Gernot Heiser recently wrote an article, "Microkernels rule!" for Embedded.com about microkernels' bad reputation. I fully agree with the message of his article: operating systems based on microkernel technology don't necessarily have to be slow and can be made more robust than their monolithic counterparts. However, Gernot mentions the Hurd and incorrectly describes its position in history:
Mach, an OS that was widely used as the basis of systems, ran into serious performance problems... There were spectacular failures, none more so than IBM's Workplace OS, which cost the company a cool two gigabucks... Needless to say, the experience with Mach and others created a bit of an image problem for microkernels (which didn't stop the GNU Hurd from repeating the mistakes of the past). However, back in 1993, Jochen Liedtke demonstrated that these performance problems weren't inherent in the microkernel concept. The Hurd did not repeat the errors of past. Work on the Hurd started in 1990. In GNU's Bulletin January, 1994 [1], you'll find an article detailing the Hurd's architecture. Workplace OS was conceived in 1991; it was deemed a failure around 1995. Jochen's article "Improving IPC by Kernel Design" was published in December 1993. Regarding, Workplace OS, its main goals were: machine independence, multiple personalities, and concurrent operation of personalities [3]. The last two goals, as far as I am aware, were never a priority in the development of the Hurd. Further, Workplace OS had already adopted many second generation microkernel features, for instance, L3's synchronous IPC. In the major "Observations and Lessons" section of [3], this is not even mentioned; management, coordination, and focus are cited as the major problems. Finally, the architectural problems that we have identified with Mach [4] are not related to IPC. The most important are the lack of resource accounting, and the bad resource management (paging decisions). Regarding the implementation that we use, the major problems are unoptimized code (e.g., when evicting a bunch of pages, Mach always sends them one at a time to the manager), and the decades old code base which was designed for machines with tens of megabytes of RAM. Neal [0] http://www.embedded.com/columns/guest/208800243 [1] http://www.gnu.org/bulletins/bull16.html#SEC13 [2] Improving IPC by kernel design Jochen Liedtke SOSP December 1993 http://portal.acm.org/citation.cfm?id=173668.168633 [3] Workplace Microkernel and OS: A Case Study Brett D. Fleisch and Mark Allan A. Co Software--Practice and Experience May 1998 http://portal.acm.org/citation.cfm?id=279869.279875 [4] A Critique of the GNU Hurd Multi-server Operating System Neal H. Walfield and Marcus Brinkmann ACM SIGOPS Operating Systems Review July 2007 http://portal.acm.org/citation.cfm?id=1278901.1278907
