Mach (kernel)
Encyclopedia
Mach is an operating system
kernel
developed at Carnegie Mellon University
to support operating system research, primarily distributed and parallel computation. Although Mach is often mentioned as one of the earliest examples of a microkernel
, not all versions of Mach are microkernels. Mach's derivatives are the basis of the modern operating system kernels in Mac OS X
(which is not a microkernel) and GNU Hurd
(which is a microkernel).
The project at Carnegie Mellon ran from 1985 to 1994, ending in apparent failure with Mach 3.0, which was finally a true microkernel
. Mach was developed as a replacement for the kernel in the BSD
version of UNIX
, so no new operating system would have to be designed around it. Today further experimental research on Mach appears to have ended, although Mach and its derivatives are in use in a number of commercial operating systems, such as NeXTSTEP
and OPENSTEP
, and most notably Mac OS X
using the XNU
operating system kernel which incorporates an earlier (non-microkernel) Mach as a major component. The Mach virtual memory management system was also adopted by the BSD developers at CSRG
, and appears in modern BSD-derived UNIX systems, such as FreeBSD
. Neither Mac OS X nor FreeBSD maintain the microkernel structure pioneered in Mach, although Mac OS X continues to offer microkernel inter-process communication
and control primitives for use directly by applications.
Mach is the logical successor to Carnegie Mellon's Accent kernel
. The lead developer on the Mach project, Richard Rashid
, has been working at Microsoft
since 1991 in various top-level positions revolving around the Microsoft Research
division. Another of the original Mach developers, Avie Tevanian
, was formerly head of software at NeXT
, then Chief Software Technology Officer at Apple Computer
until March 2006.
have tried this way. Nevertheless, those same developers lamented the loss of flexibility that the original concept offered. Another level of virtualization was sought that would make the system "work" again.
The critical abstraction
in UNIX was the pipe. What was needed was a pipe-like concept that worked at a much more general level, allowing a broad variety of information to be passed between programs. Such a system did exist using inter-process communication
(IPC): A pipe-like system that would move any information between two programs, as opposed to file-like information. While many systems, including most Unixes, had added various IPC implementations over the years, at the time these were generally special-purpose libraries only really useful for one-off tasks.
Carnegie Mellon University
started experimentation along these lines under the Accent kernel
project, using an IPC system based on shared memory
. Accent was a purely experimental system with many features, developed in an ad-hoc fashion over a period of time with changing research interests. Additionally, Accent's usefulness for research was limited because it was not UNIX-compatible, and UNIX was already the de-facto standard for almost all operating system research. Finally, Accent was tightly coupled with the hardware platform on which it was developed, and at the time in the early 1980s it appeared there would soon be an explosion of new platforms, many of them massively parallel
.
Mach started largely as an effort to produce a cleanly-defined, UNIX-based, highly portable Accent. The result was a short list of generic concepts:
Mach developed on Accent's IPC concepts, but made the system much more UNIX-like in nature, even able to run UNIX programs with little or no modification. To do this, Mach introduced the concept of a port, representing each endpoint of a two-way IPC. Ports had security and rights like files under UNIX, allowing a very UNIX-like model of protection to be applied to them. Additionally, Mach allowed any program to handle privileges that would normally be given to the operating system only, in order to allow user space
programs to handle things like interacting with hardware.
Under Mach, and like UNIX, the operating system again becomes primarily a collection of utilities. As with UNIX, Mach keeps the concept of a driver for handling the hardware. Therefore all the drivers for the present hardware have to be included in the microkernel. Other architectures based on Hardware Abstraction Layer or exokernel
s could move the drivers out of the microkernel.
The main difference with UNIX is that instead of utilities handling files, they can handle any "task". More operating system code was moved out of the kernel and into user space, resulting in a much smaller kernel and the rise of the term microkernel
. Unlike traditional systems, under Mach a process, or "task", can consist of a number of threads. While this is common in modern systems, Mach was the first system to define tasks and threads in this way. The kernel's job was reduced from essentially being the operating system to maintaining the "utilities" and scheduling their access to hardware.
The existence of ports and the use of IPC is perhaps the most fundamental difference between Mach and traditional kernels. Under UNIX, calling the kernel consists of an operation known as a syscall or trap
. The program uses a library
to place data in a well known location in memory and then causes a fault, a type of error. When the system is first started the kernel is set up to be the "handler" of all faults, so when the program causes a fault the kernel takes over, examines the information passed to it, and then carries out the instructions.
Under Mach, the IPC system was used for this role instead. In order to call system functionality, a program would ask the kernel for access to a port, then use the IPC system to send messages to that port. Although the messages were triggered by syscalls as they would be on other kernels, under Mach that was pretty much all the kernel did—handling the actual request would be up to some other program.
The use of IPC for message passing benefited support for threads and concurrency. Since tasks consisted of multiple threads, and it was the code in the threads that used the IPC mechanism, Mach was able to freeze and unfreeze threads while the message was handled. This allowed the system to be distributed over multiple processors, either using shared memory directly as in most Mach messages, or by adding code to copy the message to another processor if needed. In a traditional kernel this is difficult to implement; the system has to be sure that different programs don't try to write to the same memory from different processors. Under Mach this was well defined and easy to implement; it was the very process of accessing that memory, the ports, that was made a first-class citizen of the system.
The IPC system initially had performance problems, so a few strategies were developed to minimize the impact. Like its predecessor, Accent
, Mach used a single shared-memory mechanism for physically passing the message from one program to another. Physically copying the message would be too slow, so Mach relies on the machine's memory management unit
(MMU) to quickly map the data from one program to another. Only if the data is written to would it have to be physically copied, a process known as copy-on-write
.
