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'''Fixed priority pre-emptive scheduling''' is a scheduling system commonly used in real-time systems. With fixed priority pre-emptive scheduling, the scheduler ensures that at any given time, the processor executes the highest priority task of all those tasks that are currently ready to execute.
'''Fixed-priority preemptive scheduling''' is a [[Computer 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 pre-emptive[[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 theexecute—the [[time slice]]. This schedulerscheduling 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 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 [[RTOSreal-time operating system]]’ss (RTOSs) have pre-emptivepreemptive schedulers. Also turning off Timetime Slicingslicing effectively gives you the non-preemptive pre-emptive [[RTOS]].
Pre-emptivePreemptive scheduling is often differentiated with [[Computer multitasking#Cooperative 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]]. Fixed-priority scheduling with deferred preemption (FPDS) is a middle ground between fixed-priority pre-emptive scheduling and fixed-priority non-preemptive scheduling.<ref>{{Citation
