This paper presents a novel networking architecture designed for communication intensive parallel applications running on clusters of workstations (COWs) connected by high speed networks. This architecture permits (1) the transfer of selected communication-related functionality from the host machine to the network interface coprocessor, and (2) the exposure of this functionality directly to applications as instructions of aVirtual Communication Machine (VCM) implemented by the coprocessor. The user-level code interacts directly with the network coprocessor as the host kernel only 'connects' the application to the VCM and does not participate in the data transfers. The distinctive feature of our design is its flexibility: the integration of the network withthe applicationcan be varied to maximize performance. The resulting communication architecture is characterized by a very low overhead on the host processor, by latency and bandwidth close to the hardware limits, and by an applicatio...

There has been rapid growth in the demand for mobile communications over the past few years. This has led to intensive research and development efforts for complex PCS (personal communication service) networks. Capacity planning and performance modeling is necessary to maintain a high quality of service to the mobile subscriber while minimizing cost to the PCS provider. The need for flexible analysis tools and the high computational requirements of large PCS network simulations make it an excellent candidate for parallel simulation. Here, we describe our experiences in developing two PCS simulation models on a general purpose distributed simulation platform based on the Time Warp mechanism. These models utilize two widely used approaches to simulating PCS networks: (i) the call-initiated and (ii) the portable-initiated models. We discuss design decisions that were made in mapping these models to the Time Warp executive, and characterize the workloads resulting from these models in term...

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...

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There has been rapid growth in demand for mobile communications over the past few years that has led to intensive research and development of complex PCS (personal communication service) networks. Capacity planning and performance modeling is necessary to maintain a high quality of service to the mobile subscriber while minimizing cost. Simulation is widely used in such studies, however, because these models are extremely time consuming to execute, only small-scale PCS networks have previously been simulated. In this paper, we examine the use of the Time Warp distributed simulation mechanism in simulating large scale (1024 or more cells) PCS networks. An object-oriented, distributed discrete event simulator using Time Warp has been developed, and initial performance measurements completed. Speedups in the range of 2.8 to 7.8 using 8 Unix workstations have been obtained, enabling simulation runs that require 20 hours on a single workstation to be completed in only 3.5 hours. 1 PCS Netw...

We present a novel micro-kernel approach to parallel/distributed simulation. Using the micro-kernel approach, we develop a unified architecture for incorporating multiple types of simulation processes. The processes hold potential to employ a variety of synchronization mechanisms, and could alter their choice of mechanism dynamically. Supported mechanisms include traditional lookahead-based conservative and state saving-based optimistic execution approaches, as well as newer mechanisms such as reverse computation-based optimistic execution and aggregation-based event processing, all within a single parsimonious application programming interface (API). We also present the internal implementation and a preliminary performance evaluation of this interface in µsik, which is an efficient parallel/distributed realization of our micro-kernel architecture in C ++. 1.

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.

This paper describes issues concerning the design of an optimistic parallel discrete event simulation system that executes in environments that impose real-time constraints on the simulator's execution. Two key problems must be addressed by such a system. First the timing characteristics of the parallel simulator must be sufficiently predictable to allow one to guarantee that real-time deadlines for completing simulation computations will be met. Second, the optimistic computation must be able to interact with its surrounding environment with as little latency as possible, necessitating rapid commitment of I/O operations. To address the first question, we show that optimistic simulators that never send incorrect messages (sometimes called "aggressiveno -risk" simulators) provide sufficient predictability to allow traditional schedulability analysis techniques commonly used in realtime systems to be applied. We show that incremental state saving techniques introduce sufficient unpredic...