Messages were also checked for validity by the kernel, to avoid bad data crashing one of the many programs making up the system. Ports were deliberately modeled on the UNIX file system concepts. This allowed the user to find ports using existing file system navigation concepts, as well as assigning rights and permissions as they would on the file system.
Development under such a system would be easier. Not only would the code being worked on exist in a traditional program that could be built using existing tools, it could also be started, debugged and killed off using the same tools. With a monokernel
a bug in new code would take down the entire machine and require a reboot, whereas under Mach this would require only that the program be restarted. Additionally the user could tailor the system to include, or exclude, whatever features they required. Since the operating system was simply a collection of programs, they could add or remove parts by simply running or killing them as they would any other program.
Finally, under Mach, all of these features were deliberately designed to be extremely platform neutral. To quote one text on Mach:
There are a number of disadvantages, however. A relatively mundane one is that it is not clear how to find ports. Under UNIX this problem was solved over time as programmers agreed on a number of "well known" locations in the file system to serve various duties. While this same approach worked for Mach's ports as well, under Mach the operating system was assumed to be much more fluid, with ports appearing and disappearing all the time. Without some mechanism to find ports and the services they represented, much of this flexibility would be lost.
By 1986 the system was complete to the point of being able to run on its own on the DEC VAX. Although doing little of practical value, the goal of making a microkernel was realized. This was soon followed by versions on the IBM PC/RT and for Sun Microsystems
68030-based workstations, proving the system's portability. By 1987 the list included the Encore Multimax and Sequent Balance
machines, testing Mach's ability to run on multiprocessor systems. A public Release 1 was made that year, and Release 2 followed the next year.
Throughout this time the promise of a "true" microkernel was not yet being delivered. These early Mach versions included the majority of 4.3BSD in the kernel, a system known as POE Server, resulting in a kernel that was actually larger than the UNIX it was based on. The idea, however, was to move the UNIX layer out of the kernel into user-space, where it could be more easily worked on and even replaced outright. Unfortunately performance proved to be a major problem, and a number of architectural changes were made in order to solve this problem. Unwieldy UNIX licensing issues were also plaguing researchers, so this early effort to provide a non-licensed UNIX-like system environment continued to find use, well into the further development of Mach.
The resulting Mach 3 was released in 1990, and generated intense interest. A small team had built Mach and ported it to a number of platforms, including complex multiprocessor systems which were causing serious problems for older-style kernels. This generated considerable interest in the commercial market, where a number of companies were in the midst of considering changing hardware platforms. If the existing system could be ported to run on Mach, it would seem it would then be easy to change the platform underneath.
Mach received a major boost in visibility when the Open Software Foundation
(OSF) announced they would be hosting future versions of OSF/1 on Mach 2.5, and were investigating Mach 3 as well. Mach 2.5 was also selected for the NeXTSTEP
system and a number of commercial multiprocessor vendors. Mach 3 led to a number of efforts to port other operating systems parts for the microkernel, including IBM
's Workplace OS
and several efforts by Apple Computer
to build a cross-platform version of the Mac OS
.
For some time it appeared that every future operating system would be using the Mach as kernel by the late 1990s.
Some of Mach's more esoteric features were also based on this same IPC mechanism. For instance, Mach was able to support multi-processor machines with ease. In a traditional kernel extensive work needs to be carried out to make it reentrant or interruptible, as programs running on different processors could call into the kernel at the same time. Under Mach, the bits of the operating system are isolated in servers, which are able to run, like any other program, on any processor. Although in theory the Mach kernel would also have to be reentrant, in practice this isn't an issue because its response times are so fast it can simply wait and serve requests in turn. Mach also included a server that could forward messages not just between programs, but even over the network, which was an area of intense development in the late 1980s and early 1990s.
Unfortunately, the use of IPC for almost all tasks turned out to have serious performance impact. Benchmarks on 1997 hardware showed that Mach 3.0-based UNIX
single-server implementations were about 50% slower than native UNIX.
Studies showed the vast majority of this performance hit, 73% by one measure, was due to the overhead of the IPC. And this was measured on a system with a single large server providing the operating system; breaking the operating system down further into smaller servers would only make the problem worse. It appeared the goal of a collection-of-servers was simply not possible.
Many attempts were made to improve the performance of Mach and Mach-like microkernels, but by the mid-1990s much of the early intense interest had died. The concept of an operating system based on IPC appeared to be dead, the idea itself flawed.
In fact, further study of the exact nature of the performance problems turned up a number of interesting facts. One was that the IPC itself was not the problem: there was some overhead associated with the memory mapping needed to support it, but this added only a small amount of time to making a call. The rest, 80% of the time being spent, was due to additional tasks the kernel was running on the messages. Primary among these was the port rights checking and message validity. In benchmarks on an 486
DX-50, a standard UNIX system call took an average of 21μs
to complete, while the equivalent operation with Mach IPC averaged 114μs. Only 18μs of this was hardware related; the rest was the Mach kernel running various routines on the message. Given a syscall that does nothing, a full round-trip under BSD would require about 40μs, whereas on a user-space Mach system it would take just under 500μs.
When Mach was first being seriously used in the 2.x versions, performance was slower than traditional monolithic operating systems, perhaps as much as 25%. This cost was not considered particularly worrying, however, because the system was also offering multi-processor support and easy portability. Many felt this was an expected and acceptable cost to pay. When Mach 3 attempted to move most of the operating system into user-space, the overhead became higher still: benchmarks between Mach and Ultrix
on a MIPS R3000
showed a performance hit as great as 67% on some workloads.
