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Get To Know Mach, the Kernel of Mac OS X
An anonymous reader writes "Linux is a kernel, not an operating system. So what is Mac OS X's kernel? The Mach microkernel. The debate around Monolithic (Linux) and Micro (Mach) kernels continues, and there is a great chapter online about the Mach system from the very good book 'Operating System Concepts'. Which design is better? I report, you decide." Warning: link is to a PDF.
Huge PDF on Slashdot. This can't end well. Mirror
This appendix on Mach is from the newest edition of the classic "Operating System Concepts," Seventh Edition by Silberschatz, Galvin, and Gagne (Wiley). ISBN: 0-471-69466-5. Published December 2004.
/. them): http://www.wiley.com/WileyCDA/WileyTitle/productCd -0471694665.html
There are also free online chapters for FreeBSD and Nachos.
Link to Wiley's purchase page (given that we are
I thought the article was more than relativly informative. Personally I love my mac and I think it's about time people stop fighting over which OS is better, use the right tool for the job, be that Linux, mac, windows, whatever. Anyways, figured I'd throw in a link to some other cool stuff about mach. http://rentzsch.com/papers/overridingMacOSX The page deals with code injection and function overriding within MAC OS X. I think something like it was on here not too long ago but it's also pretty interesting stuff, I'd suggest the read.
Monolithic translates to no modules, correct?
I can't believe people are modding you up for this.
The Linux kernel is monolithic. Linux modules do not run in user-mode. They are loaded into the kernel proper.
mkLinux was an Apple-sponsored effort to run Linux on Mach. The Linux kernel was modified to run in user-mode; it basically became an executable. In fact, you could run multiple instances of the same kernel (or different kernels) simultaneously.
-mkb
That is monolithic as well, but not using the term in the same way. Monolithic essentially means made from a single piece. This CAN refer to modules as well, as the kernel modules aren't built into the kernel binary, but in the case of monolithic vs. microkernel, it doesn't refer to how the kernel is built. Rather it refers to the execution of the operating system kernel. A modular Linux kernel loads as a single executable that then loads modules into it's process space as needed to do things. This is essentially a monolithic kernel. The OS runs as a single process. Microkernel's have the OS split as seperate processes, mostly outside the core microkernel (which has the job of facilitating message passing between all these processes, and lowlevel process management). The Microkernel may or may not do I/O, sometimes seperate processes do. Hope that helps.
stop spreading the myth that Xnu is a microkernel
Well nope. You can insert newly compiled modules into a previously compiled kernel to get new features (that's how the many proprietory video drivers work, for example.) But those are
a) running in kernel space, not user space
b) communicated with by predefined hooks, rather than a generic message passing interfacing.
That's why linux modules, which are superficially like elements of a microkernel, are not really like them at all.
Athletic Scholarships to universities make as much sense as academic scholarships to sports teams.
monolithic in this case also means interface-monolithic.
basically all the interfaces are defined as symbols to the linker, and all interfaces are defined c-native.
the micro-kernels are meant to use message passing and more abstracted interfaces, as well as separate address spaces to ensure a bad module does not take down the entire kernel. Think of it like the modules run as only semi-privileged applications, handling their hardware and then giving control back to the micro-kernel which does as little as possible to arbitrate control and schedule between the subsystems and user-mode applications. Drivers are no longer fully privileged, and the entire user-space can be considered a subsystem of the kernel.
it's different, and kinda hard to design for, but i can't wait for hurd to release a linux compat layer.
The first rule of USENET is you do not talk about USENET.
Given this compatibility effort, mach is not a fair comparison, either in hurd or osx, for comparing the merits and performance between that of monolithic and microkernel achitectures because so much extra stuff was added to a design never intended for posix. Something like QNX4 and later, designed both as a microkernel and for posix, or perhaps a pure mach system running applications designed specifically for mach, might be a more fare basis to compare the value of microkernel vs monolithic architectures.
Mach on hurd is easier to grasp and test since many of the lower level mach kernel services are still represented and usable there. Apple seems to be trying to eliminate visibility of as many of the lower level mach services from application developers as possible. Yet, there are still many things that can only be done in the mach kernel on osx or darwin (such as threads that can be cancelled on socket operations or sleeps). If one wanted a bsd/posix compliant environment, I think Apple would have been far better off starting from PPC/xBSD or Linux kernels, rather than trying to rope and rebuild mach to fit into something it was never originally designed for.
