Operating System Design/Scheduling Processes/Preemption

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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, among others.

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], but comes with the disadvantage of racing conditions (where the executing process access the same resource before another (preempted) process finished using it).

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).

The AmigaOS was the first pre-emptive multitasking system available for home users (1985). Other pre-emptive operating systems include the Windows NT family (also XP or Vista), Linux, *BSD, and Mac OS X. Examples of cooperative operating systems include Windows for Workgroups (also known as Windows 3.1 or 95), NetWare, and Mac OS versions 9.x.

Linux kernels prior to Linux 2.6 were also nonpreemptive, but later releases implemented the preemptive model. Several commercial versions of UNIX are preemptive, including Solaris and IRIX.[1]

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