Bovy has re-written all our "infer d-dimensional distribution functions when you have noisy data with missing values" code in C and it appears to be much faster than the (heroic) code written by Roweis and Blanton back in the day when we were all discovering the value of
pair coding. Bovy and I spent some time discussing
split and merge, which is a method for exploring mixture-of-gaussians models when you think you might be stuck in a local minimum.
Bovy and I also discussed the problem of comparing millions of SDSS spectra to one another in finite time. We figured out that the full N-squared calculation would take a year even if we coded it in machine language, so we want to do the full comparison only after trimming the tree with some reliable heuristics. We came up with a straw plan, but I am suspicious about its reliability (that is, we don't want to trim valid leaves) and effectiveness (that is, we want to massively speed things up).