Mesa's Vulkan Driver Allows CPU Ray Tracing In Quake RTX But Don't Toss That GPU
Now that even integrated graphics are powerful enough to handle current games in full HD resolution, you might think that there's no point in CPU-based "software" rendering. Well, surprise! Any Windows PC actually includes software fallbacks for DirectX in case the GPU driver utterly fails. Most applications, including games, will refuse to run on the CPU fallback for performance reasons, but it does exist.
That's likely the exact reason that Mesa's Lavapipe software renderer exists, too: to ensure that programs don't completely fail when Vulkan hardware acceleration isn't available. To serve that mission, Lavapipe needs to support as much of Vulkan as possible, and that means implementing support for Vulkan API extensions like VK_KHR_acceleration_structure, VK_KHR_deferred_host_operations, and VK_KHR_ray_query. Wait, "ray?"
That's right: thanks to the hard work of Mesa developer Konstantin Seurer, the Lavapipe software rendering pipeline for the open-source Mesa project now supports CPU-based Vulkan ray-tracing. It's honestly quite a thing to see working at all, but naturally, the performance isn't great. Konstantin shared a screenshot (top) of Quake II RTX running on the CPU codepath, and don't be fooled by that "1 FPS" in the Rivatuner overlay—that "1629.2ms" frametime means it's actually running closer to 0.5 FPS.
Obviously, this isn't playable performance, but that's not the point of the function in the first place. With that said, we think it actually might be possible to wring quite a bit more performance out of the function if someone wanted to implement massive multi-threading support. In the screenshot, Quake II RTX is only using some 34% of the developer's CPU, so it doesn't look like it's very multi-threaded at all.
