I have decided to abort my attempt to write a 3d asteroids clone for the XBox 360. As, there already are quite a few 2d clones of the original Asteroids, I had decided to make a 3d version. The problem, as I pointed in an earlier post, is that one needs a considerably larger number of asteroids to make the game interesting. The magic number here seems to be 1000 asteroids.
I wanted collision response between asteroids. Doing collision detection is quadratic in the number of objects, unless one uses space partitioning. My experiments showed that the naive way (without space partitioning) works fine for 100 asteroids, but does not scale up.
I implemented an octree data structure to speed things up. The idea here is to partition space in boxes of varying size, where each box contains at most about 10 asteroids. Collision detection can then be done on groups of 10 asteroids.
The optimization worked fine when the game was run on the PC, but not on the XBox. The game run at 60 fps on the PC, with noticeable "hickups" every few seconds though. The game run at less than 10 fps on the XBox.
The culprit is the gargabe collector on the XBox. When looking on the net, one finds that the suggested solution is to minimize the number of live objects, which involves using structs. Unfortunately, most of the nice constructs of F#, such as discriminated unions and records, are translated to classes. Replacing these by structs is feasible, but it's not a very pleasant experience.
I also feel that I do not want to invest time developing skills to avoid garbage collection when the correct solution is to fix the garbage collector on XBox.
That does not mean I am giving up on XNA and the XBox, but I will have to find other computationally easier projects.