[Caml-list] Threading: Using and Building

[John Goerzen <jgoerzen@complete.org>]

Hello,

I am looking at using multi-threaded programs in OCaml, but have some
questions:

*** Regarding the thread support itself

Chapter 24 of the OCaml documentaion says that "The threads library is
implemented by time-sharing on a single processor.  It will not take
advantage of multi-processor machines."  That's bad.

But then later on I notice that there are two threading options: system
threads and VM-level threads.  The introductory paragraph does not seem
to apply to system threads which, in other languages at least, do not
behave that way.  So I am rather puzzled about the actual level of
thread support is here.

*** Regarding building programs and libraries with threading support

My next concern is building programs and libraries to support threading.
Chapter 24 also mentions that programs must be linked with -thread, and
all object files compiled with -thread, if the final result is to
support threads.  Alternatively, -vmthread could be substituted.

So my questions are:

 1. Few libraries out there build themselves with -thread or -vmthread.
    Is that to be considered a bug?  Is there a workaround short
    of recompiling them?

 2. Can a library or object file built with -thread or -vmthread be used
    in a non-threaded program (one that does not use -thread or
    -vmthread?)  Can it be used in a threaded program that uses the
    *other* option?  (-thread vs. -vmthread)  Does the answer to this
    question vary depending on whether C code is used, or are there
    other things that can be done in the code to increase compatibility?

 3. I am assuming that -thread and -vmthread are not universally
    supported across OCaml platforms.  Would it be correct to assume,
    then, that one should check for the presence of -thread or -vmthread
    at build time?  Do there exist platforms for which neither are
    supported?

 4. If I am developing an application...  what can I do if it is
    multi-threaded but depends on libraries that are not built in a
    multi-threaded fashion on the user's system?  What if the libraries are
    built with the wrong type of threading (-thread vs. -vmthread)?  What
    if some libraries are built with one type and some with another?

 5. If I am developing a library... what must I do to make it maximally
    compatible with non-threaded applications and both types of
    threaded applications the user may be developing?

 6. What considerations must one take into account when developing C
    interfaces that will be used in multithreaded OCaml programs?

 7. Do any of the standard build systems (OCamlMake, configure.in, etc)
    take into account the above answers in a useful way for an
    application or library developer?

 8. How do I know which, if any, standard or third-party libraries
    installed on my system are threadsafe, and which threading model
    they support?

Thanks,

John

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