GNU Hurd (usually referred to as the Hurd) is a free software computer operating system kernel, released under the GNU General Public License.

HURD is a mutually recursive acronym, standing for HIRD of Unix-Replacing Daemons, where HIRD stands for HURD of Interfaces Representing Depth. It is also a play on the words herd of gnus, reflecting how it works.

Development history

Development on the GNU operating system began in 1984 and initially made good progress. Free GNU tools started to acquire a good reputation and were often adopted in preference to inferior proprietary tools provided by system vendors. By the early 1990s, the only major component missing was the kernel.

Development on the Hurd began in 1990 after an abandoned kernel attempt in 1986, based on the research TRIX operating system developed by Professor Steve Ward and his group at MIT's Laboratory for Computer Science (LCS). According to Thomas Bushnell, the initial Hurd architect, their early plan was to adapt the 4.4BSD-Lite kernel and, in hindsight, "It is now perfectly obvious to me that this would have succeeded splendidly and the world would be a very different place today". However, in 1987, due to a lack of cooperation from the Berkeley programmers, Richard Stallman proposed instead to use the Mach microkernel developed at Carnegie-Mellon University. Work on this was delayed for three years due to uncertainty over whether CMU would release the Mach code under a suitable license.

With the release of the Linux kernel in 1991, the primary consumer of GNU's userland components soon became the Linux operating system, prompting the coining of the controversial term GNU/Linux.

Development of the Hurd has proceeded slowly. Despite an optimistic announcement by Stallman in 2002 predicting a release of GNU/Hurd later that year, the Hurd is still not considered suitable for production environments. Development in general has not met expectations, and there are still bugs and missing features. This has resulted in a poorer product than many (including Stallman) had expected.

The Debian project, among others, have worked on the Hurd project to produce binary distributions of Hurd-based GNU operating systems for PC compatible systems.

Architecture

Unlike the majority of Unix-like kernels, the Hurd builds on top of a microkernel which is responsible for providing the most basic kernel services  coordinating access to the hardware: the CPU (through process management and scheduling), RAM (via memory management), and other various input/output devices (via I/O scheduling) for sound, graphics, mass storage, etc. In theory the microkernel design would allow for all device drivers to be built as servers working in user space, but today most drivers of this kind are still contained inside GNU Mach, the currently used microkernel. That is because initially user-space drivers would have suffered from performance loss, due to the overhead of the Mach interprocess communication. With the performance of today's machines, it is possible that this overhead would no longer cause a significant performance problem.