Scheduling real-time periodic task of control system with dual priority algorithm

Since the real-time sets in control system consist of hard real-time periodic tasks and soft real-time period tasks, the dual priority algorithm is adopted for scheduling these tasks. The algorithm divides priority band into three parts, i.e. high priority band, middle priority band and low priority band. The priority of hard real-time task is set in low priority when its instance arrives and it is promoted into high priority band after a period of time. The priority of soft real-time task is set in middle priority band. The existence of the priority promotion time of hard real-time task is investigated. The method for computing the optimal priority promotion time of hard real-time task is presented. Under the condition that the lost ratio of the soft real-time task's instances β_si is in certain value (0≤β_si<1), the schedulability of tasks is analyzed. The result of simulation shows that the scheduling algorithm is effective.