unexpected malloc behavior on X86_64 Fedora 7 machines

[Flemming Andersen]

I do not know how wide spread this problem is, but I have not been able 
to install MoscowML due to an unexpected behavior of the malloc version 
used by Fedora 7 on my new Intel X86_64 machine. The installation of 
MoscowML appears to fail due to the malloc behavior as described below.

I have also found out that mallopt may be a way to control the behavior 
of malloc, but it is not very well documented in Fedora.

So I would like to know if mallopt is officially support and if it will 
continue to be so. Alternatively, I would like to know if you are able 
to suggest any other solution to the problem described below.

Description of problem with installing MoscowML under Fedora 7 on X86_64 
machine:
======================================================
I learned earlier this week from Prof. Peter Sestoft (ITU DK) who owns 
MoscowML that in my case the problem is caused by certain versions of 
malloc(), which during the installation of MoscowML alternatively 
allocate memory for the camlrunm (underlying runtime system for 
MoscowML) heap in very high addresses using mmap() or in very low 
addresses using brk(). This span appears to require a huge page_table 
(14 GB or so), which it would be silly to allocate.  I also learned from 
Peter that the problem can be avoided by either forcing high memory 
allocation using this environment variable:

    export MALLOC_MMAP_THRESHOLD_=0

or by using forcing low memory allocation using this environment variable:

    export MALLOC_MMAP_MAX_=0

However, there may be performance implications of either of these 
choices. The latter one would limit usable mosml memory to at most 3 GB, 
I think, whereas the former one has been experimentally tested to allow 
mosml to use more than 4 GB.

Also note that these environment variables affect *all* programs that 
use malloc() and hence may have mysterious side effects.

Thanks
Flemming