Why resource limits matter in Docker - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the command to limit the CPU usage of a Docker container to 50%.

Docker

docker run --cpus=[1] alpine top

Drag options to blanks, or click blank then click option'

A2

B0.5

C1

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using an integer greater than 1 to limit CPU to less than one core.

Omitting the --cpus flag.

The --cpus option limits the CPU usage. Setting it to 0.5 means the container can use up to 50% of one CPU.

2fill in blank

medium

Complete the command to limit the memory usage of a Docker container to 256 megabytes.

Docker

docker run --memory=[1] alpine top

Drag options to blanks, or click blank then click option'

A256mbs

B256mb

C256m

D256

Attempts:

3 left

💡 Hint

Common Mistakes

Using mb or mbs which are invalid units.

Not specifying any unit, which defaults to bytes and is too small.

The --memory option limits the memory. The correct suffix for megabytes is m, so 256m means 256 megabytes.

3fill in blank

hard

Fix the error in the command that tries to limit memory but uses an invalid unit.

Docker

docker run --memory=[1] alpine top

Drag options to blanks, or click blank then click option'

A512m

B512mb

C512mbs

D512

Attempts:

3 left

💡 Hint

Common Mistakes

Using mb or mbs as units.

Omitting the unit entirely.

The correct memory unit is m for megabytes. 512m is valid, while 512mb or 512mbs are invalid.

4fill in blank

hard

Fill both blanks to limit CPU to 1.5 cores and memory to 1 gigabyte.

Docker

docker run --cpus=[1] --memory=[2] alpine top

Drag options to blanks, or click blank then click option'

A1.5

B1g

C1.0g

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1.0g which is valid but less common.

Using 2 for memory which lacks a unit.

--cpus=1.5 limits CPU to one and a half cores. --memory=1g limits memory to 1 gigabyte.

5fill in blank

hard

Fill all three blanks to run a container with CPU limit 0.75, memory limit 512 megabytes, and a restart policy of always.

Docker

docker run --cpus=[1] --memory=[2] --restart=[3] alpine top

Drag options to blanks, or click blank then click option'

A0.75

B512m

Calways

Don-failure

Attempts:

3 left

💡 Hint

Common Mistakes

Using 512mb instead of 512m.

Using on-failure when the question asks for always.

The CPU limit is 0.75, memory limit is 512m for 512 megabytes, and the restart policy always makes the container restart automatically if it stops.

Practice

1. Why is it important to set resource limits like --memory and --cpus when running Docker containers?

easy

A. To prevent one container from using too many resources and slowing down the whole system

B. To make containers run faster by giving them unlimited resources

C. To allow containers to access the host's hardware directly

D. To automatically update the container software

Solution

1. Step 1: Understand resource limits purpose

   Resource limits control how much CPU and memory a container can use to avoid overloading the host system.

2. Step 2: Identify the effect of limits

   Setting limits prevents one container from hogging all resources, keeping the system stable for all containers.

3. Final Answer:

   To prevent one container from using too many resources and slowing down the whole system -> Option A

4. Quick Check:

   Resource limits = prevent overload [OK]

Hint: Limits protect system by capping container resource use [OK]

Common Mistakes:

• Thinking limits make containers faster
• Believing containers get direct hardware access
• Confusing resource limits with software updates

2. Which of the following is the correct syntax to limit a Docker container's memory to 500MB and CPU to 1.5 cores?

easy

A. docker run --memory=500MB --cpus=1.5 myimage

B. docker run --mem=500 --cpu=1.5 myimage

C. docker run --memory-limit=500MB --cpu-limit=1.5 myimage

D. docker run --memory 500MB --cpus 1.5 myimage

Solution

1. Step 1: Recall Docker resource limit flags

   The correct flags are --memory for memory and --cpus for CPU limits.

2. Step 2: Check syntax correctness

   docker run --memory=500MB --cpus=1.5 myimage uses correct flags with equal signs and units, which is the proper Docker syntax.

3. Final Answer:

   docker run --memory=500MB --cpus=1.5 myimage -> Option A

4. Quick Check:

   Correct flags and syntax = docker run --memory=500MB --cpus=1.5 myimage [OK]

Hint: Use --memory and --cpus with = and units [OK]

Common Mistakes:

• Using incorrect flag names like --mem or --cpu
• Omitting equal signs between flag and value
• Not specifying units for memory

3. What will happen if you run this command?

docker run --memory=100m --cpus=0.5 busybox sh -c "stress --vm 1 --vm-bytes 200m --vm-hang 0"

medium

A. The container will ignore the limits and run with full resources

B. The container will run normally without any issues

C. The container will use more CPU than allowed but not memory

D. The container will be killed because it tries to use more memory than allowed

Solution

1. Step 1: Analyze memory limit and stress command

   The container is limited to 100MB memory but tries to allocate 200MB, which exceeds the limit.

2. Step 2: Understand Docker behavior on limit breach

   Docker kills containers that exceed memory limits to protect the host system.

3. Final Answer:

   The container will be killed because it tries to use more memory than allowed -> Option D

4. Quick Check:

   Memory limit exceeded = container killed [OK]

Hint: Exceeding memory limit kills container [OK]

Common Mistakes:

• Assuming container runs normally despite limit
• Thinking CPU limits affect memory usage
• Believing limits are ignored

4. You set --memory=256m on a container, but it still crashes with an out-of-memory error. What is the most likely cause?

medium

A. The container is using swap memory which is unlimited by default

B. The container is running on a host with too much free memory

C. The memory limit flag was misspelled and ignored

D. The CPU limit is too low causing memory errors

Solution

1. Step 1: Check common mistakes with memory limits

   If the flag is misspelled, Docker ignores it and no limit is applied, causing crashes.

2. Step 2: Rule out other options

   Swap is limited separately, host memory does not cause container crashes, and CPU limits do not cause memory errors.

3. Final Answer:

   The memory limit flag was misspelled and ignored -> Option C

4. Quick Check:

   Misspelled flag = no limit applied [OK]

Hint: Check flag spelling if limits don't work [OK]

Common Mistakes:

• Ignoring flag spelling errors
• Confusing CPU limits with memory issues
• Assuming host memory size affects container limits

5. You want to run multiple containers on a server with 4 CPU cores and 8GB RAM. How should you set resource limits to keep the system stable?

hard

A. Do not set any limits and rely on Docker to manage resources automatically

B. Assign each container a CPU limit of 1 core and memory limit of 2GB

C. Set CPU limit to 4 cores and memory to 8GB for each container

D. Give each container unlimited CPU and memory to maximize performance

Solution

1. Step 1: Calculate fair resource distribution

   With 4 cores and 8GB RAM, dividing resources evenly means 1 core and 2GB per container if running 4 containers.

2. Step 2: Understand why limits matter for stability

   Setting limits prevents any container from using all resources, keeping the system responsive and stable.

3. Final Answer:

   Assign each container a CPU limit of 1 core and memory limit of 2GB -> Option B

4. Quick Check:

   Even resource limits = system stability [OK]

Hint: Divide total resources fairly among containers [OK]

Common Mistakes:

• Giving unlimited resources causing overload
• Setting limits higher than host capacity
• Not setting limits and risking crashes