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Torvalds on the Microkernel Debate
diegocgteleline.es writes "Linus Torvalds has chimed in on the recently flamed-up (again) micro vs monolithic kernel, but this time with an interesting and unexpected point of view. From the article: 'The real issue, and it's really fundamental, is the issue of sharing address spaces. Nothing else really matters. Everything else ends up flowing from that fundamental question: do you share the address space with the caller or put in slightly different terms: can the callee look at and change the callers state as if it were its own (and the other way around)?'"
This my favorite Linus quote from that whole thread:
"In the UNIX world, we're very used to the notion of having
many small programs that do one thing, and do it well. And
then connecting those programs with pipes, and solving
often quite complicated problems with simple and independent
building blocks. And this is considered good programming.
That's the microkernel approach. It's undeniably a really
good approach, and it makes it easy to do some complex
things using a few basic building blocks. I'm not arguing
against it at all."
He basically continues his previous argument that monolithic kernels are more efficient and easier to implement. Microkernels may seem simpler, but they have complexity in implementing all but the simple tasks. Microkernels have a more marketable name. "Microkernel" just sounds more advanced than "monolithic". He finishes off with the observation that the term "hybrid kernel" is a trick to grab marketing buzz from the microkernel side of things.
My other first post is car post.
Linus FTFA:
"The fundamental result of access space separation is that you can't share data structures. That means that you can't share locking, it means that you must copy any shared data, and that in turn means that you have a much harder time handling coherency. All your algorithms basically end up being distributed algorithms.
And anybody who tells you that distributed algorithms are "simpler" is just so full of sh*t that it's not even funny.
Microkernels are much harder to write and maintain exactly because of this issue. You can do simple things easily - and in particular, you can do things where the information only passes in one direction quite easily, but anythign else is much much harder, because there is no "shared state" (by design). And in the absense of shared state, you have a hell of a lot of problems trying to make any decision that spans more than one entity in the system.
And I'm not just saying that. This is a fact. It's a fact that has been shown in practice over and over again, not just in kernels. But it's been shown in operating systems too - and not just once. The whole "microkernels are simpler" argument is just bull, and it is clearly shown to be bull by the fact that whenever you compare the speed of development of a microkernel and a traditional kernel, the traditional kernel wins. By a huge amount, too.
The whole argument that microkernels are somehow "more secure" or "more stable" is also total crap. The fact that each individual piece is simple and secure does not make the aggregate either simple or secure."
Quick slashdoteffect there, that forum is already down. Anyhow.. mirror: http://www.mirrordot.org/stories/3f6b22ec7a7cffcf2 847b92cd5dec7e7/index.html
http://pastebin.ca/54695
HUH??
Get your facts straight.
Every popular Operating System developed in the past 15 years (and then some) apart from Linux has been either a microkernel or a hybrid kernel.
Mach, upon which Darwin and OS X are based is a microkernel. OSX and Darwin borrow some monolithic-esque features, but not quite enough to make them hybrids it would seem...
Windows NT, NetWare, ReactOS and BeOS are all Hybrid kernels. This model seems to be the most popular right now, and seems to be a reasonable compromise...
The only thing that's left are the old big-iron Unices, Solaris, MS-DOS, and Linux. In other words, Linux is the only major player left using a monolithic kernel. I don't know enough about computer science to properly make an argument one way or another, but it would see that monolithic kernels have heavily fallen out of favor in the past 15 years.
That said, perhaps a monolithic kernel is better suited to the open-source development process, which would seem counterintuitive at first because it discourages modularization, but who knows.... it could very well be true. I don't know enough to comment.
-- If you try to fail and succeed, which have you done? - Uli's moose
That would be monolithic+marketing.
MacOS X is no microkernel system. It does have Mach, sure. Mach is arguably not a microkernel by today's standards, and in any case MacOS X has a full BSD kernel bolted onto the Mach kernel. Mach and BSD are sharing address space. In other words, it's not a microkernel.
NT is the same way.
I don't know all that much about NetWare, but I'd never before heard anyone claim it to be a microkernel. It's not terribly popular anyway. (it was, but back then I'm sure it wasn't a microkernel system) ReactOS isn't much yet. BeOS died for unrelated reasons, so we really can't judge.
Monolithic kernels can be very modular. Microkernels can get really convoluted as the developers struggle with the stupid restrictions.
Here is some good readign amterial, maybe people should read and _understand_ it before posting on the subject..
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0 ,1144,0131429388,00.html
This does not mean you have to agree with the guy.
http://www.computer.org/portal/site/computer/menu
http://vig.prenhall.com/catalog/academic/product/
Sort of. In the scanerio you describe, the program was, in fact, not proved to be correct because the people who did the proof failed to take into account the real requirements for the system.
If you don't even know your requirements, no methodology to implement those requirements is going to work reliably.
I agree absolutly.
The point I was trying to make - and this is where I see the parallel - is that T seems to be trying to say that microkernel good, monolithic bad based only on elegant design, and theoretical simplicity. No doubt, it appeals to the academic in him (Go figure)
But he is ignoring the "time domain" of an operating system, if you will - it's practicality, it's ability to do usefull work in a reasonable period, and it's usability in the real world - just as Parnas' notation did.
I make no claim as to whether or not Parnas *intended* for his notation to be used for a hard real-time system - I know he was retained as a consultant on the project, but I personally neither saw nor heard of/from him during the entire time. And let me be perfectly clear - his notation was absolutly *gorgeous* and extremly usefull. I, on the other hand, having been the idiot who raised the point in the first place, wound up having to do the timing analysis based on best/worst case sensor timings, and instruction-by-instruction counting of the clock cycles required for each ISR, etc. I plugged the numbers into a programme I wrote for the purpose, and basically did nothing more than an exhaustive analysis of all possible combinations of timings. Not as elegant by far, but what the hell. Who cares if it takes two days to run, if it means you don't have to worry about glowing in the dark?
1. From AST (I'd assume you know who he is since you are interested in Linus/microkernel debate): http://www.cs.vu.nl/~ast/brown/followup/ Read the section "Microkernels Revisited":
I can't resist saying a few words about microkernels. A microkernel is a very small kernel. If the file system runs inside the kernel, it is NOT a microkernel. The microkernel should handle low-level process management, scheduling, interprocess communication, interrupt handling, and the basics of memory management and little else. ... Microsoft claimed that Windows NT 3.51 was a microkernel. It wasn't. It wasn't even close. Even they dropped the claim with NT 4.0.
2. From Windows Internals, the 4th edition, published by Microsoft Press. Page 36: Windows is similar to most Unix systems in that it's a monolithic operating system in the sense that the bulk of the operating system and device driver code shares the same kernel-mode protected memory space. Can we stop claiming Windows has a microkernel now?
I worked on a commercial microkernel OS.
The learning curve was very steep. New developers took at least half a year to be productive. A number of people never became productive and had to be fired.
Linux is really clean and tidy compared to that. Even BSD is clean and tidy compared to that microkernel OS.
Separated components tend to get complex interactions. Sharing data can be very awkward, even if you are co-located.