Static allocation of periodic tasks with precedence constraints in distributed real-time systems

Using two branch-and-bound (B&B) algorithms, an optimal solution is proposed to the problem of allocating (or assigning with the subsequent scheduling considered) periodic tasks to a set of heterogeneous processing nodes (PNs) of a distributed real-time system. The allocation objective is to minimize the maximum normalized task response time, called the system hazard, subject to precedence constraints among the tasks to be allocated. First, the task system is modeled with a task graph, which describes computation and communication modules as well as the precedence constraints among them. Second, the exact system hazard of a complete assignment is determined so that an optimal assignment can be derived. This exact cost is obtained by optimally scheduling the modules assigned to each PN with a B&B algorithm guided by the dominance relationship between simultaneously schedulable modules. Third, to reduce the amount of computation needed for an optimal assignment, a lower-bound system hazard that is obtainable with a polynomial time algorithm is derived