The Mach System

1. The Mach System A Case Study Michael R. Mahair II CPSC 550

2. What is Mach? • An operating system microkernel • Microkernel- minimal operating system kernel that provides only a basic set of operating system services • Other services provided by user-space programs called servers • Originally developed at Carnegie Mellon University to support operating system research • Replacement for the kernel in BSD (Unix) • Still the standard by which other microkernel projects are measured • In use in many commercial operating systems (including Mac OS X)

3. History (1) • Developed at CMU between 1981 and 1994 • 1986: the virtual memory and communication subsystems were running on the DEC VAX computer family, including multiprocessor versions of the VAX • 1987: first official releases – Release 0 and Release 1 • Ended with Mach 3.0 • Gained much industry attention in 1989 • The Open Software Foundation (OSF) announced that it would use Mach 2.5 as the basis for its new operating system, OSF/1

4. History (2) • In Mach 3.0, compatibility with BSD was improved • Previous versions included much of the BSD kernel • 3.0 moved the BSD code outside of the kernel, resulting in a much smaller microkernel • All Unix specific code runs in user-mode servers • Running in user-mode servers allows BSD code to be easily replaced with another operating system, or the simultaneous execution of multiple operating-system interfaces on top of the microkernel The Mach 3.0 System Image from appendix of “The Mach System”

5. History (3) • Mach 4.0 developed at University of Utah after CMU’s project finished • Since then, Mach and its derivatives have been used in a number of commercial systems, most notably Mac OS X • Encore's Multimax • NeXT OS • MachTen for the Macintoshes • Omron's Luna • DEC's OSF/1 for the DEC Alpha • IBM's OS/2 for the RS6000 based machines. • NEXTSTEP • OPENSTEP

6. Goals From CMU’s “The Mach System” web page: • Providing inter-process communication functionality at the kernel level and using it as a building block for the rest of the system. • Virtual memory support provided by the kernel and by user level servers. • Kernel level support for light-weight threads. • Support for closely and loosely coupled multi-processors and a variety of different commercially available workstations. • Micro-kernel architecture limiting the functions supported by the micro-kernel and enabling multiple user level servers to support various Application and Programming Interfaces • Maintaining at least one Unix-style API to enable the Mach system to support all the everyday uses of the project members and other researchers. • Distributing this technology to other researchers and commercial sites to use as the basis for further research or products.

7. Definitions (1) • Microkernel • Minimal operating system kernel that provides only a basic set of operating system services • Task • “An execution environment that provides the basic unit of resource allocation. It consists of a virtual address space and protected access to system resources via ports, and it may contain one or more threads.” (Silberschatz) • Thread • “The basic unit of execution and must run in the context of a task (which provides the address space). All threads within a task share the tasks’ resources (ports, memory, and so on). There is no notion of a process in Mach. Rather, a traditional process would be implemented as a task with a single thread of control.” (Silberschatz)

8. Definitions (2) • Port • “The basic object-reference mechanism in Mach and is implemented as a kernel-protected communication channel. Communication is accomplished by sending messages to ports; messages are queued at the destination port if no thread is immediately ready to receive them. Ports are protected by kernel-managed capabilities, or port rights; a task must have a port right to send a message to a port. The programmer invokes an operation on an object by sending a message to a port associated with that object. The object being represented by a port receives the messages.” (Silberschatz) • Port set • “A group of ports sharing a common message queue. A thread can receive messages for a port set and thus service multiple ports. Each received message identifies the individual port (within the set) from which it was received; the receiver can use this to identify the object referred to by the message.” (Silberschatz)

9. Definitions (3) • Message • “The basic method of communication between threads in Mach. It is a typed collection of data objects; for each object, itmay contain the actual data or a pointer to out-of-line data. Port rights are passed in messages; this is the only way to move them among tasks. (Passing a port right in shared memory does not work, because the Mach kernel will not permit the new task to use a right obtained in this manner.)” (Silberschatz) • Memory Object • “A source of memory; tasks can access it by mapping portions of an object (or the entire object) into their address spaces. The object can be managed by a user-mode external memory manager. One example is a file managed by a file server; however, a memory object can be any object for which memory-mapped access makes sense. A mapped buffer implementation of a UNIX pipe is one example.” (Silberschatz)

10. Features (1) • Microkernel Architecture • Adding a new service does not require modifying the kernel • More operations are done in user mode rather than in the kernel, providing more security • Simpler kernel design generally produces more reliable system • General enough to support multiple operating system interfaces • Support for diverse architectures, including multiprocessor with different memory access levels: • Uniform Memory Access (UMA) • Non-Uniform Memory Access (NUMA) • No Remote Memory Access (NORMA)

