GC heap allocation in 1.9

While getting the latest of trunk, I stumbled on r16194.

Matz, did you make any real measurement of the "heap free" gain that the
2K patch allows ? Given the heap growth rate in the current GC, my
experience shows that the starting heap size does not really matters, as
the heaps become huge very quickly and therefore the probability that
they are never empty is quite high. I (and one another, see ruby-core
archives) have tried to use a fixed heap size, maintaining a sorted heap
list to mitigate the performance hit (particularly in
is_pointer_to_heap, since the number of heaps explodes), with good
results.

Note: reducing the number of allocated object slots has two good
consequences. Memory, of course, but also the time needed to run
garbage_collect (that is the most important for me).

Do you have any comment ? Would you consider including a fixed-heap
approach -- in which case I would happily port my implementation on
1.9's gc.c for you to evaluate.

Sylvain