Preemptive scheduling in VS .NET

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17.2.1.2 Preemptive scheduling
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Cyclic scheduling is a completely co-operative scheduling method there is no preemption. Once a task begins execution, it must complete before
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any other task can gain control of the processor and this is the root of the major problems with cyclic scheduling. To overcome these problems, preemptive scheduling is used. A preemptive system has the following characteristics: Interrupts are used to signal external events. An interrupt will have some way, direct or indirect, of triggering a task The tasks in the system are ordered in some way; for real time systems, they are generally ordered by how urgent they are. This ordering may be constant or it may change with time depending on circumstances When a task is triggered, its urgency is compared with that of the currently executing task. If the new task is more urgent, the current task is suspended and the new task begins execution immediately. If the new task is less urgent, it is simply placed on a list of pending tasks the ready list When a task completes execution or waits for an external event to occur, it is suspended and control is passed to the most urgent task on the ready list. Note that when control is passed from one task to another, the operating system automatically saves the state of the old task so that it can resume it later. Thus the operating system effectively does the partitioning of tasks into smaller pieces for you and so tasks can simply be written as a single unit. Adding a new task is simple; all that you have to do is to assign it a priority and check that the system can still be scheduled correctly with the new task included (in the sense that each task completes within its deadline). Similarly, an increase in the execution time of any task doesn t require any extra work rearranging the schedule, just a check that the system can still be scheduled correctly. Task periods don t have to be integer multiples of each other each task simply runs when it needs to. Similarly, sporadic tasks are not a problem they only use processor time when they actually run. Preemptive scheduling also deals with non-real time tasks; these are simply placed at the bottom of the ordering and so they run whenever no time critical tasks need the processor. Preemptive scheduling solves the major problems of cyclic scheduling. However it brings some problems of its own: Resource sharing. Since tasks are started and stopped automatically by the OS it is possible that they will contend for use of a shared resource It is more dif cult to check that a set of tasks can be scheduled correctly.
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17.2.1.3 Static priority based scheduling
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This is the most common form of preemptive scheduling, at least among real time operating systems. Each task is assigned a xed priority when it is created, and these priorities determine which tasks preempt which other tasks. The actual priority values used are derived using a very simple rule from the deadlines of the various tasks. The tasks are placed in increasing deadline order that is, the task with the shortest deadline has the highest priority and so on. This is known as deadline monotonic scheduling. This result was proved in the well-known paper by Liu and Layland2 for the case where Di = Ti and the more general result for Di < Ti was proved by Leung and Whitehead.3 For the case where Di = Ti the resulting priority assignment is called rate monotonic scheduling. For this case the Liu and Layland paper also derives a simple suf cient condition for a task set to be scheduled correctly by the rate monotonic algorithm; this is possible provided that
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