11. Features (2) • Distributed operations • Network transparency to clients • Object-oriented organization • Functions with various inter-computer network speeds • Wide area networks • Local area networks • Tightly-coupled multiprocessors • Integrated memory management and inter-process communication • Efficient communication of large amounts of data • Communication-based memory management

12. Structure (1) - Overview Mach Basic Structure Image from appendix of “The Mach System”

13. Structure (2) Process Management • Mach handles tasks and threads. One task can store multiple threads. • Tasks/Threads are either Running or Suspended. • Mach provides minimal yet sufficient functionality in the kernel for handling tasks and threads. • C Threads package • A package (with OTB Mach) to do more advanced thread handling • CPU Scheduler • Only threads scheduled, not tasks • Threads assigned priority between 0 and 127 based on recent CPU usage • Threads placed in one of 32 queues based on priority • Exception Handling • A raise RPC messages is sent to the handler by the thread • Thread calls a routine to wait until the exception is handled • Handler receives information about the exception, including the thread and the task • Based on the type of exception, the handler either resumes or terminates the thread

14. Structure (3) Interprocess Communication • Ports • The kernel creates several ports for each task that is created • Ports can be collected in to Port Sets, which is useful if a thread services requests coming in on multiple ports (multiple objects) • Messages • Consists of fixed-length header and variable number of typed data objects • Network Message Server (NetMsgServer) • Handles IPC across different machines on the distributed OS • Transparent to the user • Synchronization • Port is used as a synchronization variable. Threads wishing to use the port send a “receive” command. The port waits if there is no incoming message. To return the resource after use, a thread simply sends a message to the port.

15. Structure (4) Memory Management • User Level Memory Managers • Mach allows user level memory managers by allowing memory to be created and serviced by non-kernel tasks • Memory can be paged by user level memory managers • When an object is destroyed, it is up to the memory manager to write back any changed pages to secondary storage • Shared Memory • No shared memory necessary within a task- all threads share the task’s memory • Shared memory management is still provided for other Unix constructs, such as the fork system call • Mach does not solve the problem directly, but provides the user with the ability to manage memory across tasks and across machines

16. Usage • Because Mach 3.0 uses a microkernel, only the basic features of the kernel are available • Full functionality is provided by using emulation libraries that reside outside of the kernel • Emulation Libraries are sets of routines that live in a read-only part of a program’s address space • More complex operating systems (such as BSD) can be created by using multiple emulation libraries in combination one or more servers • Run-time libraries, such as the C Threads package, can also be created to provide more functionality.

17. Applications • Encore's Multimax • NeXT OS • MachTen for the Macintoshes • Omron's Luna • DEC's OSF/1 for the DEC Alpha • IBM's OS/2 for the RS6000 based machines. • NEXTSTEP • OPENSTEP • Mac OS X (using the XNU kernel)

18. Significance • One of the earliest examples of a microkernel • Used in many commercial operating systems • Some performance problems cause Mach not to be the best choice compared to other distributed operating systems • Some of the features implemented in the kernel that caused performance problems were not always necessary • Helped lead to second generation microkernels (such as the L4 kernel), which improved many of the performance problems by moving even more functionality into the user-space

19. Summary The Mach System, especially Mach 3.0, is an excellent example of one of the earliest microkernel systems. Although it is not heavily used because of some performance problems, it helped lead the way to many of the higher performance second generation microkernels.

20. References • Coulouris, George, Jean Dollimore & Tim Kindberg. Distributed Systems: Concepts and Design. 1994. 2nd ed. p 594-597. March 3, 2007. <http://www.cdk3.net/oss/Ed2/Comparison.pdf> • Härtig, Hermann, Michael Hohmuth, JochenLiedtke, Sebastian Schönberg, and Jean Wolter. The Performance of µ-Kernel-Based Systems. 1997 by The Association for Computing Machinery, Inc. March 3, 2007. <http://os.inf.tu-dresden.de/pubs/sosp97/> • Silberschatz, Avi, Peter Baer Galvin and Greg Gagne. Operating System Concepts (appendix only- “The Mach System”). 7th ed. March 3, 2007 http://www.di.unipi.it/~scordino/sisop/mach.pdf • The Mach Project Home Page. Feb 21, 1997. Carnegie Mellon University. March 3, 2007 <http://www.cs.cmu.edu/afs/cs/project/mach/public/www/mach.html> • “The Mach System.” Feb 4, 2000. March 3, 2007. <http://library.nocrew.org/lib/os/Mach.txt> • “The Mach 4 Project.” Nov 3 1995. March 3, 2007. <http://www.cs.utah.edu/flux/mach4/html/Mach4-proj.html>