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computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
, a process is the instance of a
computer program A computer program is a sequence or set of instructions in a programming language for a computer to Execution (computing), execute. Computer programs are one component of software, which also includes software documentation, documentation and oth ...
that is being executed by one or many threads. There are many different process models, some of which are light weight, but almost all processes (even entire virtual machines) are rooted in an operating system (OS) process which comprises the program code, assigned system resources, physical and logical access permissions, and data structures to initiate, control and coordinate execution activity. Depending on the OS, a process may be made up of multiple threads of execution that execute instructions concurrently. While a computer program is a passive collection of instructions typically stored in a file on disk, a process is the execution of those instructions after being loaded from the disk into memory. Several processes may be associated with the same program; for example, opening up several instances of the same program often results in more than one process being executed. Multitasking is a method to allow multiple processes to share
processors A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...
(CPUs) and other system resources. Each CPU (core) executes a single task at a time. However, multitasking allows each processor to switch between tasks that are being executed without having to wait for each task to finish ( preemption). Depending on the operating system implementation, switches could be performed when tasks initiate and wait for completion of input/output operations, when a task voluntarily yields the CPU, on hardware interrupts, and when the operating system scheduler decides that a process has expired its fair share of CPU time (e.g, by the Completely Fair Scheduler of the Linux kernel). A common form of multitasking is provided by CPU's time-sharing that is a method for interleaving the execution of users' processes and threads, and even of independent kernel tasks – although the latter feature is feasible only in preemptive kernels such as
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...
. Preemption has an important side effect for interactive processes that are given higher priority with respect to CPU bound processes, therefore users are immediately assigned computing resources at the simple pressing of a key or when moving a mouse. Furthermore, applications like video and music reproduction are given some kind of real-time priority, preempting any other lower priority process. In time-sharing systems, context switches are performed rapidly, which makes it seem like multiple processes are being executed simultaneously on the same processor. This simultaneous execution of multiple processes is called concurrency. For security and reliability, most modern operating systems prevent direct communication between independent processes, providing strictly mediated and controlled inter-process communication functionality.


Representation

In general, a computer system process consists of (or is said to ''own'') the following resources: * An ''image'' of the executable machine code associated with a program. * Memory (typically some region of virtual memory); which includes the executable code, process-specific data (input and output), a call stack (to keep track of active subroutines and/or other events), and a heap to hold intermediate computation data generated during run time. * Operating system descriptors of resources that are allocated to the process, such as file descriptors (
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...
terminology) or handles ( Windows), and data sources and sinks. * Security attributes, such as the process owner and the process' set of permissions (allowable operations). *
Processor Processor may refer to: Computing Hardware * Processor (computing) **Central processing unit (CPU), the hardware within a computer that executes a program *** Microprocessor, a central processing unit contained on a single integrated circuit (I ...
state (
context Context may refer to: * Context (language use), the relevant constraints of the communicative situation that influence language use, language variation, and discourse summary Computing * Context (computing), the virtual environment required to s ...
), such as the content of registers and physical memory addressing. The ''state'' is typically stored in computer registers when the process is executing, and in memory otherwise. The operating system holds most of this information about active processes in data structures called process control blocks. Any subset of the resources, typically at least the processor state, may be associated with each of the process' threads in operating systems that support threads or ''child'' processes. The operating system keeps its processes separate and allocates the resources they need, so that they are less likely to interfere with each other and cause system failures (e.g., deadlock or thrashing). The operating system may also provide mechanisms for inter-process communication to enable processes to interact in safe and predictable ways.


Multitasking and process management

A multitasking operating system may just switch between processes to give the appearance of many processes executing simultaneously (that is, in parallel), though in fact only one process can be executing at any one time on a single
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...
(unless the CPU has multiple cores, then multithreading or other similar technologies can be used). It is usual to associate a single process with a main program, and child processes with any spin-off, parallel processes, which behave like asynchronous subroutines. A process is said to ''own'' resources, of which an ''image'' of its program (in memory) is one such resource. However, in multiprocessing systems ''many'' processes may run off of, or share, the same
reentrant Reentrant or re-entrant can refer to: *Re-entrant (landform), the low ground formed between two hill spurs. *Reentrancy (computing) in computer programming * Reentrant mutex in computer science *Reentry (neural circuitry) in neuroscience * Salien ...
program at the same location in memory, but each process is said to own its own ''image'' of the program. Processes are often called "tasks" in embedded operating systems. The sense of "process" (or task) is "something that takes up time", as opposed to "memory", which is "something that takes up space". The above description applies to both processes managed by an operating system, and processes as defined by process calculi. If a process requests something for which it must wait, it will be blocked. When the process is in the blocked state, it is eligible for swapping to disk, but this is transparent in a virtual memory system, where regions of a process's memory may be really on disk and not in main memory at any time. Note that even portions of active processes/tasks (executing programs) are eligible for swapping to disk, if the portions have not been used recently. Not all parts of an executing program and its data have to be in physical memory for the associated process to be active.


