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Understanding Cgroups


Containers have revolutionized the way we develop and deploy applications, providing a lightweight and portable solution to run applications consistently across different environments. In order to manage resources effectively in these environments, control groups or cgroups come into play. In this blog post, we will delve into the world of cgroups, explore their benefits, and understand how they function to enhance the performance of containerized applications.

What are Cgroups?

Cgroups, short for control groups, are a Linux kernel feature introduced in version 2.6.24. They provide a mechanism for aggregating and partitioning sets of tasks, their future children, and all associated system resources. This allows for the fine-grained control and management of resources, such as CPU, memory, I/O, and network, among the groups of tasks.

Why Cgroups Matter in Containerization

Containerization relies on the principle of isolating applications and their dependencies from the host system. This ensures that the application runs consistently, regardless of the underlying infrastructure. Cgroups play a vital role in achieving this isolation by enabling the management and allocation of system resources to containers.

Benefits of Cgroups

  1. Resource Isolation: By limiting resource usage per container, cgroups prevent one container from consuming excessive resources, ensuring that other containers and the host system remain unaffected.
  2. Fine-grained Control: Cgroups provide precise control over resource allocation, making it easy to define custom limits for each container.
  3. Improved Performance: By optimizing resource usage, cgroups can help enhance the performance of containerized applications.
  4. Enhanced Security: Isolating the resources of containers also limits their potential impact on the host system, reducing the potential attack surface.

Understanding Cgroup Hierarchy and Controllers

Cgroups are organized hierarchically, with each level representing a different degree of resource allocation. At the top of the hierarchy, there is the root cgroup, which encompasses all the tasks running on the system. Below the root, there can be multiple child cgroups that inherit properties and resource constraints from their parent cgroup.

There are several ways to view the cgroup hierarchy on a Linux system. Here are three common methods to explore and visualize the cgroup hierarchy:

Using the systemd-cgls command:

systemd-cgls is a utility provided by systemd to display the cgroup hierarchy in a tree-like format. To use this command, simply type systemd-cgls in your terminal and hit enter. The output will display a tree structure that represents the cgroup hierarchy on your system.

Examining the cgroup file system:

Cgroup hierarchies are typically mounted to the /sys/fs/cgroup directory in the Linux file system. To view the hierarchy, navigate to this directory and explore its subdirectories, which represent different controllers. Each controller directory contains subdirectories for various cgroups. You can use standard Linux file navigation commands, such as ls, cd, and tree, to explore the cgroup hierarchy.

For example, you can use the tree command to display the cgroup hierarchy for the CPU controller:

tree /sys/fs/cgroup/cpu 

Using the cgtop command:

cgtop is a utility that provides a top-like interface for monitoring cgroup resource usage in real-time. It displays the cgroup hierarchy and shows the resource usage of each cgroup. To use cgtop, simply type cgtop in your terminal and hit enter. The output will show a real-time view of your cgroup hierarchy and resource usage.

Note that cgtop may not be installed by default on your system, and you might need to install it using your distribution’s package manager.

By using one or more of these methods, you can easily view and explore the cgroup hierarchy on your Linux system.

Controllers, on the other hand, are responsible for managing specific resources within a cgroup. Some common controllers include:

  1. CPU: Manages CPU time allocation and scheduling.
  2. Memory: Controls memory usage and allocation, including swap.
  3. Block I/O: Regulates access to block devices and manages bandwidth.
  4. Network: Monitors and manages network usage.
  5. Freezer: Allows for suspending and resuming tasks within a cgroup.

Using Cgroups with Containers

Container runtime engines, such as Docker and Kubernetes, use cgroups to manage resources for their containers. When a new container is created, a corresponding cgroup is also created, and the container’s tasks are added to it. Resource limits defined in the container configuration are then applied to the cgroup, ensuring that the container adheres to the specified constraints.


Cgroups are an essential component of containerization, providing the necessary resource management and isolation capabilities that make containers so effective. By understanding how cgroups work and how they are integrated with container runtime engines, developers and system administrators can optimize the performance and security of their containerized applications.

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