 Similar approaches have since been adopted by QNX and MINIX 3.In a series of experiments, Chen and Bershad compared memory cycles per instruction (MCPI) of monolithic Ultrix with those of microkernel Mach combined with a 4.3BSD Unix server running in user space.Their results explained Mach's poorer performance by higher MCPI and demonstrated that IPC alone is not responsible for much of the system overhead, suggesting that optimizations focused exclusively on IPC will have limited impact.