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Fujimoto,
Richard M.
Fujimoto,
Richard M.
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ItemOptimistic Real-Time Simulation(Georgia Institute of Technology, 1995) Ghosh, Kaushik ; Fujimoto, Richard M. ; Schwan, KarstenOn-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 computer-intensive applications known today. Since “traditional” parellelization 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.
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ItemPORTS: Experiences with a Scheduler for Dynamic Real-Time Systems(Georgia Institute of Technology, 1994) Ghosh, Kaushik ; Fujimoto, Richard M. ; Schwan, KarstenThis paper describes several of our experiences with a real-time scheduler. Using a robot control application program, we motivate the importance of supporting multiple schedulers within the same application program. We demonstrate the utility of speculative task execution in dynamic real-time systems, and describe the implementation of a scheduler for performing speculative execution and recovery. We show that existing real-time scheduler interfaces have scope for improvement, especially when scheduling latency must be low and when multiple schedulers used by a single application must co-exist on a single processor. A new scheduler interface is specified and its basic costs are evaluated experimentally. Preliminary measurements on a KSR-1 machine are quoted. The measurements demonstrate how the execution times of temporal queries may be reduced by use of access structures to scheduler data structures. Finally, there are several overheads associated with speculative execution, and multiple schedulers in a single application. We consider the problem of on-line reconfiguration of the several overheads associated with the speculative-execution paradigm for optimal performance in the face of these overheads. Initial performance measurements of the PORTS scheduler indicate that it is possible to perform real-time scheduling with latencies approximating those of proposed specialized scheduling co-processors.
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ItemParallel Discrete Event Simulation Using Space-Time Memory(Georgia Institute of Technology, 1994) Ghosh, Kaushik ; Fujimoto, Richard M.An abstraction called space-time memory is discussed that allows parallel discrete event simulation programs using the Time Warp mechanism to be written using shared memory constructs. A few salient points concerning the implementation and use of space-time memory in parallel simulation are discussed. It is argued that this abstraction is useful from a programming standpoint for certain applications, and can yield good performance. Initial performance measurements of a prototype implementation of the abstraction on a shared-memory multiprocessor are described, and compared with a conventional, message-based implementation of Time Warp.
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ItemPORTS: A Parallel, Optimistic, Real-Time Simulator(Georgia Institute of Technology, 1993) Ghosh, Kaushik ; Panesar, Kiran S. ; Fujimoto, Richard M. ; Schwan, KarstenThis 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 sufficiently 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 first question, we show that optimistic simulators that never send incorrect messages (sometimes called "aggressive-no-risk" simulators) provide sufficient predictability to allow traditional schedulability analysis techniques commonly used in real-time systems to be applied. We show that incremental state saving techniques introduce sufficient unpredictability that they are not well-suited for real-time environments. We observe that the traditional "lowest timestamp first" 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 for 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 for the benchmark programs that were tested.