CPU limits and reservations in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to set a CPU limit of 0.5 cores for a Docker container.

Docker

docker run --cpus=[1] nginx

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

A0.5

B1

C2

D0.1

Attempts:

3 left

💡 Hint

Common Mistakes

Using an integer like 1 which allows full CPU usage.

Using a value greater than the available CPU cores.

The --cpus flag sets the maximum number of CPUs the container can use. Here, 0.5 means half a CPU core.

2fill in blank

medium

Complete the command to reserve 0.25 CPU cores for a Docker container.

Docker

docker run --cpu-reservation=[1] nginx

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

A1

B2

C0.25

D0.5

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing CPU limit with CPU reservation.

Using a value larger than the CPU limit.

The --cpu-reservation flag reserves CPU resources for the container. 0.25 means reserving a quarter of a CPU core.

3fill in blank

hard

Fix the error in the command to limit CPU usage to 1.5 cores.

Docker

docker run --cpus=[1] nginx

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

A1.5

B1,5

C1-5

D15

Attempts:

3 left

💡 Hint

Common Mistakes

Using a comma instead of a dot for decimals.

Using a dash or other invalid characters.

The CPU limit must be a decimal number with a dot, not a comma or other separator. So 1.5 is correct.

4fill in blank

hard

Fill both blanks to set a CPU limit of 2 cores and reserve 1 core for a Docker container.

Docker

docker run --cpus=[1] --cpu-reservation=[2] nginx

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

A2

B1

C0.5

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Setting reservation higher than the limit.

Using fractional values when whole cores are intended.

The --cpus flag sets the max CPU usage to 2 cores, and --cpu-reservation reserves 1 core for the container.

5fill in blank

hard

Fill all three blanks to run a container with CPU limit 1.2, CPU reservation 0.8, and name it 'webapp'.

Docker

docker run --cpus=[1] --cpu-reservation=[2] --name=[3] nginx

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

A1.2

B0.8

Cwebapp

Dnginx

Attempts:

3 left

💡 Hint

Common Mistakes

Using the image name as the container name.

Mixing up CPU limit and reservation values.

The command sets CPU limit to 1.2 cores, reserves 0.8 cores, and names the container 'webapp'.

Practice

1. What does setting a CPU limit for a Docker container do?

easy

A. It pauses the container when CPU usage is high.

B. It restricts the maximum CPU the container can use.

C. It sets the container to use all available CPUs.

D. It guarantees the container will always get that CPU amount.

Solution

1. Step 1: Understand CPU limit meaning

   A CPU limit restricts the maximum CPU a container can use, preventing it from using more than set.

2. Step 2: Differentiate from CPU reservation

   CPU reservation guarantees minimum CPU, not a maximum limit.

3. Final Answer:

   It restricts the maximum CPU the container can use. -> Option B

4. Quick Check:

   CPU limit = max CPU allowed [OK]

Hint: Limit means max CPU allowed, reservation means minimum guaranteed [OK]

Common Mistakes:

• Confusing CPU limit with CPU reservation
• Thinking CPU limit guarantees CPU
• Assuming CPU limit pauses container

2. Which of the following is the correct Docker run command to limit a container to use at most 0.5 CPUs?

easy

A. docker run --cpus=0.5 myimage

B. docker run --cpu-limit=0.5 myimage

C. docker run --cpu-reservation=0.5 myimage

D. docker run --cpu=0.5 myimage

Solution

1. Step 1: Identify correct flag for CPU limit

   The correct Docker flag to limit CPU usage is --cpus, which accepts a decimal value.

2. Step 2: Check other options for correctness

   --cpu-limit and --cpu-reservation are invalid flags; --cpu is not a valid flag either.

