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...

e-j 4r.,,D I-- " h",' _ k,) r,m '3'-. IC,-.-4 Z _ O

In a distributed memory environment the communication overhead of Time Warp as induced by the rollback procedure due to "overoptimistic" progression of the simulation is the dominating performance factor. To limit optimism to an extent that can be justified from the inherent model parallelism, an optimism control mechanism is proposed, which by maintaining a history record of virtual time differences from the time stamps carried by arriving messages, and forecasting the timestamps of forthcoming messages, probabilistically delays the execution of scheduled events to avoid potential rollback and associated communication overhead (antimessages). After investigating statistical forecast methods which express only the central tendency of the arrival process, we demonstrate that arrival processes in the context of Time Warp simulations of timed Petri nets have certain predictable and consistent ARIMA characteristics, which encourage the use of sophisticated and recursive forecast procedures...

The main performance pitfall of the Time Warp distributed discrete event simulation (DDES) protocol has been widely recognized to be the overoptimistic progression of event execution into the simulated future. The premature execution of events that eventually have to be "rolled back" due to causality violations induces memory and communication overheads as sources of performance inefficiencies. Optimistic Time Windows and self adaptive mechanisms have been proposed in the literature to control the optimism in Time Warp in order to improve or optimize its execution performance. In this work, an adaptive optimism control mechanism based on the observed model parallelism is proposed. Methodologically, logical processes (LPs) monitor the local virtual time (LVT) progression per unit CPU-time from the timestamp of arriving messages and establish a cost model for the trade-off between optimistically progressing and conservatively blocking the simulation engine. Compared to previous approache...

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.

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

Excessive rollback recoveries due to overoptimistic event execution in Time Warp simulators often degmde their runtime performance. This paper presents a two-sided throttling scheme to dynamically adjust the event execution speed of Time Warp simulators. The pro-posed throttle is based on a new concept called global progress window, which allows the individual simula-tion process to be positioned on a global time scale, thereby to accelemte OT suspend their event execution. As each simulation process can be throttled to a steady state, excessive rollback recoveries due to causality er-MTS can be avoided. To quantify the effect of roll-backs and for purpose of comparing different Time Warp implementations, we propose two new measures called RPE (number of Rollback events Per committed Event), and 8 (relative Eflectiveness in reducing roll-back overhead). OUT implementation results show that the proposed throttle effectively regulates the proceeding of each simulation process, resulting in a significant re-duction in rollback thrashing and elapsed time.

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...

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.