For example, getting the system time involves an IPC call to the user-space server maintaining system clock. The caller first traps into the kernel, causing a context switch and memory mapping. The kernel then checks that the caller has required access rights and that the message is valid. If it does, there is another context switch and memory mapping to complete the call into the user-space server. The process must then be repeated to return the results, adding up to a total of four context switches and memory mappings, plus two message verifications. This overhead rapidly compounds with more complex services, where there are often code paths passing through many servers.
This was not the only source of performance problems. Another centered on the problems of trying to handle memory properly when physical memory ran low and paging had to occur. In the traditional monolithic operating systems the authors had direct experience with which parts of the kernel called which others, allowing them to fine tune their pager to avoid paging out code that was about to be used. Under Mach this wasn't possible because the kernel had no real idea what the operating system consisted of. Instead they had to use a single one-size-fits-all solution that added to the performance problems. Mach 3 attempted to address this problem by providing a simple pager, relying on user-space pagers for better specialization. But this turned out to have little effect. In practice, any benefits it had were wiped out by the expensive IPC needed to call it in.
Other performance problems were related to Mach's support for multiprocessor
systems. From the mid-1980s to the early 1990s, commodity CPUs grew in performance at a rate of about 60% a year, but the speed of memory access grew at only 7% a year. This meant that the cost of accessing memory grew tremendously over this period, and since Mach was based on mapping memory around between programs, any "cache miss" made IPC calls slow.
Regardless of the advantages of the Mach approach, these sorts of real-world performance hits were simply not acceptable. As other teams found the same sorts of results, the early Mach enthusiasm quickly disappeared. After a short time many in the development community seemed to conclude that the entire concept of using IPC as the basis of an operating system was inherently flawed.
Most developers instead stuck with the original POE concept of a single large server providing the operating system functionality. In order to ease development, they allowed the operating system server to run either in user-space or kernel-space. This allowed them to develop in user-space and have all the advantages of the original Mach idea, and then move the debugged server into kernel-space in order to get better performance. Several operating systems have since been constructed using this method, known as co-location, among them Lites
, MkLinux
, OSF/1 and NeXTSTEP
/OPENSTEP
/Mac OS X
. The Chorus microkernel
made this a feature of the basic system, allowing servers to be raised into the kernel space using built-in mechanisms.
Mach 4 attempted to address these problems, this time with a more radical set of upgrades. In particular, it was found that program code was typically not writable, so potential hits due to copy-on-write were rare. Thus it made sense to not map the memory between programs for IPC, but instead migrate the program code being used into the local space of the program. This led to the concept of "shuttles" and it seemed performance had improved, but the developers moved on with the system in a semi-usable state. Mach 4 also introduced built-in co-location primitives, making it a part of the kernel itself.
By the mid-1990s, work on microkernel systems was largely dead, despite the market generally believing that all modern operating systems would be microkernel based by the 1990s. The only remaining widespread uses of the Mach kernel are Apple's Mac OS X and its sibling iOS, which run atop a heavily modified Mach 3 kernel.
The rest, the majority of the actual problem, was due to the kernel performing tasks such as checking the message for port access rights. While it would seem this is an important security concern, in fact, it only makes sense in a UNIX-like system. For instance, a single-user operating system running a cell phone or robot
might not need any of these features, and this is exactly the sort of system where Mach's pick-and-choose operating system would be most valuable. Likewise Mach caused problems when memory had been moved by the operating system, another task that only really makes sense if the system has more than one address space. DOS
and the early Mac OS
had a single large address space
shared by all programs, so under these systems the mapping is a waste of time.
These realizations led to a series of second generation microkernels, which further reduced the complexity of the system and placed almost all functionality in the user space. For instance, the L4 kernel (version 2) includes only seven system calls and uses 12k of memory, whereas Mach 3 includes about 140 functions and uses about 330k of memory. IPC calls under L4 on a 486DX-50 take only 5μs, faster than a UNIX syscall on the same system, and over 20 times as fast as Mach. Of course this ignores the fact that L4 is not handling permissioning or security, but by leaving this to the user-space programs, they can select as much or as little overhead as they require.
The potential performance gains of L4 are tempered by the fact that the user-space applications will often have to provide many of the functions formerly supported by the kernel. In order to test the end-to-end performance, MkLinux in co-located mode was compared with an L4 port running in user-space. L4 added about 5%-10% overhead, compared to Mach's 15%, all the more interesting considering the double context switches needed.
These newer microkernels have revitalized the industry as a whole, and projects such as the GNU Hurd
have received new attention as a result.
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
kernel
Kernel (computing)
In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...
developed at Carnegie Mellon University
Carnegie Mellon University
Carnegie Mellon University is a private research university in Pittsburgh, Pennsylvania, United States....
to support operating system research, primarily distributed and parallel computation. Although Mach is often mentioned as one of the earliest examples of a microkernel
Microkernel
In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...
, not all versions of Mach are microkernels. Mach's derivatives are the basis of the modern operating system kernels in Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
(which is not a microkernel) and GNU Hurd
GNU Hurd
GNU Hurd is a free software Unix-like replacement for the Unix kernel, released under the GNU General Public License. It has been under development since 1990 by the GNU Project of the Free Software Foundation...
(which is a microkernel).
The project at Carnegie Mellon ran from 1985 to 1994, ending in apparent failure with Mach 3.0, which was finally a true microkernel
Microkernel
In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...
. Mach was developed as a replacement for the kernel in the BSD
Berkeley Software Distribution
Berkeley Software Distribution is a Unix operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995...
version of UNIX
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
, so no new operating system would have to be designed around it. Today further experimental research on Mach appears to have ended, although Mach and its derivatives are in use in a number of commercial operating systems, such as NeXTSTEP
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
and OPENSTEP
OpenStep
OpenStep was an object-oriented application programming interface specification for an object-oriented operating system that used a non-NeXTSTEP operating system as its core, principally developed by NeXT with Sun Microsystems. OPENSTEP was a specific implementation of the OpenStep API developed...
