Re: [Caml-list] Thread behaviour

[Tom Ridge <tom.j.ridge+caml@googlemail.com>]

On 29 September 2013 13:37, Yaron Minsky <yminsky@janestreet.com> wrote:
> I think you've come away with the wrong conclusion.  Here's my summary
> of the state of play.
>
> OCaml has a single runtime lock, but _does_ support pre-emptive OS
> threads.  You can use them for concurrency but not parallelism.  Code
> that runs in a tight loop in one thread, though, can starve another
> thread, particularly if it doesn't allocate, because OCaml only gives
> up the runtime lock when it allocates.

OK, but this means that I can't think of OCaml code running in
different threads in a simple way - I have to consider the single
runtime lock at least (if I care about performance). So maybe the
question is: how should I understand the functioning of the single
runtime lock? For example, is starvation the only thing I need to
think about?

>
> You should probably avoid direct use of threads for concurrent
> programming.  Monadic concurrency libraries like lwt or async are the
> way to go.  You can definitely build message-passing libraries on top
> of these, and many have.

Could you provide some pointers to these libraries? I would be very
interested in this. One thing I didn't immediately see how to do, when
reading the Lwt docs, was to provide a user interface (connect, send,
recv etc) that did not expose the fact that co-operative threads were
in the background. It also seems to me that if someone uses my library
with their code, and their code itself uses Lwt, then there is the
possibility for strange interactions (my Lwt threads are not isolated
from their Lwt threads; with system threads and system-provided
pre-emptive scheduling, you (maybe) do get some isolation/fairness
properties).

>
> You can also build such libraries against raw threads, but I would recommend.
>
> None of which is to argue that you shouldn't use a binding of some
> non-OCaml message-passing library.  I've heard excellent things about
> zeromq,which has bindings in opam.

OK! So +1 for zeromq. I will check it out. Any votes for any other
libraries? OCamlMPI?

BTW thanks to everyone for taking the time to respond to these
questions, and apologies for my ignorance.

T