The achievements attained in accelerating the simulation of the dynamics of complex discrete event systems using parallel or distributed multiprocessing environments are comprehensively presented. While parallel discrete event simulation (DES) governs the evolution of the system over simulated time in an iterative SIMD way, distributed DES tries to spatially decompose the event structure underlying the system, and executes event occurrences in spatial subregions by logical processes (LPs) usually assigned to different (physical) processing elements. Synchronization protocols are necessary in this approach to avoid timing inconsistencies and to guarantee the preservation of event causalities across LPs. Included in the survey are discussions on the sources and levels of parallelism, synchronous vs. asynchronous simulation and principles of LP simulation. In the context of conservative LP simulation (Chandy/Misra/Bryant) deadlock avoidance and deadlock detection/recovery strategies, Con...

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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.

Many systems rely on the ability to rollback (or restore) parts of the system state to undo or recover from undesired or erroneous computations. Examples of such systems include fault tolerant systems with checkpointing, editors with undo capabilities, transaction and data base systems and optimistically synchronized parallel and distributed simulations. An essential part of such systems is the state saving mechanism. It should not only allow efficient state saving, but also support efficient state restoration in case of roll back. Furthermore, it is often a requirement that this mechanism is transparent to the user. In this paper we present a method to implement a transparent incremental state saving mechanism in an optimistically synchronized parallel discrete event simulation system based on the Time Warp mechanism. The usefulness of this approach is demonstrated by simulations of large, detailed, realistic FCA and a DCA-like cellular phone systems. 1. Introduction Many systems rel...

This paper describes results of an empirical study to evaluate the effect of communications delays on the performance of the Time Warp mechanism in order to assess the effectiveness of Time Warp in distributed computing evironments. An implementation of Time Warp on a collection of networked workstations is used in this study. Performance using synchronous and asynchronousmessage passing primitives are compared, and it is observed that Time Warp experiences much more rolled back computation when using the synchronous primitives for certain applications. Message passing is decomposed into a computation component at the senderand receiverprocessors, and a transmission delay component that represents the amount of time the message remains "in transit" within the network. The effect of each of these components on Time Warp performance is studied. It is observed that communications latency in distributed computing environments can significantly degrade the efficiency of Time Warp for applic...

It has been established elsewhere [Reyn92] that hardware to support parallel discrete event simulations (PDES) is desirable. We describe the steps leading to the implementation of a hardware-based framework to support PDES. We begin with an exploration of the criteria necessary to make such a framework both practical and useful, concluding that maintenance of sequential consistency is sufficient, while "observable" sequential consistency is more desirable but difficult to attain. We derive a functional design based on these criteria, and from that derive a prototype design. Also, we establish the utility of our design, showing that computation of critical global values, such as global virtual time, can be done in times two orders of magnitude or better than typical event times in discrete event simulations. ############################################################################################## 1. Introduction The need for special purpose hardware to support efficient parallel d...

Adaptive synchronization algorithms have been proposed to improve upon purely conservative and purely optimisitic algorithms. Experimental studies have have indeed provided encouraging results. In the spirit of previous analyses, we present the first known analytical comparison of adaptively optimisitic algorithms with the Time Warp protocol. We define a class of adaptive protocols, the asynchronous adaptive waiting protocols (AAWP's) and identify several practical protocols that belong to this class. We show that Time Warp can outperform an AAWP arbitrarily. We describe NPSI adaptive protocols, a sub-class of AAWP's, and specify a member of this sub-class, the Elastic Time Algorithm. We show that this algorithm can outperform Time Warp arbitrarily.

DEVS (Discrete Event System Specification) is a general modelling formalism with sound semantics founded on a system theoretic basis. This gives it a claim to be universal for formalisms describing Discrete Event Dynamic Systems (DEDS). This means that any other formalism such as Petri nets, which have become very popular for DEDS control can be embedded in it. Moreover, DEVS extends to the continuous case thus facilitating combined discrete/continuous modelling. The universality of DEVS is significant because it has been implemented in a variety of simulation environments, as extensions of diverse underlying Object-Oriented languages such as CLOS and C++. This gives it not only the power of formal rigor but also the practical capability of application to real world complex systems. The DEVS formalism has associated with it a characteristic abstract simulation engine architecture that can be realized in diverse sequential and parallel/distributed platforms. It is especially suitable fo...

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This paper describes the details of a hardware realization of a framework to support parallel discrete event simulation [Reyn92]. We first motivate the need for hardware to compute and disseminate critical synchronization information in all parallel simulations. We establish correctness criteria and functional requirements of the framework hardware. We then describe details of a completed prototype design which is expected to be operational Summer, 1992. Throughout this discussion, we show how our design goals of speed, scalability, adaptability, and generality have been met. Our framework offloads all parallel simulation synchronization overhead from host processors and the host communication network in a closely coupled network of high-speed computers. ############################################################################## 1. Introduction Despite the fact that parallel simulation and the potential speed-up it offers are regarded as crucial to many applications --- digital ne...