scx_tickless

This is a single user-defined scheduler used within sched_ext, which is a Linux kernel feature which enables implementing kernel thread schedulers in BPF and dynamically loading them. Read more about sched_ext.

Overview

scx_tickless is a server-oriented scheduler designed for cloud computing, virtualization, and high-performance computing workloads.

The scheduler works by routing all scheduling events through a pool of primary CPUs assigned to handle these events. This allows disabling the scheduler's tick on other CPUs, reducing OS noise.

By default, only CPU 0 is included in the pool of primary CPUs. However, the pool size can be adjusted using the --primary-domain CPUMASK option. On systems with a large number of CPUs, allocating multiple CPUs to the primary pool may be beneficial.

Tasks are placed into a global queue and the primary CPUs are responsible for distributing them to the other "tickless" CPUs. Preemption events are sent from the primary CPUs via IPC only when a "tickless" CPU is being contended by multiple tasks.

The primary CPUs also perform the check for a contended CPU and the frequency of this check can be adjusted with the --frequency FREQ option. This effectively determines the tick frequency on the "tickless" CPUs when multiple tasks are competing for them.

NOTE: in order to effectively disable ticks on the "tickless" CPUs the kernel must be booted with nohz_full. Keep in mind that nohz_full introduces syscall overhead, so this may regress latency-sensitive workloads.

Typical Use Case

Typical use cases include cloud computing, virtualization and high performance computing workloads. This scheduler is not designed for latency-sensitive workloads.

Production Ready?

This scheduler is still experimental.