CPU limits and reservations in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are CPU shares in Docker?

CPU shares define the relative priority or weight for CPU allocation among containers when the host CPU is contended. Default value is 1024; higher shares get more CPU proportionally.

beginner

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

A CPU limit sets the maximum CPU resources a container can use (e.g., via --cpus). It prevents the container from using more than the limit, protecting other containers and the host.

intermediate

How do you specify CPU shares using the Docker run command?

Use the option --cpu-shares followed by a number (default 1024), for example: docker run --cpu-shares=512 myimage gives half the priority of default.

intermediate

Explain the difference between CPU shares and CPU quota in Docker.

CPU shares set relative priority for CPU time among containers when CPU is contended (no hard limit). CPU quota (with --cpu-quota and --cpu-period) sets an absolute limit on CPU usage time.

beginner

What happens if a container tries to use more CPU than its limit?

Docker will throttle the container's CPU usage using CFS quota, ensuring it does not exceed the set limit and protecting other containers and the host system.

Which Docker option sets the relative CPU priority for a container?

A --cpuset-cpus

B --cpu-quota

C --cpus

D --cpu-shares

The --cpu-shares option sets relative priority (weight) for CPU allocation when contended.

What does the Docker --cpu-quota option control?

A Number of CPU cores assigned

B Maximum CPU time a container can use per period

C Relative CPU priority among containers

D Minimum CPU guaranteed to a container

--cpu-quota (with --cpu-period) limits the maximum CPU time a container can consume in a period.

If no CPU limits are set, what happens when multiple containers compete for CPU?

A Containers with more CPU shares get more CPU

B CPU is shared equally by default

C The first container gets all CPU

D Containers are throttled automatically

By default, containers share CPU based on --cpu-shares (1024); higher shares get proportionally more when contended.

How do you limit a container to use at most 2 CPUs using Docker run?

A --cpu-quota=2

B --cpu-shares=2

C --cpus=2

D --cpu-period=2

The --cpus=2 option limits the container to at most 2 CPUs (equivalent to quota/period settings).

What is the effect of setting CPU shares to 1024 for a container?

A It sets the container's CPU priority relative to others (default value)

B It guarantees 1024 CPU cores

C It limits the container to 1024 milliseconds of CPU time

D It reserves 1024 CPUs for the container

1024 is the default --cpu-shares value, defining relative priority, not absolute limits or guarantees.

Describe how CPU limits and shares work in Docker and why they are important.

Think about sharing a pizza: shares decide portions when fighting over slices, limits prevent taking the whole pie.

Explain the difference between CPU shares, CPU quota, and --cpus in Docker.

Shares are like voting weight, quota is a timer, --cpus is a simple cap on cores.

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