Right in between a car crash and Easter, I knew I had to write a Sunday Eve Column. So here I am, digesting vast quantities of chocolate eggs (and I don't even like chocolate), craving for coffee (for me about as special as breathing), with the goal of explaining to you my, well, obsession with microkernels. Why do I like them? Why do I think the microkernel paradigm is superior to the monolithic one? Read on.
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Member since:2005-12-03
I'm sorry, but if I have to choose between believing you in saying NT did not start out as a muK, and andy Tanenbaum saying it did, I prefer the latter. No offense.
Look Thom, I've read most of the available windows internals literature, even that leaked W2K source code, spent countless hours disassembling NTOSKRNL.EXE and even reported some kernel bugs to MS lately, and found absolutely no proof that NT was ever to be or has been a microkernel. Function calls between various executive components are so interleaved that there is absolutely no chance of decoupling them. Ever.
Noone questions Mr. Tanenbaum's authority on general OS design theory, distributed systems, compiler and automata theory etc. - but I think that in this case, he fell on MS's marketing crap. Heck, even Cutler said they're building something different from RSX-11M (which was a microkernel).
Read the relevant literature (Russinovich/Solomon is great for starters), or check out the discussions on this "microkernel issue" on comp.os.ms-windows.programmer.nt.kernel-mode.
That's a subjective matter. A kernel is hybrid when it combines both worlds; some parts live in kernelspace, while others do not. Exactly what goes into kernelspace has absolutely nothing to do with it.
Oh, it certainly has. "Hybrid microkernel" is in this context so vaguely defined term it can be applied on any OS you like. Let me put it like this: NT has inside kernel-mode (ring0) almost anything that traditional monolithic *nix kernels (Linux, Solaris, *BSD etc.) have + GUI. So can anybody tell me what the hell is so microkernelish about it, that other monolithic OSes don't have, so that it can be entitled as "hybrid microkernel"???
And since NT is very flexiable and modular, they can move stuff in and out of kernelspace relatively easily
No they can't 
I would really like to see a hacker capable of putting, lets say, Object Manager, inside user-mode server process and routing all handle-checking logic via IPC on it :-)
(hence the GUI in Vista will live in userspace again).
I thought we've demistifyed that one..
http://osnews.com/permalink.php?news_id=13007&comment_id=74945
Member since:2006-02-15
Tannenbaum's wrong on this one. Cutler et al started out designing NT along similar lines to VMS, which he had worked on at DEC. Early Microsoft documentation on the NT structure makes this non-microkernel architecture very apparent. About the time of the Seattle OSDI, Mach was gaining mind share, and Cutler came to OSDI with a presentation declaring NT to be "micro kernel based". It was, by far, the funniest presentation I've ever seen.
Early on, a "microkernel" system was thought to be one in which message-passing was used to separate functional subsystems. This comes from Mach's predecessors, especially Accent.
Mach was originally intended to be a "pure" message-passing mostly user space OS ala Accent, but DARPA demanded that Mach be Unix compatible if Rashid wanted DARPA funding. From this was born the BSD/Mach hybrid crap that ended up dying such a horrible death as OSF/1.
"microkernel" is a silly idea. It specifies an implementation technology (and one based on the hardware privilige model of a particular processor architecture at that) as a solution to an architectural problem.
That's the real reason why "microkernel" never made it out of academia into any widespread commercial use.
Member since:2005-06-29
That's the real reason why "microkernel" never made it out of academia into any widespread commercial use.
And yet there is a muK operating system we all use, we all encounter basically daily: QNX. It powers a lot of medical equipment, cars, home theater equipment, satelites, the robotic arm on the space shuttle, etc.
In fact, QNX is one of the leading RTOS's as well as one of the leading embedded operating systems, so saying that the muK never made it out of academia is wrong.
Member since:2006-02-15
I'm sorry, I thought we were discussing general purpose OSes.
The embedded universe (real embedded, not what's now coming to be called embedded) is a different beast with a different set of requirements.
QNX didn't start out as a "microkernel" -- it even predates Mach. It started out as a small well organized architecture for embedded applications. And it was well done. Also very much bare-boned.
It is the well organized architecture, and not the 'microkernel' that made QNX the market leader.
Do you, by the way, recall when QNX got around to adding 'microkernel' as a marketing bullet? 2001? when they were already, what, 20 years old.
QNX, by the way, is more like Brevix, in having a small set of base facilities, than it is like a traditional microkernel, because it does not rely on message passing.
Member since:2006-01-25
I'm sorry, but if I have to choose between believing you in saying NT did not start out as a muK, and andy Tanenbaum saying it did, I prefer the latter. No offense.
Mr. Tanenbaum never said that NT started as an muK. He would have had no way of knowing that. He was merely parroting what Microsoft was saying as they were promoting their new OS and he foolishly took them at their word. This was WAY before NT shipped btw... You might be interested to see what he has said since then:
http://www.cs.vu.nl/~ast/brown/followup/
Quote straight from the horse's mouth (about 3 quarters of the way down):
"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."
You can call it a "Hybrid" if you like, but that is nothing more than marketing bullshit. Hybrids are simply what companies call monolithic kernels to sound cool... 
Mr. Tanenbaum defines a muK the same way as most academics, a small kernel that does AT THE MOST memory management, scheduling, and IPC. EVERYTHING else, including all drivers, file systems, etc. should communicate with the kernel from userland. Period. By that definition neither NT nor OSX are even close to being microkernels.
Member since:
2005-06-29
No, NT is NOT a microkernel, and was never designed to be one.
I'm sorry, but if I have to choose between believing you in saying NT did not start out as a muK, and andy Tanenbaum saying it did, I prefer the latter. No offense.
it has even super-nonessential stuff in ring0 such as GUI, traditionally in ring3 on *NIX
That's a subjective matter. A kernel is hybrid when it combines both worlds; some parts live in kernelspace, while others do not. Exactly what goes into kernelspace has absolutely nothing to do with it. You might argue that the desktop is not supposed to be in kernelspace-- but for a desktop operating system... I'd say the desktop is important and therefore giving it a speed bump by placing it in kernelspace makes sense. The reason the GUI does not live in kernelspace in UNIX is because, well, UNIX is not a desktop operating system. Using UNIX as a benchmark for defining how a desktop operating system should handle its kernel is like asking a butcher to explain to the bakery how to make bread.
But if both higher-level services and the "microkernel" are in the same privilege level, they must be in the same address space, meaning no protection, no security, no mutual IPC - ie. no microkernel design at all
Err... Exactly, because the NT kernel is indeed not a muK, it's a hybrid. And by hybrid one means that certain parts live in kernelspace, while others don't. And since NT is very flexiable and modular, they can move stuff in and out of kernelspace relatively easily (hence the GUI in Vista will live in userspace again).