Process management (computing): Difference between revisions

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== Multiprogramming ==

In any modern operating system, there can be more than one instance of a [[computer program|program]] loaded in memory at the same time. For example, Moremore than one user can be executing the same program, with each user having separate copies of the program loaded into memory. With some programs, it is possible to have one copy loaded into memory, while several users have shared access to it so that they can each execute the same program-code. Such a program is called [[Reentrant (subroutine)|re-entrant]].{{Relevance inline|date=November 2023}} At a given instant, the [[central processing unit|processor]] can only be executing one instruction from one program, but several processes can be sustained over a period of time by assigning each process to the processor at intervals while the remainder become temporarily inactive. The execution of multiple processes over a period of time, rather than simultaneously, is known as concurrent execution.

A [[multiprogramming]] or [[Computer multitasking|multitasking]] OSO.S. is a systemOperating System that can execute many processes concurrently. Multiprogramming requires that the processor be allocated to each process for a period of time and de-allocated or issued at an appropriate moment. If the processor is de-allocated during the execution of a process, it must be done in a way that the process can restart later as efficiently as possible.

There are two ways for an OS to regain control of the processor during a program's execution in order for the OS to perform de-allocation or allocation:

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==How multiprogramming increases efficiency==

A common trait observed among processes associated with most computer programs is that they alternate between [[CPU]] cycles and [[I/O]] cycles. For the portion of the time required for CPU cycles, the process is being executed and is occupying the CPU. During the time required for I/O cycles, the process is not using the processor. Instead, it is either waiting to perform Input/Output, or is actually performing Input/Output. An example of this is reading from or writing to a file on disk. Prior to the advent of [[multiprogramming]], [[computers]] operated as single-user systems. Users of such systems quickly become aware that for much of the time that a computer was allocated to a single user -- {{snd}}for example, when a user was entering information or debugging programs -- {{snd}}the processor was idle. [[Computer scientists]] observed that the overall performance of the machine could be improved by letting a different process use the processor whenever one process was waiting for input/output. In a ''uni-programming system'', if ''N'' users were to execute programs with individual execution times of ''t''₁, ''t''₂, ..., ''t''_''N'', then the total time, ''t''_uni, to service the ''N'' processes (consecutively) of all ''N'' users would be:

: ''t''_uni = ''t''₁ + ''t''₂ + ... + ''t''_''N''.

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* Process Management Models, Scheduling, UNIX System V Release 4:

* Modern Operating Systems, Andrew Tanenbaum, Prentice Hall, (2nd Edition, 2001).

* Operating System Concepts, Silberschatz & Galvin & Gagne (httphttps://codex.cs.yale.edu/avi/os-book/OS9/slide-dir/), John Wiley & Sons, (6th Edition, 2003)

{{Operating System}}