On Wed, 1 Dec 2010, Alexander Schreiber wrote:
  On Wed, Dec 01, 2010 at 11:17:29AM -0800, Zane H.
Healy wrote:
  It does not have a BSD kernel underneath.  It
uses the Mach
 Microkernel. It has a BSD layer on top of that. 
 Which causes all kinds of "fun" for developers whose code has to be
 aware of system details like signal handling. I remember the SBCL folks
 doing some cursing about it ;-) 
It also hurts performance based on tests I've seen in the past.
Zane