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'''Fixed-priority preemptive scheduling''' is a [[Computer_multitaskingComputer multitasking|scheduling system]] commonly used in [[real-time system]]s.<ref>{{Cite journal |last=Audsley |first=Neil C. |last2=Burns |first2=Alan |last3=Davis |first3=Robert I. |last4=Tindell |first4=Ken W. |last5=Wellings |first5=Andy J. |date=1995-03-01 |title=Fixed priority pre-emptive scheduling: An historical perspective |url=https://link.springer.com/article/10.1007/BF01094342 |journal=Real-Time Systems |language=en |volume=8 |issue=2 |pages=173–198 |doi=10.1007/BF01094342 |issn=1573-1383|url-access=subscription }}</ref> With fixed priority preemptive scheduling, the scheduler ensures that at any given time, the [[central processing unit|processor]] executes the highest priority [[task (computing)|task]] of all those tasks that are currently ready to execute.
The [[Preemption (computing)|preemptive scheduler]] has a clock interrupt task that can provide the [[Scheduling (computing)|scheduler]] with options to [[Context switch|switch]] after the task has had a given period to execute—the [[time slice]]. This scheduling system has the advantage of making sure no task hogs the processor for any time longer than the time slice. However, this scheduling scheme is vulnerable to process or thread [[Resource_starvationResource starvation|lockout]]: since priority is given to higher-priority tasks, the lower-priority tasks could wait an indefinite amount of time. One common method of arbitrating this situation is aging, which gradually increments the priority of waiting processes and threads, ensuring that they will all eventually execute. Most [[Realreal-time operating system]]s (RTOSs) have preemptive schedulers. Also turning off time slicing effectively gives you the non-preemptive RTOS.
Preemptive scheduling is often differentiated with [[Computer_multitaskingComputer multitasking#Cooperative_multitaskingCooperative multitasking.2Ftime-sharing|cooperative scheduling]], in which a task can run continuously from start to end without being preempted by other tasks. To have a task switch, the task must explicitly call the scheduler. Cooperative scheduling is used in a few RTOS such as [[Salvo (RTOS)|Salvo]] or [[TinyOS]].
