Ports: a Parallel, Optimistic, Real-time Simulator Simulated System Actual System Environment Computer System Simulated Environment Real-time Simulations a Real-time System Environment Computer System Hybrid Simulation Figure 1: Real-time Systems and Real-time Simulations

This paper describes issues concerning the design of an optimistic parallel discrete event simulation system that executes in environments that impose real-time constraints on the simulator's execution. Two key problems must be addressed by such a system. First, the timing characteristics of the parallel simulator must be suuciently predictable to allow one to guarantee that real-time deadlines for completing simulation computations will be met. Second, the optimistic computation must be able to interact with its surrounding environment with as little latency as possible, necessitating rapid commitment of I/O operations. To address the rst question, we show that optimistic simulators that never send incorrect messages (sometimes called \aggressive-no-risk" simulators) provide suucient predictability to allow traditional schedulability analysis techniques commonly used in real-time systems to be applied. We show that incremental state saving techniques introduce suucient unpredictability that they are not well-suited for real-time environments. We observe that the traditional \lowest timestamp rst" scheduling policy used in many optimistic parallel simulation systems is an optimal (in the real-time sense) scheduling algorithm when event timestamps and real-time deadlines are the same. Finally, to address the question of rapid commitment of I/O operations, we utilize a continuous GVT computation scheme for shared-memory multiprocessors where a new value of GVT is computed after processing each event in the simulation. These ideas are incorporated in a parallel, optimistic, real-time simulation system called PORTS. Initial performance measurements of the shared-memory based PORTS system executing on a Kendall Square Research multiprocessor are presented. Initial performance results are encouraging, demonstrating that PORTS achieves performance approaching that of a conventional Time Warp system under several test cases.