No, you're mixing terms.
So, the linux kernel supports kernel modules, and its design is to some degree "modular" (as any project that size would have to be), but noone would claim it to be a microkernel.
--Bruce Fields
The BSD and Mach personalities run together in one and the same (kernel) address space. The BSD layer does not consists of merely some user space libraries. See e.g. this graphic.
Donate free food here
OS X's kernel, "xnu", is /based/ on Mach 3.0 and obviously shares a few concepts with Mach, but is neither a pure Microkernel, nor are all its components from Mach.
Amit Singh has a well-written page about XNU: http://www.kernelthread.com/mac/osx/arch_xnu.html
Mac OS X does not use Mach like a microkernel at all. I wish people'd get this through their thick skulls.
/. would actually edit for content.
It uses Mach and BSD in THE SAME ADDRESS SPACE. As such, it's basically as monolithic as it gets. It just happens to incoporate Mach in the kernel space and uses it for threads and IPC.
Anyone who takes 10 minutes to look at the Darwin documentation would know this.
I wish
The kernel that Apple uses in OS X is called XNU.
It uses code FROM Mach, but it is not Mach and it is not a Microkernel. NT (NT 4.0, NT 5.0 (win2k), NT 5.1 (WinXP) does not use a Microkerenl either.
The only OS that I know of that actually uses a Microkernel is GNU/Hurd.
The OS X kernel, called XNU, is a mixture of *BSD kernel code and Mach kernel code.
Yes, yes, there was a ancient debate between Linus and that other guy about Micro- vs Macro-kernels, and guess what, Linus was right.
Apple does NOT use a microkernel. It does not use Mach. It is BASED on Mach. It would be considured a kludge compared to Linux or FreeBSD, but it works out fine.
Similar to how Mustangs are based on Ford Falcons and Granadas from the late 70's and early 80's. Those cars were as much as a failure as Mach, however the Mustang is flashy and many people desire it. So go figure.
That is a true statement for the GNU project, but not for all of Linuxdom. Linux (the OS) was not started by the GNU folks. It was started as a separate project and incorporated items from the FSF (and BSD, etc.) into its release. From the beginning the whole OS has always been called "Linux" (search Google Groups for "linux 0.11 author:torvalds" and click on the "Linux information sheet" for an example of this).
Yes, RMS prefers to call the OS GNU/Linux, but that's because he's seeing things from the perspective of the GNU project incorporating the Linux kernel into their work. The rest of Linuxdom see Linux as the name of both the OS and the kernel, and qualify the name using the phrase "the Linux kernel" as an easy way to differentiate between the two.
So, the opening statement in the OS X story is false: Linux is an OS, and is used as such by folks every day. This is the reality of the situation, and it is, at best, wishful thinking on the part of folks who claim it is not to say otherwise.
..wayne..
Actually, the grandparent IS correct. I spent the last week studying the Mach and OS X designs, and I found the following things:
1. Mach is not a complete kernel. It requires someone to implement the areas which the Mach group were not researching. This has traditionally been done by compiling against BSD 4.3.
2. Mac OS X updated to the FreeBSD kernel instead of BSD 4.3 to gain a more modern kernel design with better hardware support.
3. OS 9 "Classic" is not a microkernel server, but rather a technology that Apple calls "Blue Box". Blue Box is a hardware virtualizer like VMWare that is capable of communicating directly with the OS X desktop. Using this communication, the OS 9 desktop is made to disappear, making the application appear to run on the OS X desktop.
4. The combination of Mach and FreeBSD is called "XNU" by Apple. The complete os is called Darwin, and the commercial variety with the Next and Mac APIs is called "Mac OS X".
More Info:
Mach Kernel
Wikipedia: Mach
Wikipedia: XNU
Blue Box info
Javascript + Nintendo DSi = DSiCade
Just because it runs on Mach doesn't mean that MacOS X is a microkernel architecture.
Just as an example, on the new MacMini hardware, sound level control is done in kernelspace (since HW doesn't support that anymore)! Whereas the LinuxPPC developers refuse to do things like this in kernelspace.
Actually in Linux many things are pushed out to userspace (think udev), making it much more microkernel-like than MacOSX.
(Not that Apple-Fanboys would understand anything of that)
"I can't believe people are modding you up for this."
