RHI = Render Hardware Interface. So if running on windows, the current RHI would be d3d11 or opengl.
The RHI thread is a new development so hasn’t been documented yet. Some part of the renderer’s work is visibility and scene traversal, and the rest of the work is submitting RHI commands (sending commands to the graphics hardware). With the parallel rendering work, we’re trying to split that work onto as many cores as possible. Visibility stays on the rendering thread. Scene traversal has been moved onto multiple threads with a task system.
But here’s the problem - d3d11 has botched parallel command submission. It’s much faster to submit all commands from one thread, and this thread used to be the rendering thread. This is where the RHI thread comes in, it allows us split Visibility and RHI command submission onto two different threads in d3d11.