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'''Fixed-priority pre-emptive scheduling''' is a [[Computer_multitasking|scheduling system]] commonly used in [[real-time system]]s. With fixed priority pre-emptive 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.
'''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 [[Preemption (computing)|pre-emptivepreemptive 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—theperiod to execute—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 alsois hasvulnerable the downfall ofto process or thread [[Resource_starvationResource starvation|lockout]],: assince 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 the use of aging., Agingwhich willgradually slowlyincrements increment a process/thread(s)the priority whichof iswaiting inprocesses theand waitthreads, queueensuring tothat ensurethey somewill degreeall ofeventually fairnessexecute. Most [[Realreal-time operating system]]s (RTOSs) have pre-emptivepreemptive schedulers. Also turning off time slicing effectively gives you the non-pre-emptivepreemptive RTOS.
Pre-emptivePreemptive 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]].
