Unfortunately,theperformanceofmicrokernelscansufferduetoincreased
system-function overhead. When two user-level services must communicate,
messages must be copied between the services, which reside in separate
86 Chapter 2 Operating-System Structures
address spaces. In addition, the operating system may have to switch from
one process to the next to exchange the messages. The overhead involved
in copying messages and switching between processes has been the largest
impediment to the growth of microkernel-based operating systems. Consider
the history of Windows NT : The first release had a layered microkernel organi-
zation. This version’s performance was low compared with that of Windows
95. Windows NT 4.0 partially corrected the performance problem by moving
layers from user space to kernel space and integrating them more closely.
By the time Windows XP was designed, Windows architecture had become
more monolithic than microkernel. Section 2.8.5.1 will describe how mac OS
addresses the performance issues of the Mach microkernel.