Plan 9 from Bell Labs Rob Pike Dave Presotto Sean Dorward Bob Flandrena Ken Thompson Howard Trickey Phil Winterbottom Bell Laboratories Murray Hill New Jersey 07974 USA Motivation By the mid 1980 s the trend in computing was away from large centralized time shared computers towards networks of smaller personal machines typically UNIX worksta tions People had grown weary of overloaded bureaucratic timesharing machines and were eager to move to small self maintained systems even if that meant a net loss in computing power As microcomputers became faster even that loss was recovered and this style of computing remains popular today In the rush to personal workstations though some of their weaknesses were over looked First the operating system they run UNIX is itself an old timesharing system and has had trouble adapting to ideas born after it Graphics and networking were added to UNIX well into its lifetime and remain poorly integrated and difficult to admin ister More important the early focus on having private machines made it difficult for networks of machines to serve as seamlessly as the old monolithic timesharing systems Timesharing centralized the management and amortization of costs and resources per sonal computing fractured democratized and ultimately amplified administrative prob lems The choice of an old timesharing operating system to run those personal machines made it difficult to bind things together smoothly Plan 9 began in the late 1980 s as an attempt to have it both ways to build a sys tem that was centrally administered and cost effective using cheap modern microcom puters as its computing elements The idea was to build a time sharing system out of workstations but in a novel way Different computers would handle different tasks small cheap machines in people s offices would serve as terminals providing access to large central shared resources such as computing servers and file servers For the cen tral machines the coming wave of shared memory multiprocessors seemed obvious candidates The philosophy is much like that of the Cambridge Distributed System NeHe82 The early catch phrase was to build a UNIX out of a lot of little systems not a system out of a lot of little UNIXes The problems with UNIX were too deep to fix but some of its ideas could be brought along The best was its use of the file system to coordinate naming of and Appeared in a slightly different form in Computing Systems Vol 8 3 Summer 1995 pp 221 254 2 access to resources even those such as devices not traditionally treated as files For Plan 9 we adopted this idea by designing a network level protocol called 9P to enable machines to access files on remote systems Above this we built a naming system that lets people and their computing agents build customized views of the resources in the network This is where Plan 9 first began to look different a Plan 9 user builds a private computing environment and recreates it wherever desired rather than doing all comput ing on a private machine It soon became clear that this model was richer than we had foreseen and the ideas of per process name spaces and file system like resources were extended throughout the system to processes graphics even the network itself By 1989 the system had become solid enough that some of us began using it as our exclusive computing environment This meant bringing along many of the services and applications we had used on UNIX We used this opportunity to revisit many issues not just kernel resident ones that we felt UNIX addressed badly Plan 9 has new com pilers languages libraries window systems and many new applications Many of the old tools were dropped while those brought along have been polished or rewritten Why be so all encompassing The distinction between operating system library and application is important to the operating system researcher but uninteresting to the user What matters is clean functionality By building a complete new system we were able to solve problems where we thought they should be solved For example there is no real tty driver in the kernel that is the job of the window system In the modern world multi vendor and multi architecture computing are essential yet the usual com pilers and tools assume the program is being built to run locally we needed to rethink these issues Most important though the test of a system is the computing environ ment it provides Producing a more efficient way to run the old UNIX warhorses is empty engineering we were more interested in whether the new ideas suggested by the architecture of the underlying system encourage a more effective way of working Thus although Plan 9 provides an emulation environment for running POSIX commands it is a backwater of the system The vast majority of system software is developed in the native Plan 9 environment There are benefits to having an all new system First our laboratory has a history of building experimental peripheral boards To make it easy to write device drivers we want a system that is available in source form no longer guaranteed with UNIX even in the laboratory in which it was born Also we want to redistribute our work which means the software must be locally produced For example we could have used some vendors C compilers for our system but even had we overcome the problems with cross compilation we would have difficulty redistributing the result This paper serves as an overview of the system It discusses the architecture from the lowest building blocks to the computing environment seen by users It also serves as an introduction to the rest of the Plan 9 Programmer s Manual which it accompanies More detail about topics in this paper can be found elsewhere in the manual Design The view of the system is built upon three principles First resources are named and accessed like files in a hierarchical file system Second there is a standard proto col called 9P for accessing these resources Third the disjoint hierarchies provided by different services are joined together into a single private hierarchical file name space The unusual properties of Plan 9 stem from the consistent aggressive application of these principles A large Plan 9 installation has a number of computers networked together each providing a particular class of service Shared multiprocessor servers provide comput ing cycles other large machines offer file storage These machines are located in an air conditioned machine room and are connected by high performance
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