On a somewhat tangential note:
Has there been any study of how well does the Mach VM
perform for various types of loads?  Are newer VM techniques
being tried (atop 4.4BSD)?  Has anyone tried mating VM and
garbage collection so that they perform well together?
Another idea is to modify prefetch heuristics:  If you just
fetched a page and it contains ptrs to addresses in pages
Pa, Pb ...  Pn, arrange to fetch these pages as well.
Borrowing an idea from Boehm and Demer's conservative GC,
you can treat any value that `looks' like a ptr to be a ptr.
Probably best implemented in a user mode pager.  Anyone
tried anything like this?

Now that {Net,Free}BSD are freely available people must be
trying some of these things!  Pointers to related research,
papers, etc. will be greatly appreciated.

--bakul