Re: [Caml-list] memory leak in toplevel? or, how to implement sizeof

[Jim Pryor <lists+caml@jimpryor.net>]

On Thu, Jan 06, 2011 at 10:05:32AM -0500, Pascal Zimmer wrote:
> Because the overhead parameter can vary based on which version of OCaml
> you are running, what the Gc module does, etc, I usually implement this
> kind of measurements in this way:
> 
> # let sizeof maker arg =
>     let first = Gc.((stat()).minor_words) in
>     for i = 1 to 1000000 do
>       ignore (maker arg)
>     done;
>     let second = Gc.((stat()).minor_words) in
>     (second -. first) /. 1e6;;
> 
> # sizeof (fun x -> [x]) 1;;
> - : float = 3.000022
> 
> The overhead gets amortized enough that is becomes negligible (you only
> have to rely on the fact that the loop does not do any allocation).

Thanks, that's a nice and sensible shortcut.


On Fri, Jan 07, 2011 at 01:25:42PM +0100, Damien Doligez wrote:
> The overhead is exactly the heap space used by the value returned by
> Gc.stat.  In your case it is 23 words, not 22.

Yes, that's right, the Gc.stat on my machine uses 23 words of heap, and of course we should let OCaml calculate the size for us, because as you say it will
differ on different architectures.

> 
> >    # sizeof (fun x -> x) 10;;
> >    - : int * int = (1, 10)
> > 
> > That is, one word for the int.
> 
> The int is not allocated in the heap, so the result should be 0.

There's a design choice here: we have to decide whether sizeof counts only the allocated heap space or also the pointer to it. Initially I also found it more natural to count only the allocated heap space, as you're proposing. But then when testing on embedded blocks I talked myself into thinking the other choice worked more smoothly. Looking at it again, though, I think I had just confused myself and you're right, counting only the heap space is best.

Thanks for the feedback, both of you.

-- 
Jim Pryor
profjim@jimpryor.net