M_TRIM_THRESHOLD
              When the amount of contiguous free memory at the top of
              the heap grows sufficiently large, free(3) employs sbrk(2)
              to release this memory back to the system.  (This can be
              useful in programs that continue to execute for a long
              period after freeing a significant amount of memory.)  The
              M_TRIM_THRESHOLD parameter specifies the minimum size (in
              bytes) that this block of memory must reach before sbrk(2)
              is used to trim the heap.