From a software-technology point of view, the microkernel concept is superior to large integrated kernels. On the other hand, it is widely believed that (a) microkernel based systems are inherently inefficient and (b) they are not sufficiently flexible. Contradictory to this belief, we show and support by documentary evidence that inefficiency and inflexibility of current microkernels is not inherited from the basic idea but mostly from overloading the kernel and/or from improper implementation.

Based on functional reasons, we describe some concepts which must be implemented by a microkernel and illustrate their flexibility. Then, we analyze the performance critical points. We show what performance is achievable, that the efficiency is sufficient with respect to macrokernels and why some published contradictory measurements are not evident. Furthermore, we describe some implementation techniques and illustrate why microkernels are inherently not portable, although they improve portability of the whole system.