LiquiMedia | A dynamically extensible cyclic executive

Because it involves theoretical latencies that are unbounded, and practical latencies that are large, dynamic resource allocation poses di cult problems for all real-time systems. Control applications, where the task environment and range of possible computations are both highly constrained, can solve this problem using static allocation, with all allocation occurring outside of the realtime constraints. This simple and reliable strategy is unsuited to multimedia systems, however, because they must operate successfully in extremely variable computational environments. The solution is to create a version of dynamic allocation that retains the essence of the static solution, while taking advantage of characteristics unique to multimedia. This paper discusses using this strategy for allocation of computation and communication. CPU cycles and network bandwidth are the most basic resources used in any computation. We believe that a clean abstraction that is successful with these resources will certainly extend to other resources such as input and output devices. We believe that current operating systems designed for multimedia cannot allocate computational resources at a su ciently small granularity. Current general purpose operating systems allocate a processor's time with threads. Even on the fastest systems, context switches result in delays of tens of microseconds. As a result, using the processor e ciently requires task durations a millisecond or longer. Common tasks for a multimedia operating system such as synthesizing positionalized sound or animating sprites take far less than a millisecond for each iteration. Consequently, a multimedia operating system must provide an alternative to threads for the allocation of processor time. We call our alternative performers. Performers discard the overhead of context switching by abandonning preemption. They must always yield the processor voluntarily. Forcing programmers to statically divide the application into groups of performers makes the same tradeo as a cyclic executive: additional development work in exchange for no runtime resource allocation overhead. This tradeo engenders a new problem. Unlike a traditional xed-functionality cyclic executive, a multimedia operating system needs the capability of dynamically altering which performers it's executing. Maintaining realtime execution of performers requires that the operating system attempt to execute only per-