Pre-emption as used with respect to operating systems means the ability of the operating system to preempt or stop a currently scheduled task in favour of a higher priority task. The scheduling may be one of, but not limited to, process or I/O scheduling, among others. It has been suggested that Maintenance OS be merged into this article or section. ... Scheduling is a key concept in computer multitasking and multiprocessing operating system design, and in real-time operating system design. ... I/O Scheduling which should not be confused with process scheduling. ...

Non-preemptability arises, for instance, when handling an interrupt. In this case, scheduling is avoided until the interrupt is handled. Making a scheduler preemptible has the advantage of better system responsiveness and scalability^[clarify]. In computing, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ... Scheduling is a key concept in computer multitasking and multiprocessing operating system design, and in real-time operating system design. ...

The schedulers used in most modern operating systems, such as various flavours of Unix, can preempt user processes. This is called preemptive multitasking, and is in contrast to cooperative multitasking wherein a process "gives away" its time by utilizing kernel resources or by specifically calling a kernel routine to allow other processes time to run. Some operating systems' schedulers (including Linux as of the 2.6 series) have the ability to preempt a process while it is processing a system call as well (a preemptible kernel). Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... In computing, cooperative multitasking (or non-preemptive multitasking) is a form of multitasking in which multiple tasks execute by voluntarily ceding control to other tasks at programmer-defined points within each task. ... Linux (IPA pronunciation: ) is a Unix-like computer operating system. ... In computing, a system call is the mechanism used by an application program to request service from the operating system. ...

Windows XP, Windows Vista, Linux, Unix, AmigaOS, *BSD, Mac OS X, and Windows NT are all examples of operating systems that utilize preemptive multitasking; NetWare, Windows for Workgroups, and Mac OS versions 9.x and earlier are all examples of cooperative multitasking operating systems. Windows XP is a line of operating systems developed by Microsoft for use on general-purpose computer systems, including home and business desktops, notebook computers, and media centers. ... Windows Vista is a line of graphical operating systems used on personal computers, including home and business desktops, notebook computers, Tablet PCs, and media centers. ... Linux (IPA pronunciation: ) is a Unix-like computer operating system. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... AmigaOS is the default native operating system of the Amiga personal computer. ... BSD redirects here; for other uses see BSD (disambiguation). ... Mac OS X (official IPA pronunciation: ) is a line of proprietary, graphical operating systems developed, marketed, and sold by Apple Inc. ... Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. ... NetWare is a network operating system and the set of network protocols it uses to talk to client machines on the network. ... A typical Windows 3. ... This article or section does not adequately cite its references or sources. ...

Pre-emptive multitasking

Pre-emptive multitasking is a form of multitasking. To understand the concept, compare to cooperative multiprocessing, in which only the active task (also known as process) may initiate a context switch because the task has: In computing, multitasking is a method by which multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is... A context switch is the computing process of storing and restoring the state (context) of a CPU such that multiple processes can share a single CPU resource. ...

1. completed processing.
2. become blocked on a shared resource.
3. yielded the processor to another, similarly cooperative, task.

In pre-emptive multitasking, the operating system kernel can also initiate a context switch to satisfy the scheduling policy's priority constraint, thus pre-empting the active task. In general, pre-emption means "prior seizure of". When the high priority task at that instance seize the currently running task, it is known as pre-emptive scheduling. It has been suggested that this article or section be merged into Computer_multitasking#Cooperative_multitasking. ... A kernel connects the application software to the hardware of a computer. ... A context switch is the computing process of storing and restoring the state (context) of a CPU such that multiple processes can share a single CPU resource. ...

Pre-emptive multitasking is sometimes mistakenly used when the intended meaning is more specific, referring instead to the class of scheduling policies known as time-shared scheduling, or time-sharing. Alternate uses: see Timesharing Time-sharing is an approach to interactive computing in which a single computer is used to provide apparently simultaneous interactive general-purpose computing to multiple users by sharing processor time. ...

Preemptive multitasking allows the computer system to more reliably guarantee each process a regular "slice" of operating time. It also allows the system to rapidly deal with important external events like incoming data, which might require the immediate attention of one or another process.

At any specific time, processes can be grouped into two categories: those that are waiting for input or output (called "I/O bound"), and those that are fully utilizing the CPU ("CPU bound"). In early systems, processes would often "poll", or "busywait" while waiting for requested input (such as disk, keyboard or network input). During this time, the process was not performing useful work, but still maintained complete control of the CPU. With the advent of interrupts and preemptive multitasking, these I/O bound processes could be "blocked", or put on hold, pending the arrival of the necessary data, allowing other processes to utilize the CPU. As the arrival of the requested data would generate an interrupt, blocked processes could be guaranteed a timely return to execution. IO bound refers to a condition in which the time it takes to complete a computation is determined principally by the speed of IO operations. ... CPU bound refers to a condition where the time to complete a computation is determined principally by the speed of the central processor and main memory. ... It has been suggested that this article or section be merged with Polling (computer science). ...

Although multitasking techniques were originally developed to allow multiple users to share a single machine, it soon became apparent that multitasking was useful regardless of the number of users. Many operating systems, from mainframes down to single-user personal computers, have recognized the usefulness of multitasking support for a variety of reasons. Multitasking makes it possible for a single user to run multiple applications at the same time, or to run "background" processes while retaining control of the computer.

In simple terms: Pre-emptive multitasking involves the use of an interrupt mechanism which suspends the currently executing process and invokes a scheduler to determine which process should execute next. Therefore all processes will get some amount of CPU time at any given time.

Time slice

The period of time for which a process is allowed to run in a preemptive multitasking system is generally called the "time slice". The scheduler is run once every time slice to choose the next process to run. If the time slice is too short then the scheduler will consume too much processing time but if it is too long then processes may not be able to respond to external events quickly enough.

An interrupt is scheduled to allow the operating system kernel to switch between processes when their time slices expire, effectively allowing the processor’s time to be shared between a number of tasks, giving the illusion that it is dealing with these tasks simultaneously, or concurrently. The operating system which controls such a design is called a multi-tasking system. In computing, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ... It has been suggested that Maintenance OS be merged into this article or section. ... A kernel connects the application software to the hardware of a computer. ...

External links

• Introduction to Preemption
• Perils of Preemption (and Alternatives)