What is the state of OCaml's parallelization abilities?

Question

I'm interested in using OCaml for a project, however I'm not sure about where its parallelization capabilities are anymore. Is there a message passing ability in OCaml? Is OCaml able to efficiently use more than 1 CPU?

Most of what I have read on the subject was written in 2002-2006, and I haven't seen anything more recent.

Thanks!

Answer 1

This 2009 issue of the Caml weekly news ("CWN", a digest of interesting messages from the caml list) shows that:

• the official party line on threads and Ocaml hasn't changed. A notable quote:

  (...) in general, the whole standard library is not thread-safe. Probably that should be stated in the documentation for the threads library, but there isn't much point in documenting it per standard library module. -- X. Leroy

  (for how Ocaml threads can still be useful, see a remark by the culprit himself in another question on SO)

• the most frequently adopted paradigm for parallelism is message-passing, and of note is X. Leroy's OcamlMPI, providing bindings for programming in SPMD style against the MPI standard. The same CWN issue I pointed to above provides references to examples, and numerous other related projects.

• another message-passing solution is JoCaml, pioneering new style of concurrent communications known as join calculus. Note that it is binary-compatible with OCaml compilers.

• that did not prevent the confection of a runtime whose GC is ok with parallelism, though: see a discussion of OCAML4MC in this other issue of the CWN.

There is also:

• Netmulticore - multi-processing sharing ocaml values via mapped shared memory.

• CamlP3l - compiler for Caml parallel programs.

• OCaml-Java - an OCaml compiler that emits Java bytecode

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I haven't followed more recent discussions about Ocaml & parallel programming, though. I'm leaving this CW so that others can update what I mention. It would be great if this question could reach the same level of completeness as the analogous one for Haskell.

Answer 2

At present, the OCaml runtime does not support running across multiple cores in parallel, so a single OCaml process cannot take advantage of multiple cores. This is unlikely to change directly; the direction the OCaml developers are most interested in taking for increased parallelism seems to be allowing multiple OCaml runtimes to run in parallel in a single process; this will allow for very fast message passing, but will not allow multiple threads to run in parallel in a shared-memory configuration. The major hangup is the garbage collector; some years ago, the team experimented with a concurrent GC, but it introduced unacceptable slowdowns in the single-threaded case.

There are a couple of projects, namely Functory and OCamlnet, which provide multicore-happy parallelism by using multiple processes.

In general, the OCaml community tends to favor message passing approaches, which can be done across process boundaries (like OCamlnet does), over single-process shared-memory multithreading. If your program can be split into multiple processes (many can!), then yes, you can efficiently use multiple CPUs.