Could a Cray XK6 run a real-time raytracer?

Question

I heard about Cray's new supercomputer -- the XK6 -- today, but I am a little confused about where the bottlenecks are. Is it in interconnec? Can an XK6 configured with, say, 500,000 16-core processors achieve a graphic fidelity comprable to Toy Story 3 in real-time? By "real-time", I mean 60fps, or around 16.7 milliseconds per frame.

Answer 1

No. Pure computation is surprisingly little of what it takes to render a film frame from Toy Story 3 or a similar modern animated (or VFX) film. Those scenes may reference many hundreds of GB of texture, and even if you could know exactly which subset of that texture will be needed for a frame, it may be tens of GB, which still needs to be read from disk and/or transferred over a network. GPUs or massively parallel distributed computation doesn't speed that up. Furthermore, rendering is only the very last step... preparing the geometric input for a frame (simulating the fluids, cloth and hair, tessellating the geometry, reading and interpreting large scenes from disk) can be substantial.

So, just pulling numbers out of the air (but these are moderately realistic), say it takes 30 minutes to prepare the scene (load the models, tessellate it, some minor sims, etc.), and 4.5 hours to render (of which, say, 30 minutes is reading texture and other resources from disk, leaving 4 hours of "ray tracing" and other computation). If the XK6 made the ray tracing infinitely fast, it would only speed the total process up by 5x (1 hour is still hard-to-serialize prep and I/O). That's Amdahl's Law for you.

Now, you're probably asking yourself, "how do games go so fast?" They do it in two ways: (1) they drastically reduce the data set (texture size, geometric resolution, etc.) to make it all fit on the GPU and be reasonably fast to load levels (which, curiously, you the user are not counting when you think of the rendering as happening in "real time"); (2) they spare no expense in computation, tricks, and human labor to optimize the scenes and algorithms before they ship the disks, so that when it's in front of the player it can render quickly.

So, in summary, if you are asking if the total computational power of the XK6 is enough to compute in real time all the pure math required to render a film frame, then yes, it probably is. But if you are asking if an XK6 could actually render the movie in realtime given the kinds of inputs the renderer needs, then no, it couldn't. Would an XK6 be of any use to people rendering those movie frames? No, it probably wouldn't be worth the trouble of reprogramming all the software (hundreds of man years) from the ground up.