from 1.93:
          * Following is a brief list of features that distinguish this malloc
  	  * implementation:
  	  *
  	  *   + Multiple arenas are used if there are multiple CPUs, which reduces lock
  	  *     contention and cache sloshing.
  	  *
  	  *   + Cache line sharing between arenas is avoided for internal data
  	  *     structures.
  	  *
  	  *   + Memory is managed in chunks, rather than as individual pages.
  	  *
  	  *   + Allocations are region-based; internal region size is a discrete
  	  *     multiple of a quantum that is appropriate for alignment constraints.
  	  *     This applies to allocations that are up to half the chunk size.
  	  *
  	  *   + Coalescence of regions is delayed in order to reduce overhead and
  	  *     fragmentation.
  	  *
  	  *   + realloc() always moves data, in order to reduce fragmentation.
  	  *
  	  *   + Red-black trees are used to sort large regions.
  	  *
  	  *   + Data structures for huge allocations are stored separately from
  	  *     allocations, which reduces thrashing during low memory conditions.
  	  *