapp > usage.txt

D. docker stats --no-stream --format 'table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}' webapp > usage.txt

Solution

1. Step 1: Choose the correct command and options

   To get a one-time snapshot, use --no-stream. To format output with container name, CPU, and memory, use --format with a table layout.

2. Step 2: Redirect output to a file

   Use shell redirection > usage.txt to save the output to a file named usage.txt.

3. Final Answer:

   docker stats --no-stream --format 'table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}' webapp > usage.txt -> Option D

4. Quick Check:

   One-time formatted output saved to file = docker stats --no-stream --format 'table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}' webapp > usage.txt [OK]

Hint: Use --no-stream and --format, redirect output to file [OK]

Common Mistakes:

• Using invalid docker monitor command
• Assuming --output option exists
• Using --stream=false instead of --no-stream