Microkernel

In computer science, a microkernel (also known as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, thread management, and inter-process communication (IPC). If the hardware provides multiple rings or CPU modes, the microkernel is the only software executing at the most privileged level (generally referred to as supervisor or kernel mode). Traditional operating system functions, such as device drivers, protocol stacks and file systems, are removed from the microkernel to run in user space. In source code size, microkernels tend to be under 10,000 lines of code, as a general rule. MINIX's kernel, for example has fewer than 6,000 lines of code.

Microkernels developed in the 1980s as a response to changes in the computer world, and several challenges adapting existing "mono-kernels" to these new systems. New device drivers, protocol stacks, file systems and other low-level systems were being developed all the time, code that was normally located in the monolithic kernel, and thus required considerable work and careful code management to work on. Microkernels were developed with the idea that all of these services would be implemented as user-space programs, like any other, allowing them to be worked on monolithically and started and stopped like any other program. This would not only allow these services to be more easily worked on, but also separated the kernel code to allow it to be finely tuned without worrying about unintended side effects. Moreover, it would allow entirely new operating systems to be "built up" on a common core, aiding OS research.

Microkernels were a very hot topic in the 1980s when the first usable local area networks were being introduced. The same mechanisms that allowed the kernel to be distributed into user space also allowed the system to be distributed across network links. The first microkernels, notably Mach, proved to have disappointing performance, but the inherent advantages appeared so great that it was a major line of research into the late 1990s. However, during this time the speed of computers grew greatly in relation to networking systems, and the disadvantages in performance came to overwhelm the advantages in development terms. Many attempts were made to adapt the existing systems to have better performance, but the overhead was always considerable and most of these efforts required the user-space programs to be moved back into the kernel. By 2000, most large-scale (Mach-like) efforts had ended, although OpenStep used an adapted Mach kernel called XNU, which is now used in the OS known as Darwin, which is the open source part of Mac OS X. As of 2012, the Mach-based GNU Hurd is also functional and its inclusion in testing versions of Arch Linux and Debian in progress.

Although major work on microkernels had largely ended, experimenters continued development. It has since been shown that many of the performance problems of earlier designs were not a fundamental requirement of the concept, but instead due to the designer's desire to use single-purpose systems to implement as many of these services as possible. Using a more pragmatic approach to the problem, including assembly code and relying on the processor to enforce concepts normally supported in software led to a new series of microkernels with dramatically improved performance.

Microkernels are closely related to exokernels. They also have much in common with hypervisors, but the latter make no claim to minimality and are specialized to supporting virtual machines; indeed, the L4 microkernel frequently finds use in a hypervisor capacity.