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TopThe research reviewed here can be traced back over the last few years, where distinguished researches and projects have demonstrated the effectiveness of hierarchical scheduling techniques of periodic tasks in RTOS.
It was initially proposed by researchers in (Deng, & Liu, (1997, December)), where they proposed a two level hierarchical scheduling framework considering the maximum task execution time (WCET), and it was proved to be resources wasting.
In (Davis, & Burns, (2005, December)), authors applied a fixed priority preemptive scheduling in HSF global and local levels considering sporadic, periodic and deferrable servers (Inam, Mäki-Turja, Sjödin, Ashjaei, & Afshar, (2011, September)) in their work. It guaranteed that tasks in highest levels would meet their deadlines while lowest level tasks may suffer from deadlines misses since it was based on (WCET) in their calculations. Hence critical tasks are assigned to high levels to ensure meeting their deadlines. In (Zhang, & Burns, (2007, December)), authors proposed an analysis of hierarchical EDF preemptive scheduling, depending on the earliest deadline first (EDF) scheduler introduced by (Liu & Layland, 1973). They presented a schedulability test for different hard real time application tasks when the local scheduler is EDF and the global scheduler is EDF or fixed priority. EDF scheduling is proved to have a better performance than fixed-priority scheduling in terms of deadline miss. However, it is difficult to ensure that critical tasks will meet their deadlines as it depends on the tasks deadlines which varing over the run time.