A large class of real-time applications involve tasks which are periodic in nature. All the information needed for scheduling in such a system is static and is known a priori. Scheduling of a periodic task set in a real-time system with a single processor is a well studied topic. These aperiodic real-time tasks are usually activated by unexpected events to which the system must respond within a bounded time. Aperiodic tasks, triggered by external events, may constitute a large fraction of real-time tasks in some embedded systems which interact with unpredictable external environment.
Aperiodic tasks make scheduling more complex and scheduling in such systems has to be dynamic.
Such systems have been studied only recently. Following the rapid developments in networking technology and the availability of VLSI chips with substantial computing power at low cost in the recent years, distributed systems for real-time applications are now envisaged as realisable, in many new areas. Such a system has the potential of higher performance and greater fault tolerance. Scheduling in a distributed real-time system is complicated by the fact that it has to be considered in conjunction with the issue of placement, ie, on which processor is a particular task to be executed. The efficiency of performance and correctness of such a system depends on how good the heuristic is.