Understanding the Completely Fair Scheduler (CFS) in Linux

Introduction

Welcome to DevelopersCoffee! If you’re new here, let’s delve into a core concept of the Linux operating system: The Completely Fair Scheduler (CFS). This is the default process scheduler in Linux. A process scheduler is like a traffic cop for your computer. It decides which tasks get to use the CPU, when they get to use it, and for how long.

What is CFS?

CFS, or Completely Fair Scheduler, aims to allocate CPU time to tasks in a way that is fair. It’s not about giving each task the same amount of time, but giving them time proportional to their weight or priority. This ensures that no task hogs the CPU while others are waiting their turn.

How Does it Work?

Time-Sharing and Virtual Runtime

CFS works on a time-sharing principle. Each task is allocated a “virtual runtime,” which is essentially a value that keeps track of how long a task has been running on the CPU. The main goal is to minimize this value, meaning tasks that have been waiting longer or have used less CPU time are prioritized.

Skip List Data Structure

Contrary to popular belief, CFS doesn’t use a Red-Black Tree for its internal workings. Instead, it uses a data structure known as a “skip list.” Skip lists are simple but offer quick operations for insertions, deletions, and lookups. This efficiency is key for a scheduler, which needs to quickly decide which task should run next.

Examples

To understand how CFS works, let’s consider an example with three tasks: A, B, and C.

1. Round 1: Task A runs for 10 ms, then B runs for 10 ms, and finally, C runs for 10 ms.
2. Round 2: Now it’s A’s turn again. But this time, A only gets 5 ms because it had 10 ms in the first round.
3. Round 2 Continued: Task B and Task C would also each get 5 ms in the second round to balance the overall time and make it fair.

In this way, each task gets a fair shot at using the CPU, thanks to the virtual runtime and the skip list that helps in managing these tasks efficiently.

Conclusion

The Completely Fair Scheduler (CFS) in Linux is designed to allocate CPU time in a way that is both fair and efficient. Whether you’re new to Linux or a seasoned user, understanding CFS will give you a deeper insight into how your system manages tasks.

References

1. Linux Kernel Archives
2. Understanding the Linux Kernel Scheduler

I hope this blog post helps everyone, especially the new members of DevelopersCoffee, to understand what CFS is and how it works!