Process states

An operating system kernel that allows multitasking needs processes to have certain states. Names for these states are not standardised, but they have similar functionality. * First, the process is "created" by being loaded from a secondary storage device ( hard disk drive, CD-ROM, etc.) into main memory. After that the
process scheduler In computing, scheduling is the action of assigning ''resources'' to perform ''tasks''. The ''resources'' may be processors, network links or expansion cards. The ''tasks'' may be threads, processes or data flows. The scheduling activity is ...
assigns it the "waiting" state. * While the process is "waiting", it waits for the scheduler to do a so-called context switch. The context switch loads the process into the processor and changes the state to "running" while the previously "running" process is stored in a "waiting" state. * If a process in the "running" state needs to wait for a resource (wait for user input or file to open, for example), it is assigned the "blocked" state. The process state is changed back to "waiting" when the process no longer needs to wait (in a blocked state). * Once the process finishes execution, or is terminated by the operating system, it is no longer needed. The process is removed instantly or is moved to the "terminated" state. When removed, it just waits to be removed from main memory. (particularly chapter 3, section 3.2, "process states", including figure 3.9 "process state transition with suspend states")


Inter-process communication

When processes need to communicate with each other they must share parts of their address spaces or use other forms of inter-process communication (IPC). For instance in a shell pipeline, the output of the first process need to pass to the second one, and so on; another example is a task that can be decomposed into cooperating but partially independent processes which can run at once (i.e., using concurrency, or true parallelism – the latter model is a particular case of concurrent execution and is feasible whenever enough CPU cores are available for all the processes that are ready to run). It is even possible for two or more processes to be running on different machines that may run different operating system (OS), therefore some mechanisms for communication and synchronization (called communications protocols for distributed computing) are needed (e.g., the Message Passing Interface, often simply called
MPI MPI or Mpi may refer to: Science and technology Biology and medicine * Magnetic particle imaging, an emerging non-invasive tomographic technique * Myocardial perfusion imaging, a nuclear medicine procedure that illustrates the function of the hear ...
).


History

By the early 1960s, computer control software had evolved from monitor control software, for example
IBSYS IBSYS is the discontinued tape-based operating system that IBM supplied with its IBM 709, IBM 7090 and IBM 7094 computers. A similar operating system (but with several significant differences), also called IBSYS, was provided with IBM 7040 a ...
, to executive control software. Over time, computers got faster while computer time was still neither cheap nor fully utilized; such an environment made multiprogramming possible and necessary. Multiprogramming means that several programs run concurrently. At first, more than one program ran on a single processor, as a result of underlying uniprocessor computer architecture, and they shared scarce and limited hardware resources; consequently, the concurrency was of a ''serial'' nature. On later systems with multiple processors, multiple programs may run concurrently in '' parallel''. Programs consist of sequences of instructions for processors. A single processor can run only one instruction at a time: it is impossible to run more programs at the same time. A program might need some resource, such as an input device, which has a large delay, or a program might start some slow operation, such as sending output to a printer. This would lead to processor being "idle" (unused). To keep the processor busy at all times, the execution of such a program is halted and the operating system switches the processor to run another program. To the user, it will appear that the programs run at the same time (hence the term "parallel"). Shortly thereafter, the notion of a "program" was expanded to the notion of an "executing program and its context". The concept of a process was born, which also became necessary with the invention of re-entrant code. Threads came somewhat later. However, with the advent of concepts such as time-sharing, computer networks, and multiple-CPU shared memory computers, the old "multiprogramming" gave way to true multitasking, multiprocessing and, later, multithreading.


See also

*
Background process A background process is a computer process that runs ''behind the scenes'' (i.e., in the background) and without user intervention. Typical tasks for these processes include logging, system monitoring, scheduling, and user notification. The backgr ...
* Child process * Exit * Fork * Light-weight process *
Orphan process An orphan process is a computer process whose parent process has finished or terminated, though it remains running itself. Unix-like In a Unix-like operating system any orphaned process will be immediately adopted by an implementation-defined s ...
* Parent process * Process group * Wait * Working directory * Zombie process


Notes


References


Further reading

* Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau (2014).
Operating Systems: Three Easy Pieces
. Arpaci-Dusseau Books. Relevant chapters
Abstraction: The ProcessThe Process API
* Gary D. Knott (1974)
A proposal for certain process management and intercommunication primitives
' ACM SIGOPS Operating Systems Review. Volume 8, Issue 4 (October 1974). pp. 7 – 44


External links

*
Online Resources For Process InformationComputer Process Information Database and Forum
{{DEFAULTSORT:Process (computing) Concurrent computing Operating system technology