Re: [Caml-list] Native threads under Debian 3.0r0

[Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>]

From: Markus Mottl <markus@oefai.at>
> 
> There indeed seems to be something broken about threads (at least native
> ones). Things work fine with byte-code and the toplevel, but native
> code + native threads don't.  I have managed to reproduced this problem
> under Linux and Solaris with OCaml 3.06, but also with an older version
> (3.04+10 - 2002-04-18) on an Alpha (Digital Unix).

This behaviour is normal, and I believe documented.
Looking around line 270 in otherlibs/systhreads/posix.c gives the
explanation.
You cannot switch between caml threads just anywhere, you
have to choose a safe place to do that. The way to do that is to raise
a flag requesting a thread change, and wait for the code to actually
release control.
In bytecode "something_to_do" is used, and checked pretty often: at
every function call and every loop iteration.
However, this would mean too much overhead for native code, so
switching is only checked when the GC is called. To make it work even
on slowly allocating programs, the young generation is declared "full"
every once in a while, but there must still be some allocation for it to
happen.

So the answer is: a native code thread should be allocating some
data, otherwise it cannot be interrupted. The problem does not exist
for bytecode.

---------------------------------------------------------------------------
Jacques Garrigue      Kyoto University     garrigue at kurims.kyoto-u.ac.jp
		<A HREF=http://wwwfun.kurims.kyoto-u.ac.jp/~garrigue/>JG</A>


> > $ cat toto.ml
> > open Thread
> > 
> > let _ =
> >   let code1 () =
> > 	let x = ref 0 in
> > 	while true do
> > 	  incr x;
> > 	  Printf.printf "alpha %d\n" !x;
> > 	  flush stdout
> > 	done
> >   in
> >   let thread1 = Thread.create code1 ()
> >   in
> >   while true do
> > 	()
> >   done
> > 
> > compiled with
> > 
> > $ ocamlopt.opt -thread unix.cmxa threads.cmxa toto.ml -o toto
> > 
> > and run as
> > 
> > $ ./toto
> > 
> > give me something like
> > 
> > alpha 0
> > alpha 1
> > alpha 2
> > alpha 3
> > alpha 4
> > alpha 5
> > alpha 6
> > ...
> > 
> > ?
> > 
> > Instead, the program doesn't output anything, and when I hit Ctrl-C,
> > it leaves some processes running, which can only be killed with
> > SIGKILL.
> > 
> > The program works when compiled as bytecode. System info :
> > 
> > $ uname -a
> > Linux gogol 2.4.18 #1 Thu Aug 22 20:42:28 CEST 2002 i586 unknown
> > $ ls -l /lib/libpthread*
> > -rw-r--r--    1 root     root       102172 Apr 28 11:57 /lib/libpthread-0.9.so
> > 
> > However, even in bytecode, when I use the Graphics module, some subprocesses
> > refuse to die when I interrupt the main program.
> > 
> > Any clues ? Thanks very much.
> > --
> > Berke Durak
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