In computing, a process is the Instance (computer science), instance of a computer program that is being executed by one or many thread (computing), 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 Concurrency (computer science), concurrently. While a computer program is a passive collection of Instruction set, 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. Computer multitasking, Multitasking is a method to allow multiple processes to share Central processing unit, processors (CPUs) and other system resources. Each CPU (core) executes a single Task (computing), task at a time. However, multitasking allows each processor to context switch, switch between tasks that are being executed without having to wait for each task to finish (Preemption (computing), 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 Kernel (operating system), kernels such as Linux kernel, Linux. 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 (computer science), concurrency. For security and reliability, most modern operating systems prevent direct inter-process communication, 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 Memory management#Dynamic memory allocation, 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 terminology) or Handle (computing), handles (Microsoft Windows, Windows), and data sources and sinks. * Computer security, Security attributes, such as the process owner and the process' set of permissions (allowable operations). * Central processing unit, Processor state (context (computing), context), such as the content of processor register, 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' Thread (computer science), 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 (computer science), 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 Computer multitasking, multitasking operating system may just switch between processes to give the appearance of many processes Execution (computing), executing simultaneously (that is, in Parallel computing, parallel), though in fact only one process can be executing at any one time on a single Central processing unit, CPU (unless the CPU has multiple cores, then Thread (computing)#Multithreading, 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 Asynchrony (computer programming), 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 Reentrancy (computing), reentrant 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 system, 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 calculus, process calculi. If a process requests something for which it must wait, it will be blocked. When the process is in the Process state, 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 Computer data storage#Primary storage, 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 (operating system), kernel that allows multitasking needs processes to have process states, certain states. Names for these states are not standardised, but they have similar functionality. * First, the process is "created" by being loaded from a Auxiliary memory, secondary storage device (hard disk drive, CD-ROM, etc.) into main memory. After that the Scheduling (computing), process scheduler assigns it the "waiting" state. * While the process is "waiting", it waits for the scheduling (computing), 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 Message Passing Interface, MPI).

History

By the early 1960s, computer control software had evolved from monitor control software, for example IBM 7090/94 IBSYS, IBSYS, to executive control software. Over time, computers got faster while Time-sharing, computer time was still neither cheap nor fully utilized; such an environment made Computer multitasking#Multiprogramming, multiprogramming possible and necessary. Multiprogramming means that several programs run Concurrency (computer science), concurrently. At first, more than one program ran on a single processor, as a result of underlying uniprocessor system, uniprocessor computer architecture, and they shared scarce and limited hardware resources; consequently, the concurrency was of a ''serial'' nature. On later systems with Multiprocessing, multiple processors, multiple programs may run concurrently in ''Parallel computing, 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 System resource, 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 Reentrancy (computing), re-entrant code. Thread (computer science), 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 Computer multitasking, multitasking, multiprocessing and, later, Thread (computing)#Multithreading, multithreading.

Notes

References

External links

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