My experience has been building parallel and distributed programs using multiple runtime instances that communicate via message passing. There are libraries that help automate this, link our own Async_parallel (https://blogs.janestreet.com/async-parallel/). One advantage of this approach is that it scales beyond a single box.

There is work going on at OCaml Labs on a parallel runtime. I suspect it will be useful, but in the end, message passing is I think a better idiom than shared memory threads for parallel programming. When the true parallel runtime lands, I'm not sure that we'll actually use it much for running truly parallel threads.