Preemption (computing)
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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 etc.
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.
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).
Linux, Unix, *BSD, Mac OS X, and Windows NT are all examples of operating systems that utilize preemptive multitasking; Netware, Windows for Workgroups, and Macintosh System 9 are all examples of cooperative multitasking operating systems.
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