axle OS (GitHub, blog) is a hobby microkernel and userspace which includes many home-grown utilities such as an x86_64 assembler + ELF linker, a TCP/IP/(ARP/DNS/NIC/etc)]( stack, SATA support, a TrueType renderer, a GameBoy emulator, and more. Everything is built around message passing, from process launches and virtual memory operations, to driver events and GUI updates.

axle OS lacks a GPU driver, but features a compositing desktop window manager with transparency effects and animations. Since the compositor runs on the CPU, I’ve put significant effort into making redraws as efficient and targeted as possible to create a smooth and responsive experience.

In this talk, I’ll give a tour of axle’s CPU-bound compositor from first principles. We’ll go on a journey of live visualisations, seeing how each successive optimization allows the compositor to perform progressively less work per frame, building up towards a general strategy for redraws that comprehensively covers screen updates.

Developing a compositor that plays optimisation tricks involves lots of testing, which can be onerous in an OS that’s primarily developed in an emulator and which must boot itself before the compositor can run. Therefore, we’ll also take a look at a host-side userspace harness I made for the compositor: the compositor can run on my host-native macOS, or as a part of the full axle OS distribution. I developed a simulator which allows me to record user interaction (such as dragging a window around with a mouse), capture the composited frames, and write a test suite that replays these events and ensures the composited frames don’t deviate from the correct output.

We’ll investigate R-trees, different compositing strategies, client request rate limiting, and the various types of redraws that the compositor must be able to handle. This talk aims to be an engaging and ‘interactive’ experience for the audience, with lots of guiding visualisations motivating each optimization we make to our compositor, following the journey towards axle OS’s contemporary CPU compositor.