I can. Maybe you're too young to remember when the term monolithic was commonly used to describe a kernel which, instead of using loadable modules, was linked as a single binary image. This was, and is, a valid use of the word. Here's an example.
The first time I heard someone say that Solaris is monolithic, I thought that they were saying that, like SysVR3, it didn't support loadable modules. I didn't realize that, with the development of microkernels, the term "monolithic kernel" had started to be used in a different context.
> I agree that way back when, Linux was the name of the kernel, period
Not so. Here a posting from Linux Torvalds about Linux -- from the beginning the term was used as both the name for the kernel and the whole OS:
LINUX INFORMATION SHEET
(last updated 13 Dec 1991)
1. WHAT IS LINUX 0.11
LINUX 0.11 is a freely distributable UNIX clone. It implements a
subset of System V and POSIX functionality. LINUX has been written
from scratch, and therefore does not contain any AT&T or MINIX
code--not in the kernel, the compiler, the utilities, or the libraries.
For this reason it can be made available with the complete source code
via anonymous FTP. LINUX runs only on 386/486 AT-bus machines; porting
to non-Intel architectures is likely to be difficult, as the kernel
makes extensive use of 386 memory management and task primitives.
[...]
2. LINUX features
- System call compatible with a subset of System V and POSIX
- Full multiprogramming (multiple programs can run at once)
- Memory paging with copy-on-write
- Demand loading of executables
- Page sharing of executables
- ANSI compliant C compiler (gcc)
- A complete set of compiler writing tools
(bison as yacc-replacement, flex as lex replacement)
- The GNU 'Bourne again' shell (bash)
- Micro emacs
- most utilities you need for development
(cat, cp, kermit, ls, make, etc.)
- Over 200 library procedures (atoi, fork, malloc, read, stdio, etc.)
- Currently 4 national keyboards: Finnish/US/German/French
- Full source code (in C) for the OS is freely distributable
- [...]
..wayne..
First of all, having modules or not has no effect on being monolithic. The entire kernel is a single process that simply executes code, wether its compiled into the kernel, or loaded into the kernel as a module makes no difference here. Microkernels actually have seperate processes for different parts of the kernel, and they cannot execute code from each other, they must communicate back and forth using some sort of message passing system.
And second, no BSD based kernel forces you to use modules. Have you actually tried any BSD? Modules are entirely optional, just like linux. In fact, openbsd's kernel only has support for modules, but nothing is actually compiled as a module, and using modules is unsupported.
> QNX uses shared memory to pass messages
So does Mach, and it's slow. I've never seen real-world measures to suggest that QnX is fast. All we know is that the performance of the OS itself is good, and that's a VERY DIFFERENT measure.
The slow performance is due to a number of problems:
1) not all MMU's are really suited to this task. Many are slower to set up than just copying the memory around. Sun found this to be at around 5k, below that, it was faster to just copy memory physically.
2) MMUs/VM are based on pages, 2 or 4k typically. Thus passing in a single 32-bit int parameter causes big page hits. You can tune this out, but it's still annoying.
3) Each copy takes TWO context switches - one to switch into the kernel to copy the memory across ports, another back out to the called program. This means that even the simplest "system calls" are twice as slow as under a monokernel, AT BEST.
4) Additionally the data has to be examined to see if it contains ports being passed around, and if so, they have to be translated because the port codes are private to a program (and thus different in the other one).
5) Using mapped memory ignores all the hardware specific solutions to these problems, many of which are built into modern processors.
It's exactly the sort of one-size-fits-all solution that you'd expect from a research project. One that doesn't work in the real world. One that should have been replaced, and was in L4, Spring, etc.
For instance, Spring included three different IPC systems, each tuned to certain types of data, each used in different ways on different CPUs. The "fast-path" used a half-switch into the kernel by mapping off registers, allowing IPC to degenerate into register passing largely identical to a procedure call. As long as the message fit within the limitations -- 8 registers, no port identifiers, etc. -- it was faster than a traditional Unix trap. These limitations seem serious, but were in fact used for 80% of calls and 60% of returns (you often say "getDiskSector(integer value)" which could fit into the fast-path, and get back 2k of data which wouldn't).
Maury
XNU is essentially a monolithic kernel, much like Linux. The real differences, in my opinion, lie in the IOKit object oriented driver API, whereas Linux has no real driver API and drivers have complete access to all kernel functions as drivers are simply kernel modules.