- problems:
    - making libxml allocate the document
      tree from a shared memory segment
        - libxml provides hooks for 
	  malloc replacements

    - libxml is quite 
    	- memory hungry 
	- malloc and free friendly
	  (called probably at least
	   twice for each function)
	- won't using shared memory
	  actually slow down (slower
	  malloc and free)?
	    - we could: keep a normal
	      memory cache of ``candidates''
	      to be inserted in the cache.
	      When a ``candidate'' shows
	      to be actually called with
	      an higher frequency than other
	      cached documents, the next
	      time it is used it is allocated
	      in the shared memory pool.

    - multithreaded related. I probably
      cannot pass any additional argument
      to the malloc routines
        - use a global tsd which mantains
	  the descriptor used by the malloc
	  library

   
- apache1 -> must rely on an external library: mm
- apache2 -> provides wrappers that work both
	when the model is multithreaded and
	when it is not