How many CPUs does a docker container use?

Introduction

Docker containers are a popular way to package and deploy applications. One of the key features of containerization is the ability to isolate resources and limit their usage. In terms of CPU, it is important to know how many CPUs a container can use, as this can greatly impact the performance of the application running inside the container. This article will cover the concepts of CPU resource allocation in Docker, determining the number of CPU available to a container, configuring CPU resources for a container, and advanced techniques for managing CPU resources. Understanding these concepts will help ensure that your containers have the right CPU resources to run efficiently.

CPU usage is an important aspect of running applications in Docker containers. When a container is created, it is allocated a certain amount of CPU resources from the host machine. Understanding and managing CPU usage in a Docker container is crucial for ensuring that the container has enough resources to run efficiently and for preventing resource contention with other containers running on the same host.

Understanding CPU usage in Docker containers

Docker uses host resources, including CPU, for containers. When a container is created, it is allocated a certain amount of CPU resources from the host machine. By default, containers have no specific CPU limits and can use as much CPU as the host machine has available. To view the CPU usage of a running container, you can use the docker stats command. This command shows the CPU usage for all running containers.

docker stats <container_name> 

Another command to view the CPU usage of a running container is the docker top command.

docker top <container_name> 

This command shows the running processes inside a container and their resource usage.

Setting CPU limits in a Docker container

There are several ways to set CPU limits in Docker containers −

• Using the --cpus flag in the docker run command: This flag is used to set the number of CPUs available to the container as a fraction of the total number of CPUs on the host machine.

docker run --name my-container --cpus 0.5 <image> 

• Using the --cpu-quota and --cpu-period flags in the docker run command: These flags are used to set an absolute limit on the container's CPU usage in microseconds per second.

docker run --name my-container --cpu-quota 100000 --cpu-period 100000 <image> 

• Using the --cpu-shares flag in the docker run command: This flag is used to set the relative CPU shares for a container. This means that if multiple containers are running on a host, the container with a higher value of CPU shares will get a larger proportion of CPU resources.

docker run --name my-container --cpu-shares 512 <image> 

• Using the --cpuset-cpus flag in the docker run command: This flag is used to specify the specific CPUs that a container should be bound to.

docker run --name my-container --cpuset-cpus 0,1 <image> 

• Using the docker update command to update CPU limits after container creation −

docker update --cpus 0.5 my-container 

• Using the docker service create command for swarm services, one can set CPU limits for services

docker service create --name my-service --config-rm --cpu-quota 100000 --cpu-period 100000 <image>

• Using the docker-compose file, one can set CPU limits

services:
  my-service:
    image: <image>
    deploy:
      resources:
        limits:
          cpus: '0.5'

It is important to note that, the way to limit CPU usage depends on the version of Docker you are running and the host operating system. Some methods may not be available on all platforms, and some may have different default values.

Best practices for managing CPU usage in Docker containers

To ensure that your containers have the resources they need to run efficiently, you should monitor their CPU usage and set appropriate limits. Here are some tips for managing CPU usage in Docker containers −

• Start by setting a base CPU limit for all containers, using the --cpu-shares flag or the cpus configuration option in Docker Compose. This will ensure that all containers have a minimum amount of CPU resources available to them.

• Use the docker stats and docker top commands regularly to monitor the CPU usage of your containers. This will help you identify if any of your containers are using more CPU than they need.

• Use the --cpu-quota flag or the cpus configuration option in Docker Compose to set an upper limit on the CPU usage of your containers. This will prevent any one container from monopolizing the host's CPU resources.

• Use the --cpu-period flag in the docker run command to specify the length of a CPU period, in microseconds. This can be useful for fine-tuning the CPU usage of your containers.

Conclusion

Managing CPU usage in Docker containers is an important aspect of running applications in containers. By understanding how Docker uses host resources for containers, and by setting appropriate limits on CPU usage, you can ensure that your containers have the resources they need to run efficiently. Additionally, monitoring CPU usage with the docker stats and docker top commands can help you identify any issues with CPU usage and take action to resolve them.