Optimistic Real-Time Simulation

On-line simulations play a critical role in understanding, supervising, and controlling large-scale systems. For instance, in military training applications, users are presented with complex scenarios that consist of both actual and simulated devices interacting in real-time. Troubleshooting of complex telephone networks may utilize on-line simulations to play`what if' games when evaluating alternative solution strategies. Future command and control applications are envisioned to present end users with data originating simultaneously from actual observations and from simulations. The discrete event simulations (DES) employed with such real-world applications are among the most compute-intensive applications known today. Since \traditional" parallelization of DES is of limited utility due to irregular, complex data dependencies, such parallelization commonly employs speculative execution. Unfortunately, it is unclear whether speculatively executed DES can operate correctly in the presence of the real-time constraints imposed by electro-mechanical systems and human participants. This paper describes a prototype real-time simulation system using speculative execution methods on a high performance, parallel machine. In addition, we explain the technical problems that must be addressed for systems that contain both simulated and actual real-time devices, and we describe appropriate solution methods. For illustrative purposes, problems, solutions, and the real-time simulation testbed are exposed using a sample application in which a simulation interacts with real-time software to control a robotic device.