, and most notably Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
using the XNU
XNU
XNU is the computer operating system kernel that Apple Inc. acquired and developed for use in the Mac OS X operating system and released as free and open source software as part of the Darwin operating system...
operating system kernel which incorporates an earlier (non-microkernel) Mach as a major component. The Mach virtual memory management system was also adopted by the BSD developers at CSRG
Computer Systems Research Group
The Computer Systems Research Group was a research group at the University of California, Berkeley that was dedicated to enhancing AT&T Unix operating system and funded by Defense Advanced Research Projects Agency.- History :...
, and appears in modern BSD-derived UNIX systems, such as FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
. Neither Mac OS X nor FreeBSD maintain the microkernel structure pioneered in Mach, although Mac OS X continues to offer microkernel inter-process communication
Inter-process communication
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...
and control primitives for use directly by applications.
Mach is the logical successor to Carnegie Mellon's Accent kernel
Accent kernel
Accent was an operating system kernel developed at Carnegie Mellon University . Accent was developed as a follow-on to the Aleph kernel developed at the University of Rochester, fixing several of its problems and re-targeting its hardware support for networks of workstation machines instead of...
. The lead developer on the Mach project, Richard Rashid
Richard Rashid
Richard F. Rashid oversees Microsoft Research's worldwide operations. Previously, he was the director of Microsoft Research. He joined Microsoft Research in 1991, and was promoted to vice president in 1994. In 2000, he became senior vice president...
, has been working at Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
since 1991 in various top-level positions revolving around the Microsoft Research
Microsoft Research
Microsoft Research is the research division of Microsoft created in 1991 for developing various computer science ideas and integrating them into Microsoft products. It currently employs Turing Award winners C.A.R. Hoare, Butler Lampson, and Charles P...
division. Another of the original Mach developers, Avie Tevanian
Avie Tevanian
Avadis "Avie" Tevanian is a former Senior Vice President of Software Engineering at Apple Computer from 1997 to 2003, and a former Chief Software Technology Officer from 2003 to 2006. He is a member of the board of embedded software tools company Green Hills Software. Tevanian was responsible for...
, was formerly head of software at NeXT
NeXT
Next, Inc. was an American computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations intended for the higher education and business markets...
, then Chief Software Technology Officer at Apple Computer
Apple Computer
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...
until March 2006.
Name
According to Tevanian, "Mach" comes from a mispronounciation. As he and others headed to lunch on a rainy Pittsburgh day avoiding mud puddles, Tevanian jokingly suggested that they name their new microkernel MUCK, for "Multi-User Communication Kernel" or "Multiprocessor Universal Communication Kernel". An Italian colleague mispronounced MUCK as "Mach", which Rashid liked.Mach concepts
Since Mach was designed as a "drop-in" replacement for the traditional UNIX kernel, this discussion focuses on what distinguishes Mach from UNIX. It became clear early that UNIX's concept of everything-as-a-file might not be practical on modern systems, although some systems such as Plan 9 from Bell LabsPlan 9 from Bell Labs
Plan 9 from Bell Labs is a distributed operating system. It was developed primarily for research purposes as the successor to Unix by the Computing Sciences Research Center at Bell Labs between the mid-1980s and 2002...
have tried this way. Nevertheless, those same developers lamented the loss of flexibility that the original concept offered. Another level of virtualization was sought that would make the system "work" again.
The critical abstraction
Abstraction (computer science)
In computer science, abstraction is the process by which data and programs are defined with a representation similar to its pictorial meaning as rooted in the more complex realm of human life and language with their higher need of summarization and categorization , while hiding away the...
in UNIX was the pipe. What was needed was a pipe-like concept that worked at a much more general level, allowing a broad variety of information to be passed between programs. Such a system did exist using inter-process communication
Inter-process communication
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...
(IPC): A pipe-like system that would move any information between two programs, as opposed to file-like information. While many systems, including most Unixes, had added various IPC implementations over the years, at the time these were generally special-purpose libraries only really useful for one-off tasks.
Carnegie Mellon University
Carnegie Mellon University
Carnegie Mellon University is a private research university in Pittsburgh, Pennsylvania, United States....
started experimentation along these lines under the Accent kernel
Accent kernel
Accent was an operating system kernel developed at Carnegie Mellon University . Accent was developed as a follow-on to the Aleph kernel developed at the University of Rochester, fixing several of its problems and re-targeting its hardware support for networks of workstation machines instead of...
project, using an IPC system based on shared memory
Shared memory
In computing, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Depending on context, programs may run on a single processor or on multiple separate processors...
. Accent was a purely experimental system with many features, developed in an ad-hoc fashion over a period of time with changing research interests. Additionally, Accent's usefulness for research was limited because it was not UNIX-compatible, and UNIX was already the de-facto standard for almost all operating system research. Finally, Accent was tightly coupled with the hardware platform on which it was developed, and at the time in the early 1980s it appeared there would soon be an explosion of new platforms, many of them massively parallel
Massively parallel
Massively parallel is a description which appears in computer science, life sciences, medical diagnostics, and other fields.A massively parallel computer is a distributed memory computer system which consists of many individual nodes, each of which is essentially an independent computer in itself,...
.
Mach started largely as an effort to produce a cleanly-defined, UNIX-based, highly portable Accent. The result was a short list of generic concepts:
- a "task" is an object consisting of a set of system resources that enable "threads" to run
- a "threadThread (computer science)In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
" is a single unit of execution, exists within a context of a task and shares the task's resources - a "portComputer port (software)In computer programming, port has a wide range of meanings.A software port is a virtual/logical data connection that can be used by programs to exchange data directly, instead of going through a file or other temporary storage location...
