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The Great Microkernel Debate Continues
ficken writes "The great conversation about micro vs. monolithic kernel is still alive and well. Andy Tanenbaum weighs in with another article about the virtues of microkernels. From the article: 'Over the years there have been endless postings on forums such as Slashdot about how microkernels are slow, how microkernels are hard to program, how they aren't in use commercially, and a lot of other nonsense. Virtually all of these postings have come from people who don't have a clue what a microkernel is or what one can do. I think it would raise the level of discussion if people making such postings would first try a microkernel-based operating system and then make postings like "I tried an OS based on a microkernel and I observed X, Y, and Z first hand." Has a lot more credibility.'"
And the date at the bottom of the article is "12 May 2006". The same article has been linked from slashdot before too. We really haven't argued about this for a while...
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while you are quite correct the question becomes why should the CPU handle those instructions? It is like USB 2.0 versus firewire 400. Firewire while "slower" burst rate has a higher steady rate precisely because it offloads some instructions.
SCSI, firewire are examples of good tech working for you. The CPU should output instructions to devices smart enough to be able to work on their own. Leaving more cycles available to do things that actually matter.
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If you read the article, Tannenbaum reminds everyone of how Microsoft paid Ken Brown to write a book accusing Linus of stealing the Minix microkernel. FTFA:
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OS X, strictly speaking, is a hybrid kernel. Essentially, NeXT mashed together Carnegie Mellon's microkernel Mach with BSD (a monolithic kernel) - yielding the overwhelmingly originally named XNU kernel. (X is Not Unix). So in short - yes , OS X is a microkernel based OS, but is just as much a monolithic kernel based OS.
The secret to creativity is knowing how to hide your sources. - Albert Einstein
The only reason this debate is going on is because CPUs do not have the concept of modules. If they did, then each module would not be able to crash the rest of the modules.
If you wonder how to do modules without sacrificing the flat address space, it's quite easy: In most CPU designs, each page descriptor has a user/supervisor bit which defines if the contents of a page are accessible by the other pages. Instead of this bit, CPUs must use the target address to look up module information from another table. In other words, the CPU must maintain a map of addresses to modules, and use this map to provide security access.
This design is not as slow as it initially might seem. Modern CPUs are very fast, and they already contain many such maps: the Translation Lookaside Buffer, the Global Descriptor Table cache, the Local Descriptor Table cache, Victim Caches, Trace Caches, you name it.
Depends. Ever hear of FUSE? It's been showing up in quite a few distros for the capabilities it buys by running outside of kernel space. It's become so important, that it has been ported to BSD, Solaris, and Mac OS X.
What does it do? Why, it's a monolithic driver that provides an interface to support userspace filesystem drivers. i.e. A microkernel in practice, if not in definition. Ergo, the grandparent's point about a slow migration.
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He mentions them because they meet his design goal: they are highly-reliable operating systems used in mission-critical applications. (Here, "mission" might be, "Bombing the fuck out of people.") He is building his case that it's easier to design a bullet-proof OS using a microkernel, as opposed to a monolithic kernel.
And he's right. If your goal is reliability and security, a microkernel is a better design. Both goals rely on limiting the amount of time (and the amount of code) spent in kernel space. "Process isolation" is the mantra.
NeXTStep was a hybrid kernel. It was *almost* a microkernel (based on Mach). And, it was *highly* usable. It had the most usable UI in the industry, and still does in its current reincarnation as OS-X.
I think microkernels still have legs.
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