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Research in parallel discrete event simulation indicates that neither purely conservative nor purely optimistic synchronization algorithms will perform well consistently. We survey several new approaches that attempt to improve performance by limiting optimistic execution. In most of these, the criterion for limiting optimism is static or based on local information, which conflicts with the dynamic nature of discrete event simulations. We contend that an adaptive approach based on low cost near-perfect system state information is the most likely to yield a consistently efficient synchronization algorithm. We suggest a framework by which NPSI (near-perfect state information) adaptive protocols could be designed and describe the first such protocol - Elastic Time Algorithm. We present performance results from an implementation of this algorithm which show that adaptive protocols based on the use of NPSI are promising. In particular, we show that NPSI adaptive protocols have th...

This paper reviews issues concerning the design of adaptive protocols for parallel discrete event simulation (PDES). The need for adaptive protocols are motivated in the background of the classical synchronization problem that has driven much of the research in this field. Traditional conservative and optimistic protocols and their hybrid variants --- that form the basis of adaptive protocols --- are also discussed. Adaptive synchronization protocols are reviewed with special reference to their characteristics regarding the aspects of the simulation state that influence the adaptive decisions and the control parameters used. Finally, adaptive load management strategies and their relationship to the synchronization protocol are discussed. Keywords: Simulation, Computers, Methodology.

We describe a new algorithm, called Filter, that limits the propagation of erroneous computations in optimistic discrete-event distributed simulations. In the proposed algorithm, each message carries a bounded amount of dependency information that describes the assumptions made in the generation of the message, and, in addition, processes keep track of straggler events that have occurred in the system. This knowledge is used by processes to \ lter " out messages that depend on a preempted state by discarding them upon receipt. We describe the algorithm and its use in conjunction with time-warp, suggest several ways of reducing its potential overhead by adjusting the extent of ltering, and point out several interesting performance tradeo s that we are currently exploring. 1

It is well known that Time Warp may suffer from poor performance due to excessive rollbacks caused by overly optimistic execution. Here we present a simple flow control mechanism using only local information and GVT that limits the number of uncommitted messages generated by a processor, thus throttling overly optimistic TW execution. The flow control scheme is analogous to traditional networking flow control mechanisms. A "window" of messages defines the maximumnumber of uncommitted messages allowed to be scheduled by a process. Committing messages is analogous to acknowledgments in networking flow control. The initial size of the window is calculated using a simple analytical model that estimates the instantaneous number of messages that a process will eventually commit. This window is expanded so that the process may progress up to the next commit point (generally the next fossil collection), and to accommodate optimistic execution. The expansions to the window are based on monitori...

The optimistic approach to parallel discrete event simulation (PDES) has led to a number of algorithms capable of fully exploiting the inherent parallelism of discrete event systems. On the other hand, these parallel algorithms, as well as most implementations of the Time Warp mechanism were designed to suit a specific parallel architecture, therefore suffering from lack of portability. This paper proposes the bulk synchronous parallel (BSP) model as a target platform for the design of portable parallel algorithms for optimistic simulation. After an overview of the main directions in PDES, the paper describes the Time Warp mechanism, presenting the most important issues related to optimistic simulation. A class of BSP algorithms for GVT computation is introduced and analysed in terms of the the BSP cost model. Then, two BSP algorithms for optimistic PDES are discussed; the first algorithm aims at avoiding recursive rollbacks in aggressive-cancellation Time Warp implementations, while t...

Efficient computer simulation of complex physical phenomena has long been challenging due to their multiphysics and multiscale nature. In contrast to traditional time-stepped execution methods, the authors describe an approach using optimistic parallel discrete event simulation (PDES) and reverse computation techniques to execute plasma physics codes. They show that reverse computationbased optimistic parallel execution can significantly reduce the execution time of an example plasma simulation without requiring a significant amount of additional memory compared to conservative execution techniques. The authors describe an application-level reverse computation technique that is efficient and suitable for complex scientific simulations.

This paper addresses issues of implementation and performance optimization of simulations designed to model spatially explicit problems with the use of parallel discrete event simulation. A simulation system is presented that uses the optimistic protocol and runs on a distributed memory machine—the IBM SP. The efficiency of parallel discrete event simulations that use the optimistic protocol is strongly dependent on the overhead incurred by rollbacks. This paper introduces a novel approach to rollback processing which limits the number of events rolled back as a result of a straggler or antimessage. The method, called Breadth-First Rollback (BFR), is suitable for spatially explicit problems where the space is discretized and distributed among processes and simulation objects move freely in the space. The BFR uses incremental state saving, allowing the recovery of causal relationships between events during rollback. These relationships are then used to determine which events need to be rolled back. This paper presents an application of BFR to the simulation of Lyme disease. Our results demonstrate and almost linear speedup—a

Simulator : : : : : : : : : : : : : : : : : : : : : : : : : 87 Appendix E. Examples of Human Performance Process Hierarchical Decomposition 92 Appendix F. Scalable Coherent Interfaces 96 Contents (continued) Chapter Page Appendix G. Synopses of Selected High Performance Parallel Machines 98 Appendix H. Glossary of Acronyms 102 References 105 List of Figures Figure Page 1.1 A Holarchy : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 17 2.1 Possible Paths for Human Performance Process Model Creation : : : : : : : 21 6.1 Numerical Aerodynamics Simulation Results for Embarassingly Parallel Benchmarks : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 40 6.2 CM2: Numerical Aerodynamics Simulation Benchmark Results : : : : : : : 41 6.3 Human Performance Process and Architectures : : : : : : : : : : : : : : : : 42 8.1 Heterogeneous Computing Environment : : : : : : : : : : : : : : : : : : : : 50 9.1 High Performance Systems Metrics : : :...

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