CPU Shares for Docker containers

In this article we’ll explain CPU shares, so you can understand how to set them in Docker.

CPU shares (cpu_share) are a feature of Linux Control Groups (cgroup). CPU shares control how much CPU time a process in a container can use.

Container in this context means a set of processes running in the same cgroup. This definition is applicable to:

  • Docker containers
  • Pods in Kubernetes
  • Other systems such as systemd that use cgroups

CPU Shares are relative

A container’s cpu_share is a relative value used for scheduling CPU time between different containers. The cpu_share has no meaning in isolation. For example, setting a cpu_share of 512 gives you no information about how much CPU time the container will get. Setting one container’s cpu_share to 512 and another container’s to 1024 means that the second container will get double the amount of CPU time as the first. That said, it still gives no information about the actual amount of CPU time each container will get, only the relative amount between them.

For example, if we have three containers on a machine with 1 CPU.

  1. 1024 shares
  2. 512 shares
  3. 512 shares

Then each container will get:

  1. 0.5 CPU
  2. 0.25 CPU
  3. 0.25 CPU

If the same three containers deployed to a machine with 2 CPUs.

Then each container will get:

  1. 1 CPU
  2. 0.5 CPU
  3. 0.5 CPU

The key takeaway is that cpu_shares are relative, they give you no information without knowing what other containers are running on the machine and the machine’s resources.

CPU shares enforcement occurs during CPU contention

The above figures for how much CPU time a container has access to are only valid if every container wants to execute at the same time.

If only one container is active it can use all the CPU. If more than one container executes at once then the relative cpu_shares dictate how much CPU time each container has access to. The cpu_shares of inactive containers are irrelevant.

The key take away is that cpu_shares allow for high utilisation. Any container, regardless of its cpu_shares, can use a machine’s CPU resources if other containers are inactive. During CPU contention cpu_shares specify how much CPU time each container gets.

CPU Shares in Docker

Docker uses a base value of 1024 for cpu_shares. To give a container relatively more CPU set --cpu-shares for Docker run to a value greater than 1024. To give a container relatively less CPU time set --cpu-shares to lower than 1024.

If --cpus or --cpu-quota is set then even if there is no contention for CPU time the container will be limited based. This can be good for predictable resource usage but is generally bad for utilisation.

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