raytracing with CUDA

Question

I'm currently implementing a raytracer. Since raytracing is extremely computation heavy and since I am going to be looking into CUDA programming anyway, I was wondering if anyone has any experience with combining the two. I can't really tell if the computational models match and I would like to know what to expect. I get the impression that it's not exactly a match made in heaven, but a decent speed increasy would be better than nothing.

Answer 1

One thing to be very wary of in CUDA is that divergent control flow in your kernel code absolutely KILLS performance, due to the structure of the underlying GPU hardware. GPUs typically have massively data-parallel workloads with highly-coherent control flow (i.e. you have a couple million pixels, each of which (or at least large swaths of which) will be operated on by the exact same shader program, even taking the same direction through all the branches. This enables them to make some hardware optimizations, like only having a single instruction cache, fetch unit, and decode logic for each group of 32 threads. In the ideal case, which is common in graphics, they can broadcast the same instruction to all 32 sets of execution units in the same cycle (this is known as SIMD, or Single-Instruction Multiple-Data). They can emulate MIMD (Multiple-Instruction) and SPMD (Single-Program), but when threads within a Streaming Multiprocessor (SM) diverge (take different code paths out of a branch), the issue logic actually switches between each code path on a cycle-by-cycle basis. You can imagine that, in the worst case, where all threads are on separate paths, your hardware utilization just went down by a factor of 32, effectively killing any benefit you would've had by running on a GPU over a CPU, particularly considering the overhead associated with marshalling the dataset from the CPU, over PCIe, to the GPU.

That said, ray-tracing, while data-parallel in some sense, has widely-diverging control flow for even modestly-complex scenes. Even if you manage to map a bunch of tightly-spaced rays that you cast out right next to each other onto the same SM, the data and instruction locality you have for the initial bounce won't hold for very long. For instance, imagine all 32 highly-coherent rays bouncing off a sphere. They will all go in fairly different directions after this bounce, and will probably hit objects made out of different materials, with different lighting conditions, and so forth. Every material and set of lighting, occlusion, etc. conditions has its own instruction stream associated with it (to compute refraction, reflection, absorption, etc.), and so it becomes quite difficult to run the same instruction stream on even a significant fraction of the threads in an SM. This problem, with the current state of the art in ray-tracing code, reduces your GPU utilization by a factor of 16-32, which may make performance unacceptable for your application, especially if it's real-time (e.g. a game). It still might be superior to a CPU for e.g. a render farm.

There is an emerging class of MIMD or SPMD accelerators being looked at now in the research community. I would look at these as logical platforms for software, real-time raytracing.

If you're interested in the algorithms involved and mapping them to code, check out POVRay. Also look into photon mapping, it's an interesting technique that even goes one step closer to representing physical reality than raytracing.