Linux Scheduling Classes

A Linux system may run a web server, a background backup, an audio engine, and a control loop at the same time. These workloads do not merely need different numeric priorities. They need different rules for deciding who should run.

Linux handles this by organizing scheduling behavior into scheduling classes. Each class implements a scheduling model for a family of workloads. The kernel first considers which class has eligible runnable work, then lets that class choose one of its threads.

This distinction explains why increasing the nice value of a normal process cannot make it outrank a real-time thread, why SCHED_FIFO behaves differently from SCHED_RR, and why the user-visible SCHED_IDLE policy is not the same thing as the kernel's idle thread.

This chapter explains how Linux separates policies, priorities, and scheduling classes, compares the fair, real-time, deadline, and extensible classes, and shows how to inspect and configure scheduling policy safely.