cgroups

Hector is studying to become a GNU/Linux System Administrator. He knows nothing about cgroups. Please answer the following questions for Hector. Use the term "Free Software" vice "open source", and "GNU/Linux" vice "Linux" unles you are referring to the Linux kernel. Hector's workstation is a modern computer running the latest version of Fedora GNU/Linux.

  1. What are cgroups and what problem do they solve?
  2. What is the history of cgroups? When was the concept created, and by whom?
  3. On a modern Fedora GNU/Linux workstation or server, what commands related to cgroups are available?
  4. Please give and fully explain several commands related to cgroups. Also show the simulated commandline output of the commands.

What are cgroups?

Cgroups, short for control groups, are a feature of the GNU/Linux kernel that allows you to allocate, prioritize, deny, manage, and monitor system resources like CPU time, memory, network bandwidth, and disk I/O for a collection of processes. They are a key component for implementing resource management and isolation, a fundamental aspect in server administration and system optimization.

Problem Solved by Cgroups

Cgroups solve the problem of uncontrolled resource monopolization by processes. Without cgroups, a single process or a group of processes could consume disproportionate amounts of system resources, leading to system instability or degraded performance for other tasks. By using cgroups, a system administrator can:

  • Limit the resources a process or a group of processes can use.
  • Prioritize certain processes over others.
  • Isolate processes and their resource usage.
  • Automatically start or stop processes based on resource usage or other triggers.

History of Cgroups

  • Creation: The concept of cgroups was originally introduced by engineers at Google, primarily Paul Menage and Rohit Seth, around 2006.
  • Integration into the Kernel: Cgroups were merged into the GNU/Linux kernel in 2007, starting from version 2.6.24.
  • Evolution: Cgroups have undergone significant changes and improvements over the years. The original implementation, now known as cgroups v1, has been largely superseded by cgroups v2, which offers a more unified and simplified hierarchy.

Cgroups Commands in Fedora GNU/Linux

On a modern Fedora GNU/Linux workstation or server, the following commands related to cgroups are commonly used:

  1. cgcreate: Create a new cgroup.
  2. cgdelete: Remove a cgroup.
  3. cgset: Modify settings for a cgroup.
  4. cgget: Retrieve settings of a cgroup.
  5. cgclassify: Move running processes into a cgroup.
  6. cgexec: Execute a command in the context of a cgroup.
  7. systemd-cgtop: Show top-like statistics for cgroups managed by systemd.

Examples and Simulated Outputs

  1. Creating a New Cgroup

    sudo cgcreate -g cpu,memory:/mygroup

    This creates a new cgroup named 'mygroup' under the 'cpu' and 'memory' subsystems.

  2. Assigning Resource Limits

    sudo cgset -r cpu.shares=512 mygroup sudo cgset -r memory.limit_in_bytes=500M mygroup

    This sets the CPU shares for 'mygroup' to 512 and limits its memory usage to 500 MB.

  3. Moving a Process to a Cgroup

    sudo cgclassify -g cpu,memory:mygroup 1234

    This moves the process with PID 1234 to 'mygroup'.

  4. Running a Command in a Cgroup

    sudo cgexec -g cpu,memory:mygroup /usr/bin/myapp

    This executes '/usr/bin/myapp' in the context of 'mygroup'.

  5. Checking Resource Usage

    systemd-cgtop

    This command displays a top-like interface showing the resource usage of cgroups.

The actual output of these commands would depend on the current state of your system and the specific processes you are managing with cgroups.

Note

Remember, Hector, that working with cgroups requires root privileges, so you will often need to use 'sudo' for these commands. Additionally, the specific syntax and capabilities can vary depending on the version of the GNU/Linux kernel and the tools available on your Fedora system. It's always a good idea to consult the man pages (man <command>) for the most accurate and detailed information.