Weak Hashtbl are hashtbl which doesn't old the keys and such as if one key can be reclaime the binding in the hashtbl is reclaimed also. The tricky part appear when your values can contains pointer to the keys. This presentation describes the problems. It present the best solution I can come with the actual ocaml runtime solution 3 in the presentation. In fact it's not possible to have a perfect solution with the actual ocaml runtime which work if the values can point to keys and also to the hashtbl. We implement a known solution ephemerons (Hayes 97) in the ocaml runtime, here the patch.