Recently there has been a great deal of interest in performance evaluation of parallel simulation. Most work is devoted to the time complexity and assumes that the amount of memory available for parallel simulation is unlimited. This paper studies the space complexity of parallel simulation. Our goal is to design an efficient memory management protocol which guarantees that the memory consumption of parallel simulation is of the same order as sequential simulation. (Such an algorithm is referred to as optimal.) We first derive the relationships among the space complexities of sequential simulation, Chandy-Misra simulation, and Time Warp simulation. We show that Chandy-Misra may consume more storage than sequential simulation, or vice versa. Then we show that Time Warp never consumes less memory than sequential simulation. Then we describe cancelback, an optimal Time Warp memory management protocol proposed by Jefferson. Although cancelback is considered as a complete solution for the storage management problem in Time Warp, some efficiency issues in implementing this algorithm must be considered. In this paper, we propose an optimal algorithm called artificial rollback. We show that this algorithm is easy to implement and analyze. An implementation of artificial rollback is given, which is integrated with processor scheduling to adjust the memory consumption rate based on the amount of free storage available in the system.

Time management is required in simulations to ensure temporal aspects of the system under investigation are correctly reproduced by the simulation model. This paper describes the time management services that have been defined in the High Level Architecture. The need for time management services is discussed, as well as design rationales that lead to the current definition of the HLA time management services. These services are described, highlighting information that must flow between federates and the Runtime Infrastructure (RTI) software in order to efficiently implement time management algorithms. 1. Introduction The Defense Modeling and Simulation Office (DMSO), through its High Level Architecture (HLA) initiative, is addressing the continuing need for interoperability between new and existing simulations within the U. S. Department of Defense. The HLA builds upon and generalizes the results of the Distributed Interactive Simulation (DIS) effort (DIS Steering Committee 1994) and...

Complex models may have model components distributed over a network and generally require significant execution times. The field of parallel and distributed simulation has grown over the past fifteen years to accommodate the need of simulating the complex models using a distributed versus sequential method. In particular, asynchronous parallel discrete event simulation (PDES) has been widely studied, and yet we envision greater acceptance of this methodology as more readers are exposed to PDES introductions that carefully integrate real-world applications. With this in mind, we present two key methodologies (con- servative and optimistic) which have been adopted as solutions to PDES systems. We discuss PDES terminology and methodology under the umbrella of the personal communications services application.

Most work to date in parallel and distributed discrete event simulation is based on assigning precise time stamps to events, and time stamp order event processing. An alternative approach is examined where modelers use time intervals rather than precise time stamps to specify uncertainty as to when events occur. Partial orderings called approximate time (AT) and approximate time causal (ATC) order are proposed and synchronization algorithms developed that exploit these specifications to yield more efficient execution on parallel and distributed computers. Performance measurements of the AT-ordering mechanism on a cluster of workstations demonstrate as much as twenty-fold performance improvement compared to time stamp ordering with negligible impact on the results computed by the simulation. The context for much of this work is federated simulation systems that provided the initial motivation for this work. These results demonstrate that exploiting temporal uncertainty inhere...

Parallel discrete event simulation offers significant speedup over the traditional sequential event list algorithm. A number of conservative and optimistic algorithms have been proposed and studied for parallel simulation. We examine the problem of transparent execution of a simulation model using conservative algorithms, and present experimental results on the performance of these transparent implementations. The conservative algorithms implemented and compared include the null message algorithm, the conditional-event algorithm, and a new algorithm which is a combination of these. We describe how dynamic topology can be supported by conservative algorithms. Language constructs to express lookahead are discussed. Finally, performance measurements on a variety of benchmarks are presented, along with a study of the relationship between model characteristics like lookahead, communication topology and the performance of conservative algorithms. 1 INTRODUCTION Distributed(or parallel) simu...

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.

this paper describes our approach from the system point of view. The programming interface is described in detail in the next section, following which the design and salient implementation aspects are discussed. Representative results from a few simulation systems are then reported, and the concluding section discusses some of the critical issues in such an approach, the implications for applications other than stochastic simulation, and ongoing and future work

Distributedsynchronizationforparallelsimulationisgenerallyclassifiedasbeingeitheroptimisticorconservative. Whileconsiderableinvestigationshavebeenconducted toanalyzeandoptimizeeachofthesesynchronization strategies,verylittlestudyonthedefinitionandstrictness ofcausalityhavebeenconducted.Dowereallyneed topreservecausalityinalltypesofsimulations?This paperattemptstoanswerthisquestion.Wearguethat significantperformancegainscanbemadebyreconsideringthisdefinitiontodecideiftheparallelsimulation needstopreservecausality.Weinvestigatethefeasibility ofunsynchronizedparallelsimulationthroughtheuseof severalqueuingmodelsimulationsandpresentacomparativeanalysisbetweenunsynchronizedandTimeWarp simulation.

A distributed simulation is said to be repeatable if successive executions utilizing the same inputs produce exactly the same outputs. Repeatability is a highly desirable property, particularly for analytic simulation models. This paper discusses the question of repeatability in distributed simulations in general, and in the context of the High Level Architecture in particular. Specifically, allowing zero lookahead, a feature not supported in the baseline HLA, has important ramifications with respect to achieving repeatable executions. Extensions to the existing time management services in the HLA are proposed that allow (1) repeatable executions, (2) federate control over the ordering of simultaneous events, and (3) zero lookahead. The extensions proposed in this paper are currently under consideration by DMSO for possible future inclusion in the High Level Architecture time management services. 1. INTRODUCTION It is sometimes important that repeated executions of a simulation progra...

Parallel discrete event simulation with conservative synchronization algorithms has been used as a high performance alternative to sequential simulation. In this paper we examine the performance of a set of parallel conservative algorithms that have been implemented in Maisie, an algorithm-independent simulation language. The algorithms include the null message algorithm, the conditional event algorithm, and a new algorithm called Accelerated Null Message algorithm that combines the preceding alternatives. The experiments presented in this paper study the performance of the algorithms as a function of model connectivity, computation granularity, and lookahead properties -- model characteristics that have the most significant impact on the performance of conservative protocols. A novel contribution of this paper is the introduction of the Ideal Simulation Protocol (or ISP) as a common reference point to compare the efficiency of the algorithms.