# scx_layered

This is a single user-defined scheduler used within [sched_ext](https://github.com/sched-ext/scx/tree/main), which is a Linux kernel feature which enables implementing kernel thread schedulers in BPF and dynamically loading them. [Read more about sched_ext](https://github.com/sched-ext/scx/tree/main).

## Overview

A highly configurable multi-layer BPF / user space hybrid scheduler.

scx_layered allows the user to classify tasks into multiple layers, and apply
different scheduling policies to those layers. For example, a layer could be
created of all tasks that are part of the `user.slice` cgroup slice, and a
policy could be specified that ensures that the layer is given at least 80% CPU
utilization for some subset of CPUs on the system.

## How To Install

Available as a [Rust crate](https://crates.io/crates/scx_layered): `cargo add scx_layered`

## Typical Use Case

scx_layered is designed to be highly customizable, and can be targeted for
specific applications. For example, if you had a high-priority service that
required priority access to all but 1 physical core to ensure acceptable p99
latencies, you could specify that the service would get priority access to all
but 1 core on the system. If that service ends up not utilizing all of those
cores, they could be used by other layers until they're needed.

## Production Ready?

Yes. If tuned correctly, scx_layered should be performant across various CPU
architectures and workloads.

That said, you may run into an issue with infeasible weights, where a task with
a very high weight may cause the scheduler to incorrectly leave cores idle
because it thinks they're necessary to accommodate the compute for a single
task. This can also happen in CFS, and should soon be addressed for
scx_layered.