Re: [Caml-list] Garbage collection and caml_adjust_gc_speed

[Damien Doligez <damien.doligez@inria.fr>]

On 2006-08-29, at 06:16, Christopher Kauffman wrote:

> I am finishing up my first major research application written in  
> OCaml. The code is a scientific application which does a moderate  
> amount of floating point computations for which I have employed the  
> Bigarray library and the Lacaml package.

[...]

> What concerns me is the large overhead I seem to be getting from  
> the first function, 'caml_adjust_gc_speed', which I assume is  
> related to the garbage collector. Over 4 million calls to this  
> function seems a little much. I attempted to play with a garbage  
> collection parameter, the value of control.space_overhead in the Gc  
> module. According to the manual, this affects the major GC speed  
> and increasing the value is supposed to cut down on the  
> aggressiveness of the GC. Setting space_overhead to 99 did not  
> change number of calls to 'caml_adjust_gc_speed'.

caml_adjust_gc_speed is called by caml_alloc_custom to tell it about
resources (in this case, memory) outside the heap that depend on the
allocated block.

None of the GC parameter can reduce this number.  I guess your program
allocated over 4 million bigarrays.

What I find surprising is that this simple function would dominate
the run time of your program.  I suspect some problem with gprof.
Could you give us more details about the computations performed by
your program?

-- Damien