Add ray tracer example

[athas]

I don't know if you're interested in more examples, but if so, here's one.

Ray tracing is interesting both because of the BVH construction (which is a delightful divide-and-conquer problem), but also because the big parallel loop over all the pixels presents interesting load balancing issues.

Also, I'm really impressed with how fast MPL runs this thing.

[shwestrick]

Nice! Is this a benchmark from the Futhark project?

And yes, we are looking for more examples. I've been slowly migrating what we have already into this repo so you'll see more things here soon. If you have other things you'd like to add, please submit more PRs :)

[athas]

No, this is a port of a parallel Haskell program I wrote for a course in parallel functional programming. I was disappointed at the poor performance I got (it did not scale beyond 4 cores, and none of my students managed to do better), and wanted to see if MPL could handle it better. And it does: I get almost linear speedup up to 8 cores (which is what I have on this machine).

[shwestrick]

Great. I'm glad to hear it's working well on your machine.

I just ran some tests on our 72-core and was able to get decent speedup (25x) on larger image sizes. By the way, I made some tweaks (7cd68af) like lowering the grain size, since each ray cast seems to be a pretty beefy amount of work on its own.