Existing online multiplayer games typically use a client-server model, which introduces added latency as well as a single bottleneck and single point of failure to the game. Distributed multiplayer games minimize latency and remove the bottleneck, but require special synchronization mechanisms to provide a consistent game for all players. Current synchronization methods have been borrowed from distributed military simulations and are not optimized for the requirements of fast-paced multiplayer games. In this paper we present a new synchronization mechanism, trailing state synchronization (TSS), which is designed around the requirements of distributed first-person shooter games. We look at TSS...

In this paper, we introduce a new Time Warp system called ROSS: Rensselaer's Optimistic Simulation System. ROSS is an extremely modular kernel that is capable of achieving event rates as high as 1,250,000 events per second when simulating a wireless telephone network model (PCS) on a quad processor PC server. In a head-to-head comparison, we observe that ROSS out performs the Georgia Tech Time Warp (GTW) system by up to 180% on a quad processor PC server and up to 200% on the SGI Origin 2000 . ROSS only requires a small constant amount of memory buffers greater than the amount needed by the sequential simulation for a constant number of processors. ROSS demonstrates for the first time that stable, highly-efficient execution using little memory above what the sequential model would require is possible for low-event granularity simulation models. The driving force behind these high-performance and low memory utilization results is the coupling of an efficient pointer-based implementation framework, Fujimoto 's fast GVT algorithm for shared memory multiprocessors, reverse computation and the introduction of Kernel Processes (KPs). KPs lower fossil collection overheads by aggregating processed event lists. This aspect allows fossil collection to be done with greater frequency, thus lowering the overall memory necessary to sustain stable, efficient parallel execution. These characteristics make ROSS an ideal system for use in large-scale networking simulation models. The principle conclusion drawn from this study is that the performance of an optimistic simulator is largely determined by its memory usage. 1

Parallel simulation is emerging as the dominant technique for studying parallel computers. However, the interconnection networks of these machines can be modeled at many different levels of abstraction, allowing researchers to trade off accuracy and performance. In this paper, we use the Wisconsin Wind Tunnel, a parallel simulator for cache-coherent shared-memory machines, to study the trade-offs of accuracy versus performance for six different network simulation models. We evaluate these models for a variety of parallel applications, cachecoherence protocols, and topologies. We show that only the two most expensive models---which model contention at individual links---are robust in the presence of high network loads or non-uniform traffic patterns.

We present a parallel simulator — BigSim — for predicting performance of machines with a very large number of processors. The simulator provides the ability to make performance predictions for machines such as Blue-Gene/L, based on actual execution of real applications. We present this capability using case-studies of some application benchmarks. Such a simulator is useful to evaluate the performance of specific applications on such machines even before they are built. A sequential simulator may be too slow or infeasible. However, a parallel simulator faces problems of causality violations. We describe our scheme based on ideas from parallel discrete event simulation and utilize inherent determinacy of many parallel applications. We also explore techniques for optimizing such parallel simulations of machines with large number of processors on existing machines with fewer number of processors. 1 1

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.

This paper examines memory management issues associated with Time Warp synchronized parallel simulation on distributed memory machines. The paper begins with a summary of the techniques which have been previously proposed for memory management on various parallel processor memory structures. It then concentrates the discussion on parallel simulation executing on a distributed memory computer -- a system comprised of separate computers, interconnected by a communications network. An important characteristic of the software developed for such systems is the fact that the dynamic memory is allocated from a pool of memory that is shared by all of the processes at a given processor.

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...

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

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...