" is a protected message queueMessage queueIn computer science, message queues and mailboxes are software-engineering components used for interprocess communication, or for inter-thread communication within the same process. They use a queue for messaging – the passing of control or of content...
for communication between tasks; tasks own send and receive rights to each port - "messages" are collections of typed data objects, they can only be sent to ports - not specific tasks or threads
Mach developed on Accent's IPC concepts, but made the system much more UNIX-like in nature, even able to run UNIX programs with little or no modification. To do this, Mach introduced the concept of a port, representing each endpoint of a two-way IPC. Ports had security and rights like files under UNIX, allowing a very UNIX-like model of protection to be applied to them. Additionally, Mach allowed any program to handle privileges that would normally be given to the operating system only, in order to allow user space
User space
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
programs to handle things like interacting with hardware.
Under Mach, and like UNIX, the operating system again becomes primarily a collection of utilities. As with UNIX, Mach keeps the concept of a driver for handling the hardware. Therefore all the drivers for the present hardware have to be included in the microkernel. Other architectures based on Hardware Abstraction Layer or exokernel
Exokernel
Exokernel is an operating system kernel developed by the MIT Parallel and Distributed Operating Systems group, and also a class of similar operating systems....
s could move the drivers out of the microkernel.
The main difference with UNIX is that instead of utilities handling files, they can handle any "task". More operating system code was moved out of the kernel and into user space, resulting in a much smaller kernel and the rise of the term microkernel
Microkernel
In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...
. Unlike traditional systems, under Mach a process, or "task", can consist of a number of threads. While this is common in modern systems, Mach was the first system to define tasks and threads in this way. The kernel's job was reduced from essentially being the operating system to maintaining the "utilities" and scheduling their access to hardware.
The existence of ports and the use of IPC is perhaps the most fundamental difference between Mach and traditional kernels. Under UNIX, calling the kernel consists of an operation known as a syscall or trap
Signal (computing)
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
. The program uses a library
Library (computer science)
In computer science, a library is a collection of resources used to develop software. These may include pre-written code and subroutines, classes, values or type specifications....
to place data in a well known location in memory and then causes a fault, a type of error. When the system is first started the kernel is set up to be the "handler" of all faults, so when the program causes a fault the kernel takes over, examines the information passed to it, and then carries out the instructions.
Under Mach, the IPC system was used for this role instead. In order to call system functionality, a program would ask the kernel for access to a port, then use the IPC system to send messages to that port. Although the messages were triggered by syscalls as they would be on other kernels, under Mach that was pretty much all the kernel did—handling the actual request would be up to some other program.
The use of IPC for message passing benefited support for threads and concurrency. Since tasks consisted of multiple threads, and it was the code in the threads that used the IPC mechanism, Mach was able to freeze and unfreeze threads while the message was handled. This allowed the system to be distributed over multiple processors, either using shared memory directly as in most Mach messages, or by adding code to copy the message to another processor if needed. In a traditional kernel this is difficult to implement; the system has to be sure that different programs don't try to write to the same memory from different processors. Under Mach this was well defined and easy to implement; it was the very process of accessing that memory, the ports, that was made a first-class citizen of the system.
The IPC system initially had performance problems, so a few strategies were developed to minimize the impact. Like its predecessor, Accent
Accent kernel
Accent was an operating system kernel developed at Carnegie Mellon University . Accent was developed as a follow-on to the Aleph kernel developed at the University of Rochester, fixing several of its problems and re-targeting its hardware support for networks of workstation machines instead of...
, Mach used a single shared-memory mechanism for physically passing the message from one program to another. Physically copying the message would be too slow, so Mach relies on the machine's memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
(MMU) to quickly map the data from one program to another. Only if the data is written to would it have to be physically copied, a process known as copy-on-write
Copy-on-write
Copy-on-write is an optimization strategy used in computer programming. The fundamental idea is that if multiple callers ask for resources which are initially indistinguishable, they can all be given pointers to the same resource...
.
Messages were also checked for validity by the kernel, to avoid bad data crashing one of the many programs making up the system. Ports were deliberately modeled on the UNIX file system concepts. This allowed the user to find ports using existing file system navigation concepts, as well as assigning rights and permissions as they would on the file system.
Development under such a system would be easier. Not only would the code being worked on exist in a traditional program that could be built using existing tools, it could also be started, debugged and killed off using the same tools. With a monokernel
Monolithic kernel
A monolithic kernel is an operating system architecture where the entire operating system is working in the kernel space and alone as supervisor mode...
a bug in new code would take down the entire machine and require a reboot, whereas under Mach this would require only that the program be restarted. Additionally the user could tailor the system to include, or exclude, whatever features they required. Since the operating system was simply a collection of programs, they could add or remove parts by simply running or killing them as they would any other program.
Finally, under Mach, all of these features were deliberately designed to be extremely platform neutral. To quote one text on Mach:
- Unlike UNIX, which was developed without regard for multiprocessing, Mach incorporates multiprocessing support throughout. Its multiprocessing support is also exceedingly flexible, ranging from shared memory systems to systems with no memory shared between processors. Mach is designed to run on computer systems ranging from one to thousands of processors. In addition, Mach is easily ported to many varied computer architectures. A key goal of Mach is to be a distributed system capable of functioning on heterogeneous hardware. (Appendix B, Operating System Concepts)
There are a number of disadvantages, however. A relatively mundane one is that it is not clear how to find ports. Under UNIX this problem was solved over time as programmers agreed on a number of "well known" locations in the file system to serve various duties. While this same approach worked for Mach's ports as well, under Mach the operating system was assumed to be much more fluid, with ports appearing and disappearing all the time. Without some mechanism to find ports and the services they represented, much of this flexibility would be lost.
Development
Mach was initially hosted as additional code written directly into the existing 4.2BSD kernel, allowing the team to work on the system long before it was complete. Work started with the already functional Accent IPC/port system, and moved on to the other key portions of the OS, tasks and threads and virtual memory. As portions were completed various parts of the BSD system were re-written to call into Mach, and a change to 4.3BSD was also made during this process.By 1986 the system was complete to the point of being able to run on its own on the DEC VAX. Although doing little of practical value, the goal of making a microkernel was realized. This was soon followed by versions on the IBM PC/RT and for Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
68030-based workstations, proving the system's portability. By 1987 the list included the Encore Multimax and Sequent Balance
Sequent Computer Systems
Sequent Computer Systems, or Sequent, was a computer company that designed and manufactured multiprocessing computer systems. They were among the pioneers in high-performance symmetric multiprocessing open systems, innovating in both hardware and software Sequent Computer Systems, or Sequent, was...
machines, testing Mach's ability to run on multiprocessor systems. A public Release 1 was made that year, and Release 2 followed the next year.
Throughout this time the promise of a "true" microkernel was not yet being delivered. These early Mach versions included the majority of 4.3BSD in the kernel, a system known as POE Server, resulting in a kernel that was actually larger than the UNIX it was based on. The idea, however, was to move the UNIX layer out of the kernel into user-space, where it could be more easily worked on and even replaced outright. Unfortunately performance proved to be a major problem, and a number of architectural changes were made in order to solve this problem. Unwieldy UNIX licensing issues were also plaguing researchers, so this early effort to provide a non-licensed UNIX-like system environment continued to find use, well into the further development of Mach.
The resulting Mach 3 was released in 1990, and generated intense interest. A small team had built Mach and ported it to a number of platforms, including complex multiprocessor systems which were causing serious problems for older-style kernels. This generated considerable interest in the commercial market, where a number of companies were in the midst of considering changing hardware platforms. If the existing system could be ported to run on Mach, it would seem it would then be easy to change the platform underneath.
Mach received a major boost in visibility when the Open Software Foundation
Open Software Foundation
The Open Software Foundation was a not-for-profit organization founded in 1988 under the U.S. National Cooperative Research Act of 1984 to create an open standard for an implementation of the UNIX operating system.-History:...
(OSF) announced they would be hosting future versions of OSF/1 on Mach 2.5, and were investigating Mach 3 as well. Mach 2.5 was also selected for the NeXTSTEP
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
system and a number of commercial multiprocessor vendors. Mach 3 led to a number of efforts to port other operating systems parts for the microkernel, including IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
's Workplace OS
Workplace OS
Workplace OS was born in 1991 as an ambitious plan by IBM to create a new computer operating system. The goal of Workplace OS was to improve software portability and reduce maintenance costs of IBM's software by using a common microkernel base for all of IBM's operating systems.At the base of...
and several efforts by Apple Computer
Apple Computer
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...
to build a cross-platform version of the Mac OS
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...
.
For some time it appeared that every future operating system would be using the Mach as kernel by the late 1990s.
Performance problems
Mach was originally intended to be a replacement for classical monolithic UNIX, and for this reason contained many UNIX-like ideas. For instance, Mach used a permissioning and security system patterned on UNIX's file system. Since the kernel was privileged (running in kernel-space) over other OS servers and software, it was possible for malfunctioning or malicious programs to send it commands that would cause damage to the system, and for this reason the kernel checked every message for validity. Additionally most of the operating system functionality was to be located in user-space programs, so this meant there needed to be some way for the kernel to grant these programs additional privileges, to operate on hardware for instance.Some of Mach's more esoteric features were also based on this same IPC mechanism. For instance, Mach was able to support multi-processor machines with ease. In a traditional kernel extensive work needs to be carried out to make it reentrant or interruptible, as programs running on different processors could call into the kernel at the same time. Under Mach, the bits of the operating system are isolated in servers, which are able to run, like any other program, on any processor. Although in theory the Mach kernel would also have to be reentrant, in practice this isn't an issue because its response times are so fast it can simply wait and serve requests in turn. Mach also included a server that could forward messages not just between programs, but even over the network, which was an area of intense development in the late 1980s and early 1990s.
Unfortunately, the use of IPC for almost all tasks turned out to have serious performance impact. Benchmarks on 1997 hardware showed that Mach 3.0-based UNIX
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
single-server implementations were about 50% slower than native UNIX.
Studies showed the vast majority of this performance hit, 73% by one measure, was due to the overhead of the IPC. And this was measured on a system with a single large server providing the operating system; breaking the operating system down further into smaller servers would only make the problem worse. It appeared the goal of a collection-of-servers was simply not possible.
Many attempts were made to improve the performance of Mach and Mach-like microkernels, but by the mid-1990s much of the early intense interest had died. The concept of an operating system based on IPC appeared to be dead, the idea itself flawed.
In fact, further study of the exact nature of the performance problems turned up a number of interesting facts. One was that the IPC itself was not the problem: there was some overhead associated with the memory mapping needed to support it, but this added only a small amount of time to making a call. The rest, 80% of the time being spent, was due to additional tasks the kernel was running on the messages. Primary among these was the port rights checking and message validity. In benchmarks on an 486
Intel 80486
The Intel 80486 microprocessor was a higher performance follow up on the Intel 80386. Introduced in 1989, it was the first tightly pipelined x86 design as well as the first x86 chip to use more than a million transistors, due to a large on-chip cache and an integrated floating point unit...
DX-50, a standard UNIX system call took an average of 21μs
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
to complete, while the equivalent operation with Mach IPC averaged 114μs. Only 18μs of this was hardware related; the rest was the Mach kernel running various routines on the message. Given a syscall that does nothing, a full round-trip under BSD would require about 40μs, whereas on a user-space Mach system it would take just under 500μs.
When Mach was first being seriously used in the 2.x versions, performance was slower than traditional monolithic operating systems, perhaps as much as 25%. This cost was not considered particularly worrying, however, because the system was also offering multi-processor support and easy portability. Many felt this was an expected and acceptable cost to pay. When Mach 3 attempted to move most of the operating system into user-space, the overhead became higher still: benchmarks between Mach and Ultrix
Ultrix
Ultrix was the brand name of Digital Equipment Corporation's native Unix systems. While ultrix is the Latin word for avenger, the name was chosen solely for its sound.-History:...
on a MIPS R3000
R3000
The R3000 is a microprocessor chip set developed by MIPS Computer Systems that implemented the MIPS I instruction set architecture . Introduced in June 1988, it was the second MIPS implementation, succeeding the R2000 as the flagship MIPS microprocessor...
showed a performance hit as great as 67% on some workloads.
For example, getting the system time involves an IPC call to the user-space server maintaining system clock. The caller first traps into the kernel, causing a context switch and memory mapping. The kernel then checks that the caller has required access rights and that the message is valid. If it does, there is another context switch and memory mapping to complete the call into the user-space server. The process must then be repeated to return the results, adding up to a total of four context switches and memory mappings, plus two message verifications. This overhead rapidly compounds with more complex services, where there are often code paths passing through many servers.
This was not the only source of performance problems. Another centered on the problems of trying to handle memory properly when physical memory ran low and paging had to occur. In the traditional monolithic operating systems the authors had direct experience with which parts of the kernel called which others, allowing them to fine tune their pager to avoid paging out code that was about to be used. Under Mach this wasn't possible because the kernel had no real idea what the operating system consisted of. Instead they had to use a single one-size-fits-all solution that added to the performance problems. Mach 3 attempted to address this problem by providing a simple pager, relying on user-space pagers for better specialization. But this turned out to have little effect. In practice, any benefits it had were wiped out by the expensive IPC needed to call it in.
Other performance problems were related to Mach's support for multiprocessor
Multiprocessor
Computer system having two or more processing units each sharing main memory and peripherals, in order to simultaneously process programs.Sometimes the term Multiprocessor is confused with the term Multiprocessing....
systems. From the mid-1980s to the early 1990s, commodity CPUs grew in performance at a rate of about 60% a year, but the speed of memory access grew at only 7% a year. This meant that the cost of accessing memory grew tremendously over this period, and since Mach was based on mapping memory around between programs, any "cache miss" made IPC calls slow.
Regardless of the advantages of the Mach approach, these sorts of real-world performance hits were simply not acceptable. As other teams found the same sorts of results, the early Mach enthusiasm quickly disappeared. After a short time many in the development community seemed to conclude that the entire concept of using IPC as the basis of an operating system was inherently flawed.
Potential solutions
IPC overhead is a major issue for Mach 3 systems. However, the concept of a multi-server operating system is still promising, though it still requires some research. The developers have to be careful to isolate code into modules that do not call from server to server. For instance, the majority of the networking code would be placed in a single server, thereby minimizing IPC for normal networking tasks.Most developers instead stuck with the original POE concept of a single large server providing the operating system functionality. In order to ease development, they allowed the operating system server to run either in user-space or kernel-space. This allowed them to develop in user-space and have all the advantages of the original Mach idea, and then move the debugged server into kernel-space in order to get better performance. Several operating systems have since been constructed using this method, known as co-location, among them Lites
Lites
Lites was a Unix-like operating system, based on 4.4BSD and the Mach microkernel.Specifically, Lites was a multi-threaded server and emulation library that provided unix functionality to a Mach based system...
, MkLinux
MkLinux
MkLinux is an open source computer operating system started by the OSF Research Institute and Apple Computer in February 1996 to port Linux to the PowerPC platform, and Macintosh computers...
, OSF/1 and NeXTSTEP
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
/OPENSTEP
OpenStep
OpenStep was an object-oriented application programming interface specification for an object-oriented operating system that used a non-NeXTSTEP operating system as its core, principally developed by NeXT with Sun Microsystems. OPENSTEP was a specific implementation of the OpenStep API developed...
/Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
. The Chorus microkernel
ChorusOS
ChorusOS is a microkernel real-time operating system designed for embedded systems. Sun Microsystems acquired Chorus Systèmes, the company which created ChorusOS, in 1997. Sun no longer supports ChorusOS. The founders of Chorus Systems started a new company called Jaluna in August 2002. Jaluna has...
made this a feature of the basic system, allowing servers to be raised into the kernel space using built-in mechanisms.
Mach 4 attempted to address these problems, this time with a more radical set of upgrades. In particular, it was found that program code was typically not writable, so potential hits due to copy-on-write were rare. Thus it made sense to not map the memory between programs for IPC, but instead migrate the program code being used into the local space of the program. This led to the concept of "shuttles" and it seemed performance had improved, but the developers moved on with the system in a semi-usable state. Mach 4 also introduced built-in co-location primitives, making it a part of the kernel itself.
By the mid-1990s, work on microkernel systems was largely dead, despite the market generally believing that all modern operating systems would be microkernel based by the 1990s. The only remaining widespread uses of the Mach kernel are Apple's Mac OS X and its sibling iOS, which run atop a heavily modified Mach 3 kernel.
Second-generation microkernels
Further analysis demonstrated that the IPC performance problem was not as obvious as it seemed. Recall that a single-side of a syscall took 20μs under BSD and 114μs on Mach running on the same system. Of the 114, 11 were due to the context switch, identical to BSD. An additional 18 were used by the MMU to map the message between user-space and kernel space. This adds up to only 31μs, longer than a traditional syscall, but not by much.The rest, the majority of the actual problem, was due to the kernel performing tasks such as checking the message for port access rights. While it would seem this is an important security concern, in fact, it only makes sense in a UNIX-like system. For instance, a single-user operating system running a cell phone or robot
Robot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...
might not need any of these features, and this is exactly the sort of system where Mach's pick-and-choose operating system would be most valuable. Likewise Mach caused problems when memory had been moved by the operating system, another task that only really makes sense if the system has more than one address space. DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
and the early Mac OS
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...
had a single large address space
Single address space operating system
In computer science, a single address space operating system is a type of operating system with simple memory management which uses only one globally shared virtual address space.- List of SASOS Projects :...
shared by all programs, so under these systems the mapping is a waste of time.
These realizations led to a series of second generation microkernels, which further reduced the complexity of the system and placed almost all functionality in the user space. For instance, the L4 kernel (version 2) includes only seven system calls and uses 12k of memory, whereas Mach 3 includes about 140 functions and uses about 330k of memory. IPC calls under L4 on a 486DX-50 take only 5μs, faster than a UNIX syscall on the same system, and over 20 times as fast as Mach. Of course this ignores the fact that L4 is not handling permissioning or security, but by leaving this to the user-space programs, they can select as much or as little overhead as they require.
The potential performance gains of L4 are tempered by the fact that the user-space applications will often have to provide many of the functions formerly supported by the kernel. In order to test the end-to-end performance, MkLinux in co-located mode was compared with an L4 port running in user-space. L4 added about 5%-10% overhead, compared to Mach's 15%, all the more interesting considering the double context switches needed.
These newer microkernels have revitalized the industry as a whole, and projects such as the GNU Hurd
GNU Hurd
GNU Hurd is a free software Unix-like replacement for the Unix kernel, released under the GNU General Public License. It has been under development since 1990 by the GNU Project of the Free Software Foundation...
have received new attention as a result.
Operating systems and kernels based on Mach
- GNU HurdGNU HurdGNU Hurd is a free software Unix-like replacement for the Unix kernel, released under the GNU General Public License. It has been under development since 1990 by the GNU Project of the Free Software Foundation...
(based on GNU MachGNU MachGNU Mach is an implementation of the Mach microkernel. It is the default microkernel in the GNU Hurd operating system. GNU Mach runs on IA-32 machines. GNU Mach is maintained by developers on the GNU project...
) - LitesLitesLites was a Unix-like operating system, based on 4.4BSD and the Mach microkernel.Specifically, Lites was a multi-threaded server and emulation library that provided unix functionality to a Mach based system...
- MkLinuxMkLinuxMkLinux is an open source computer operating system started by the OSF Research Institute and Apple Computer in February 1996 to port Linux to the PowerPC platform, and Macintosh computers...
- mtXinuMtXinumt Xinu was a software company founded in 1983 that produced two operating systems. Its slogan "We know Unix™ backwards and forwards" was an allusion to the company's name and abilities....
- MachTenMachTenMachTen is a Unix-like operating system from Tenon Intersystems that runs as an application program on Apple Macintosh computers running Mac OS....
- MacMachMacMachMacMach is a computer operating system from the early 1990s. Architecturally, it consisted of 4.3BSD code running on the Mach microkernel, with the Macintosh System 7 running experimentally as a Mach task. The entire system ran on Macs based on the Motorola 68k family of microprocessors....
- NEXTSTEPNEXTSTEPNeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
- OSF/1
- Workplace OSWorkplace OSWorkplace OS was born in 1991 as an ambitious plan by IBM to create a new computer operating system. The goal of Workplace OS was to improve software portability and reduce maintenance costs of IBM's software by using a common microkernel base for all of IBM's operating systems.At the base of...
- UNICOS MAXUnicosUNICOS is the name of a range of Unix-like operating system variants developed by Cray for its supercomputers. UNICOS is the successor of the Cray Operating System . It provides network clustering and source code compatibility layers for some other Unixes. UNICOS was originally introduced in 1985...
- KylinKylinNamed after the mythical beast qilin, Kylin is an operating system developed by academics at the National University of Defense Technology in the People's Republic of China, and approved for use by the People's Liberation Army. Based on Mach and FreeBSD, it aims to add an extra level of security...
- XNUXNUXNU is the computer operating system kernel that Apple Inc. acquired and developed for use in the Mac OS X operating system and released as free and open source software as part of the Darwin operating system...
and DarwinDarwin (operating system)Darwin is an open source POSIX-compliant computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects....
, the basis of Mac OS XMac OS XMac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
and iOS
See also
- MicrokernelMicrokernelIn computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...
- L4 microkernel familyL4 microkernel familyL4 is a family of second-generation microkernels, generally used to implement Unix-like operating systems, but also used in a variety of other systems.L4 was a response to the poor performance of earlier microkernel-base operating systems...
- EROS microkernel family
- MERT
External links
- The Mach project at Carnegie Mellon
- The Mach System – Appendix to Operating System Concepts (8th ed) by Avi Silberschatz, Peter Baer Galvin and Greg Gagne
- A comparison of Mach, Amoeba and Chorus
- Towards Real Microkernels – Contains numerous performance measurements, including those quoted in the article
- The Performance of µ-Kernel-Based Systems – Contains an excellent performance comparison of Linux running as a monokernel, on Mach 3 and on L4
- Mach kernel source code - Browsable version of the Mach Kernel source code on the FreeBSD/Linux kernel cross reference site