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Re: GC with finalisation?
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Date: 1999-08-30 (13:57)
From: Markus Mottl <mottl@m...>
Subject: Re: GC with finalisation?
> >No, you need not care about references, because this will be done by the
> >GC of OCaml. 
> 	We must be firing past each other. I have to 'model'
> all the references one python object has to other python
> objects using the Syms. No python object will be collected
> directly by ocaml, since they're all registered in a strong
> array, just to prevent this!

It's not the objects which are collected - it's their "Syms". The objects
may exist anywhere - but must exist in the finalizing closures of the
strong array.

> 	Instead, every binding/unbinding of references 
> in the python objects has to be modelled using the Syms,
> so that inspecting the weak array of Syms, which ARE
> collected by ocaml, will allow me to invoke the finaliser,
> and THEN remove the final strong reference, to allow the
> now finalised object to be collected by ocaml.
> 	I can't just make a weak array of Syms, without
> the Syms refering to each other, because they'd 
> all get collected immediately!

Where is the problem when several Syms get collected at once? As long
as objects (better: their handlers) get registered in the right order,
they will be finalized in the right order: simply, because the objects
contain a "Sym" to other objects (via the closure!) and thus, not
even the handlers of the other objects will be reclaimed before the
containing object!

This can, of course, lead to cycles which will never be reclaimed by
the OCaml-GC when the objects in the closures reference themselves
mutually. But this can be easily resolved by a "meta-GC":

You finalize all objects of reclaimed handlers. Then you put all
objects that are still available into a set.  Now you traverse all
reachable objects from the root and fold'n'filter them out of the
set. The remaining set contains all objects which should be reclaimed
but reference themselves mutually -> these can now be removed from the
weak array and be finalized...

This last and a bit more time consuming pass (finding of cyclic
structures) needs not be conducted every time you do "normal"
finalization. You could use a "countdown" until this "cycle collection"
is launched.

> 	Right. And the 'Syms' will depend only on other Syms:
> 	type Sym = Sym list ref;;

I have rather thought about something like:

  type 'a sym = Sym of 'a

  class some_python_class = object
    method finalize = print_endline "Finalized!"

  let weak_ar = Weak.create 100
  let strong_ar = Array.create 100 (Obj.magic 0)
  let ar_size = ref (-1)

  let register () =
    let my_obj = new some_python_class in
    let obj_handler = Sym my_obj in
    Weak.set weak_ar 0 (Some obj_handler);
    Array.set strong_ar 0 (fun _ -> my_obj#finalize);
    incr ar_size;

  let finalize () =
    for i = 0 to !ar_size do
      if not (Weak.check weak_ar i) then strong_ar.(i) ()

  let _ =
    ignore (register ());  (* Discards object_handler *)
    Gc.full_major ();
    finalize ()

This is a fast hack - eg. "finalize" doesn't remove the closure after
finalization, etc...

But the main idea should be clear. If you "register" the objects in the
order of their creation (naturally), they will also be finalized in the
right order if you traverse the weak array.

Note: if you remove the "ignore" in the main function and replace it by
some binding, the object will not get finalized! - This proves that it
is really due to the garbage collection that finalization happens.

The "full major collection" is called to force the object handler (Sym)
to be reclaimed. You need not do this during normal execution, but you
will have to before the program exits as it immediately does in this
small example.

Ah, and you could even catch exceptions in function "finalize"!

> >This guarantees that reclaimed elements of the weak array can be finalized
> >at once when traversing the array.
> 	The dependencies change dynamically, so it is not
> possible to order the Sym array like this: the ordering is not
> invariant.

The dependencies exist through object handlers - as long as the "real
object" is not reclaimed (it won't be as long as there is a closure
containing it), it will not "release" depending objects (better: their
"Sym"-handle). Thus, it is guaranteed that finalization respects this
order. Time of object creation is "only" the second "finalization order

Markus Mottl

Markus Mottl, mottl@miss.wu-wien.ac.at, http://miss.wu-wien.ac.at/~mottl