CPU limits and reservations in Docker - Commands & Configuration

Introduction

Sometimes your computer runs many programs at once, and some use too much power. CPU limits and reservations help control how much computer power each program can use so everything runs smoothly without slowing down.

When you want to make sure a critical app always has enough CPU power to work well.

When running multiple containers on one machine and you want to prevent one from using all CPU.

When testing how your app behaves with limited CPU resources.

When you want to avoid your server becoming slow because one container uses too much CPU.

When you want to plan resource use before adding more containers to your system.

Config File - docker-compose.yml

docker-compose.yml

version: '3.8'
services:
  my-app:
    image: nginx:1.23
    deploy:
      resources:
        limits:
          cpus: '0.5'
        reservations:
          cpus: '0.25'

This file tells Docker to run an Nginx app with CPU limits and reservations.

limits.cpus means the app can use up to half a CPU core.

reservations.cpus means Docker tries to keep at least a quarter of a CPU core available for this app.

Commands

This command starts the app using the docker-compose file and applies the CPU limits and reservations defined there.

Terminal

docker stack deploy -c docker-compose.yml my-stack

Expected Output

Creating network my-stack_default

Creating service my-stack_my-app

→

-c - Specifies the compose file to use

This command lists all running services to check if our app started successfully.

Terminal

docker service ls

Expected Output

ID                  NAME                MODE                REPLICAS            IMAGE               PORTS

abc123def456        my-stack_my-app     replicated          1/1                 nginx:1.23

This command shows the CPU limits and reservations set for the running service in JSON format.

Terminal

docker service inspect my-stack_my-app --format '{{json .Spec.TaskTemplate.Resources}}'

Expected Output

{"Limits":{"NanoCPUs":500000000},"Reservations":{"NanoCPUs":250000000}}

→

--format - Formats output to show only resource info in JSON

Key Concept

If you remember nothing else from this pattern, remember: CPU limits stop a container from using too much CPU, and reservations guarantee it some CPU to run smoothly.

Common Mistakes

Setting CPU limits without reservations

The container might get less CPU than it needs and slow down because no CPU is guaranteed.

Always set reservations along with limits to reserve CPU for your container.

Using CPU values greater than the host's CPU count

Docker cannot allocate more CPU than physically available, causing errors or ignored settings.

Set CPU limits and reservations within the total CPU cores your machine has.

Using CPU limits in docker run without swarm mode

CPU limits in the deploy section only work with Docker Swarm stacks, not with simple docker run commands.

Use --cpus flag with docker run for limits outside swarm mode.

Summary

Create a docker-compose.yml file with CPU limits and reservations under deploy.resources.

Deploy the stack with docker stack deploy to apply CPU controls.

Verify the service is running and check CPU settings with docker service inspect.

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