CPU Quota for Docker and Kubernetes

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

CPU quota (cpu_qota) is a feature of Linux Control Groups (cgroup). CPU quota 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 Quota is a absolute value

Whereas cpu_shares are a relative value, cpu_quota is the number of microseconds of CPU time a container can use per cpu_period. For example configuring:

  • cpu_quota to 50,000
  • cpu_period to 100,000

The container will be allocated 50,000 microseconds per 100,000 microsecond period. A bit like (see below) the use of 0.5 CPUs. Quota can be greater than the period. For example:

  • cpu_quota to 200,000
  • cpu_period to 100,000

Now the container can use 200,000 microseconds of CPU time every 100,000 microseconds. To use the CPU time there will either need to be multiple processes in the container, or a multi-threaded process. This configuration is a bit like (see below) having 2 CPUs.

The default cpu_period on most platforms is 100,000 microseconds or 100 milliseconds.

Once your time is up

After a container uses its cpu_quota it is throttled for the remainder of the cpu_period. A multi-threaded process, or a container with multiple processes can use its quota up in a shorter time than its quota. Multiple threads or processes in the same container executing on different CPUs in parallel all use up the quota. For example:

  • cpu_quota to 200,000
  • cpu_period to 100,000
  • Container with a process with 10 threads

The 10 threads can use 200,000 microseconds of CPU quota on 10 different CPUs in 20,000 microseconds. Linux then throttles the container for the remaining 80,000 microseconds of the period. Tail latency for multi-threaded containers that use cpu_quota are commonly high due to this throttling.

Takeaways

cpu_quota allows setting an upper bound on the amount of CPU time a container gets. Linux enforces the limit even if CPU time is available. Quotas can hinder utilisation while providing a predictable upper bounds on CPU time.

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