AW: [Caml-list] What is triggering a lot of GC work?

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am 25.02.2013 03:08:14 schrieb(en) Francois Berenger:
> Hello,
> 
> Is there a way to profile a program in order
> to know which places in the source code
> trigger a lot of garbage collection work?
> 
> I've seen some profiling traces of OCaml programs
> of mine, sometimes the trace is very flat,
> and the obvious things are only GC-related.
> 
> I think it may mean some performance-critical part
> is written in a functional style and may benefit
> from some more imperative style.

This is really a hard question, and I fear an allocation profiler  
cannot always answer it. Imperative style means to use assignments, and  
assignments have often to go through caml_modify, and are not as cheap  
as you would think. In contrast, allocating something new can usually  
avoid caml_modify.

This can have counter-intuitive consequences. Yesterday I sped an  
imperative program up by adding allocations! The idea is so strange  
that I need to report it here. The program uses an array for storing  
intermediate values. Originally, there was only one such array, and  
sooner or later this array was moved to the major heap by the GC.  
Assigning the elements of an array in the major heap with young values  
is the most expensive form of assignment - the array elements are  
temporarily registered as roots by the OCaml runtime. So my idea was to  
create a fresh copy of the array now and then so it is more often in  
the minor heap (the array was quite small). Assignments within the  
minor heap are cheaper - no root registration. The program was 10%  
faster finally.

My general experience is that optimizing the memory behavior is one of  
the most difficult tasks, especially because the OCaml runtime is  
designed for functional programming, and short-living allocations are  
really cheap. Usual rules like "assignment is cheaper than new  
allocation" just do not hold. It depends.

Gerd


> 
> Regards,
> F.
> 
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
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