Re: [Caml-list] A question about GC.

[Guillaume Yziquel <guillaume.yziquel@gmx.ch>]

Le Monday 13 Jun 2011 à 14:34:07 (+0300), ygrek a écrit :
> On Mon, 13 Jun 2011 12:37:46 +0200
> Philippe Wang <mail@philippewang.info> wrote:
> 
> > As far as I know, when OCaml's GC is not working as expected, it's
> > (almost) always because there are blocks allocated outside OCaml's
> > heap (a.k.a. custom blocks).
> 
> Custom blocks are allocated on ocaml heap.

No, not always. Nothing stops you from putting custom blocks outside of
the ocaml heap. It might even sometimes make sense. Though usually
you're fine allocating custom blocks on the ocaml heap with a pointer to
out of heap data.

> > How big are custom blocks? The bigger they are, the worst the behavior
> > tends to be.
> 
> Why?

Because the GC relies on the two last arguments of caml_alloc_custom to
properly evaluate the memory impact of allocating a custom block
together with the data it refers to. It's easy to get it wrong for large
data, and thus miscalibrate the GC on these blocks.

However, the fact that grey values seem to accumulate doesn't seem to
fit with such an simple explanation. But maybe Philippe is thinking
about something else when talking about "big custom blocks allocated
outside of the ocaml heap".

-- 
     Guillaume Yziquel