pre-emptive multitasking

pre-emptive multitasking

(operating system, parallel)
A type of multitasking where the scheduler can interrupt and suspend ("swap out") the currently running task in order to start or continue running ("swap in") another task. The tasks under pre-emptive multitasking can be written as though they were the only task and the scheduler decides when to swap them. The scheduler must ensure that when swapping tasks, sufficient state is saved and restored that tasks do not interfere.

The length of time for which a process runs is known as its "time slice" and may depend on the task's priority or its use of resources such as memory and I/O.

OS/2, Unix and the Amiga use pre-emptive multitasking.

This contrasts with cooperative multitasking where each task must include calls to allow it to be descheduled periodically.
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preemptive multitasking

A multitasking method that shares processing time with all running programs. Preemptive multitasking creates a time-shared environment in which running programs receive a recurring slice of time from the CPU. Depending on the operating system, the time slice may be the same for all programs or it may be adjustable to meet the current mix of programs and users. For example, background programs can be given more CPU time no matter how heavy the foreground load and vice versa. In addition, the OS is able to grab the machine cycles that a modem or network program needs for uninterrupted processing.

Mainframe operating systems have employed preemptive multitasking for decades. Desktop operating systems began to utilize this architecture starting with Windows 95 and Mac OS X. Contrast with non-preemptive multitasking. See multitasking.
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