Radeon "sisched" Scheduler Is Made Obsolete By RADV's ACO Back-End

It's been years since last hearing anything about sisched as the SI machine instruction scheduler that started out for the RadeonSI OpenGL driver and was ultimately supported by the RADV Vulkan driver too.

Years ago, SISCHED helped offer better open-source AMD Radeon Linux gaming performance but those days are over. The scheduler was made part of the AMDGPU LLVM back-end and that sisched code hasn't seen any new work in ages. Now with Valve's ACO taking off so well since its mainlining in Mesa 19.3 as an alternative to the AMDGPU LLVM back-end, it pretty much nails the coffin on SISCHED.

Valve open-source developer Samuel Pitoiset summed it up today with this RADV commit that dropped SISCHED from being enabled by default for The Talos Principle, "sisched is completely unmaintained, it used to give few more FPS in the past but with ACO, it's now obsolete. It seems even faster without sisched now."

SISCHED was never enabled by default but was selectively white-listed or with the RADV_PERFTEST=sisched environment variable. The Valve ACO back-end in Mesa 20.1-devel remains off by default but can be toggled with the RADV_PERFTEST=aco environment variable. We've generally found ACO to indeed work quite well on 19.3/20.0 and really helps the Linux gaming performance even back to old AMD GCN 1.0 hardware.