What is the name of the Linux kernel's Completely Fair Scheduler?

The Linux kernel scheduler is called the Completely Fair Scheduler.

Usually abbreviated CFS, it was created by Ingo Molnár and merged into Linux 2.6.23, released in October 2007. CFS was designed for ordinary tasks that need a fair share of CPU time, balancing throughput with interactive responsiveness.

Instead of assigning every process a fixed time slice, CFS tracks virtual runtime, or `vruntime`. The runnable task with the lowest accumulated virtual runtime is generally selected next. Its run queue used a time-ordered red-black tree, making insertion and removal efficient even as workloads grew.

The O(1) scheduler was CFS’s predecessor, not a synonym for it. Real-time schedulers such as FIFO, round-robin, and deadline scheduling serve different scheduling classes. A modern historical detail is that Linux 6.6 replaced CFS as the default fair scheduler with EEVDF, or Earliest Eligible Virtual Deadline First; CFS remains the canonical name associated with this major Linux scheduling design.