First, I want to make clear here that real-time ray tracing for games is still in the research phase. If you would build a game only for ray tracing then it could be less work than building a game only for rasterization. There are many advantages that ease the development process like automatic culling techniques, simpler development of interaction between special effects and also simpler development of special effects. What you need for that is of course a ray tracer with some kind of API. Intel is developing this currently for its own research. Companies would provide some kind of ray tracing library then can be used as comfortable (or more comfortable) like DirectX. Therefore the game developers would NOT need to hassle with any threading work for ray tracing. These things are all well hidden in a well implemented ray tracer. Btw, the performance increase using ray tracing for rendering the image is almost linear to the number of cores. Therefore you can expect a good speed benefit with future many-core systems.