Operating System Design/Scheduling Processes/Preemption
Preemption as used with respect to operating systems means the ability of the operating system to preempt (that is, stop or pause) a currently scheduled task in favour of a higher priority task. The resource being scheduled may be the processor or I/O, among others.
Non-preemptability arises, for instance, when handling an interrupt. In this case, scheduling is avoided until the interrupt is handled.
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 preemptive multitasking system available for home users (1985). Other preemptive 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.
The person currently at the cashier may be interrupted and have to wait for another customer before finishing.
In the Linux kernel, the scheduler is called after each timer interrupt (that is, quite a few times per second). It determines what process to run next based on a variety of factors, including priority, time already run, etc. The implementation of preemption in other kernels is likely to be similar.
Advantages and Disadvantages
Making a scheduler preemptible has the advantage of better system responsiveness and scalability, but comes with the disadvantage of race conditions (where the executing process accesses the same resource before another preempted process finished using it).