Re: [Caml-list] Memory leak in ocaml runtime with backtraces+threads?

[Gabriel Scherer <gabriel.scherer@gmail.com>]

I did not previously know of that problem, sorry.

Could you submit a bugreport on the Mantis bugtracker?
  http://caml.inria.fr/mantis/


On Mon, Apr 11, 2016 at 7:27 AM, Rob Hoes <Rob.Hoes@citrix.com> wrote:
> Hello,
>
> I have noticed that an OCaml program with backtrace recording enabled seems to leak memory each time a new thread is created and completes.
>
> For example, the following program, compiled with the `-g` option, will very quickly eat up all your memory:
>
> let _ =
>   Printexc.record_backtrace true;
>   let rec loop () =
>     let t = Thread.create (fun () ->
>       try
>         raise Not_found
>       with Not_found ->
>         ()
>     ) () in
>     Thread.join t;
>     loop ()
>   in
>   loop ()
>
> I’ve tried this on various compilers (4.02.3, 4.01.0 and 3.12.1), on Linux and Mac, native and bytecode, and see the problem on all of them.
>
> Valgrind reports that the allocation of `caml_backtrace_buffer` in the function `caml_stash_backtrace` in asmrun/backtrace_prim.c is the culprit (https://github.com/ocaml/ocaml/blob/trunk/asmrun/backtrace_prim.c#L98):
>
>   if (caml_backtrace_buffer == NULL) {
>     Assert(caml_backtrace_pos == 0);
>     caml_backtrace_buffer = malloc(BACKTRACE_BUFFER_SIZE
>                                    * sizeof(backtrace_slot));
>     if (caml_backtrace_buffer == NULL) return;
>   }
>
> The `caml_backtrace_buffer` variable is a global variable initialised to NULL in byterun/backtrace.c. However, it seems to be local to a thread (how does it become thread local?), is set to NULL whenever a thread is created (where?), and a new malloc happens when an exception is raised. It is not freed when the thread completes.
>
> Has anyone seen this before, or have suggestions for how to fix it?
>
> Thanks,
> Rob
>
>
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