3. Final Answer:

   docker run --cpus=0.5 myimage -> Option A

4. Quick Check:

   Use --cpus to set CPU limit [OK]

Hint: Use --cpus flag to limit CPU usage in docker run [OK]

Common Mistakes:

• Using non-existent flags like --cpu-limit
• Confusing CPU limit with CPU reservation flags
• Using --cpu instead of --cpus

3. Given the command docker service create --cpus=1.5 --reserve-cpu=1 myimage, what does this configuration mean?

medium

A. The container can use up to 1.5 CPUs and is guaranteed 1 CPU.

B. The container is limited to 1 CPU and has no guaranteed CPU.

C. The container can use up to 1 CPU and is guaranteed 1.5 CPUs.

D. The container is guaranteed 1.5 CPUs and limited to 1 CPU.

Solution

1. Step 1: Interpret --cpus flag

   --cpus=1.5 means the container can use up to 1.5 CPUs maximum.

2. Step 2: Interpret --reserve-cpu flag

   --reserve-cpu=1 means the container is guaranteed at least 1 CPU.

3. Final Answer:

   The container can use up to 1.5 CPUs and is guaranteed 1 CPU. -> Option A

4. Quick Check:

   --cpus = max CPU, --reserve-cpu = guaranteed CPU [OK]

Hint: Max CPU = --cpus, guaranteed CPU = --reserve-cpu [OK]

Common Mistakes:

• Swapping meanings of limit and reservation
• Assuming reservation is max CPU
• Ignoring one of the flags

4. You run docker service create --reserve-cpu=2 myimage on a host with only 1 CPU. What is the likely outcome?

medium

A. Docker will reserve 2 CPUs regardless of host capacity.

B. Docker will throw an error and refuse to start the container.

C. Docker will reserve CPU as best as possible but cannot guarantee 2 CPUs.

D. Docker will limit the container to 1 CPU automatically.

Solution

1. Step 1: Understand CPU reservation vs host capacity

   CPU reservation requests minimum CPU but Docker cannot guarantee more CPUs than host has.

2. Step 2: Behavior when reservation exceeds host CPUs

   Docker tries to reserve as much as possible but cannot guarantee more than available CPUs; it does not error out.

3. Final Answer:

   Docker will reserve CPU as best as possible but cannot guarantee 2 CPUs. -> Option C

4. Quick Check:

   Reservation tries best effort, no error if host CPU insufficient [OK]

Hint: Reservation can't exceed host CPU, but no error thrown [OK]

Common Mistakes:

• Expecting error when reservation > host CPU
• Thinking Docker limits container automatically
• Confusing reservation with limit

5. You want to run two containers on a 4-CPU host. Container A needs at least 1 CPU but can use up to 2 CPUs. Container B needs at least 1.5 CPUs but can use up to 2.5 CPUs. Which of the following flag sets correctly set CPU limits and reservations to meet these requirements without overcommitting the host?

hard

A. Container A: --reserve-cpu=1.5 --cpus=2, Container B: --reserve-cpu=1 --cpus=2.5

B. Container A: --reserve-cpu=2 --cpus=1, Container B: --reserve-cpu=2.5 --cpus=1.5

C. Container A: --reserve-cpu=1 --cpus=1.5, Container B: --reserve-cpu=1.5 --cpus=2

D. Container A: --reserve-cpu=1 --cpus=2, Container B: --reserve-cpu=1.5 --cpus=2.5

Solution

1. Step 1: Check CPU reservations sum

   Container A reserves 1 CPU, Container B reserves 1.5 CPUs; total 2.5 CPUs < 4 CPUs host, so no overcommit.

2. Step 2: Check CPU limits sum

   Container A max 2 CPUs, Container B max 2.5 CPUs; total 4.5 CPUs > 4 CPUs host, but limits can exceed host CPUs as they are max caps, not guaranteed.

3. Step 3: Verify options

   Container A: --reserve-cpu=1 --cpus=2, Container B: --reserve-cpu=1.5 --cpus=2.5 matches requirements exactly. Others swap reservation and limit or exceed reservation beyond requirements.

4. Final Answer:

   Container A: --reserve-cpu=1 --cpus=2, Container B: --reserve-cpu=1.5 --cpus=2.5 -> Option D

5. Quick Check:

   Reservations sum < host CPUs, limits set max usage [OK]

Hint: Sum reservations ≤ host CPUs; limits can be higher [OK]

Common Mistakes:

• Swapping reservation and limit values
• Overcommitting CPU reservations beyond host capacity
• Confusing reservation with limit in commands