Re: [Caml-list] Presumed bug in OCaml's garbage collector or in its Weak module...

[Alain Frisch <alain.frisch@lexifi.com>]

On 21/06/2016 14:43, François Bobot wrote:
> However it would be interesting to keep the nice property that
> `get_copy` give us for the implementation of weak structure, without
> using unsafe features. The nice property is that a value is not kept
> alive just because it collides with a value that is often accessed.
> Perhaps a kind of transient roots, which are considered as root only at
> the end of the each part of mark phase, would allow to mark the value as
> few time as possible. But the API would not be pretty...

Not directly related to this thread, but it might be worth mentioning 
that for our use case (hash-consing terms), disabling the shallow copy 
(i.e. using get instead of get_copy) lead to very significant 
performance gains.  The main explanation is probably that it reduces 
quite a bit the pressure on the GC (more than the avoided cost of the 
copy itself, I guess).  Another possible explanation is that keeping 
some values a bit longer in the weak set is actually a good thing in our 
usage scenario (we keep some derived values, computed only when the term 
is first inserted in the weak set); this is also exacerbated by the 
lower rate of the GC (the first explanation).  We are "saved" by the 
fact that phases that exercises the weak set a lot are alternating with 
other GC-intensive phases (which guarantee that the weak set are 
eventually pruned).

Perhaps the "official" Weak.Make should at least allow to use get 
instead of get_copy?


Alain