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Fixed-priority preemptive scheduling is a scheduling system commonly used in real-time systems.With fixed priority preemptive 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.
In preemptive multitasking, the operating system kernel can also initiate a context switch to satisfy the scheduling policy's priority constraint, thus preempting the active task. In general, preemption means "prior seizure of". When the high-priority task at that instance seizes the currently running task, it is known as preemptive scheduling.
It is commonly accepted that an implementation of fixed-priority pre-emptive scheduling (FPS) is simpler than a dynamic priority scheduler, like the EDF. However, when comparing the maximum usage of an optimal scheduling under fixed priority (with the priority of each thread given by the rate-monotonic scheduling ), the EDF can reach 100% while ...
For example, Windows NT/XP/Vista uses a multilevel feedback queue, a combination of fixed-priority preemptive scheduling, round-robin, and first in, first out algorithms. In this system, threads can dynamically increase or decrease in priority depending on if it has been serviced already, or if it has been waiting extensively.
A Round Robin preemptive scheduling example with quantum=3. Round-robin (RR) is one of the algorithms employed by process and network schedulers in computing. [1] [2] As the term is generally used, time slices (also known as time quanta) [3] are assigned to each process in equal portions and in circular order, handling all processes without priority (also known as cyclic executive).
The priority ceiling protocol [11] enhances the basic priority inheritance protocol by assigning a ceiling priority to each semaphore, which is the priority of the highest job that will ever access that semaphore. A job cannot preempt a lower priority critical section if its priority is lower than the ceiling priority for that section.
Deadline-monotonic priority assignment is a priority assignment policy used with fixed-priority pre-emptive scheduling. With deadline-monotonic priority assignment, tasks are assigned priorities according to their deadlines. The task with the shortest deadline is assigned the highest priority. [1]
Linux is an example of a monolithic-kernel operating system with kernel preemption. The main benefit of kernel preemption is that it solves two issues that would otherwise be problematic for monolithic kernels, in which the kernel consists of one large binary. [5] Without kernel preemption, two major issues exist for monolithic and hybrid kernels: