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

[Rob Hoes <Rob.Hoes@citrix.com>]

Sure: http://caml.inria.fr/mantis/view.php?id=7220

Thanks,
Rob

> On 11 Apr 2016, at 13:01, Gabriel Scherer <gabriel.scherer@gmail.com> wrote:
> 
> 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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