Re: string vs vect + GC question

[Xavier Leroy <Xavier.Leroy@inria.fr>]

Concerning your performance comparison between arrays and strings:

> let s = make_string 65536 `a`;;
> let f n =
>   for i = 0 to n do for j = 0 to n do s.[i*256+j] <- s.[i*256+j] done done;; 
> let v = make_matrix 256 256 `a`;;
> let g n =
>   for i = 0 to n do for j = 0 to n do v.(i).(j) <- v.(i).(j) done done;;

> COMMENT: For 2D arrays, strings win the game. This is quite surprising
>          since Caml Light is not supposed to be very good in doing the
>          explicit arithmetic involved in the index computation. Vects seem
>          to be penalized by the double indirection. 

The main reason why arrays of arrays are less efficient is that each
access performs two bound checks, instead of one for the string-based
version. With array bound checking turned off (option -O fast), both
versions run at the same speed.

Also, if you're really doing matrices of bytes, the string-based
encoding is 4 times more compact (8 times on an Alpha) than the
array-based representation, which improves greatly cache behavior and
I/O speed.

As for your memory management question:

> I have a basic question regarding the behaviour of the Caml-Light Garbage
> collector: when an object becomes hidden, because of a its name is rebound at
> the top level, is it automatically reclaimed by the gc ?...
> For example:
>	#let m = make_vect 1024 0;;
>	m : int vect = [|0; 0; 0; 0; 0; ... |]
>	#let m = 0;;
>	m : int = 0   (* <- Will the space allocated for the vector be reclaimed ?*)

No, it will not be reclaimed. That's because all toplevel definitions
are entered in a global table of values, and always referenced through
that table. Rebinding an identifier does not guarantee that its
previous value is no longer needed, e.g.:

        let m = make_vect 1024 0;;
        let f x = m.(x);;
        let m = 0;;

"f" still accesses the previous value of m in the global table. 

Note that this behavior is specific to the toplevel: locally-bound
structures will always become garbage when leaving the scope of their
definition.

- Xavier Leroy