Showing papers by "Richard M. Fujimoto published in 2000"

Parallel and Distributed Simulation Systems

Abstract: Originating from basic research conducted in the 1970's and 1980's, the parallel and distributed simulation field has matured over the last few decades. Today, operational systems have been fielded for applications such as military training, analysis of communication networks, and air traffic control systems, to mention a few. The article gives an overview of technologies to distribute the execution of simulation programs over multiple computer systems. Particular emphasis is placed on synchronization (also called time management) algorithms as well as data distribution techniques.

Design of high performance RTI software

Abstract: This paper describes the implementation of RTI-Kit, a modular software package to realize runtime infrastructure (RTI) software for distributed simulations such as those for the High Level Architecture. RTI-Kit software spans a wide variety of computing platforms, ranging from tightly coupled machines such as shared memory multiprocessors and cluster computers to distributed workstations connected via a local area or wide area network. The time management, data distribution management, and underlying algorithms and software are described.

Efficient execution of Time Warp programs on heterogeneous, NOW platforms

Abstract: Time Warp is an optimistic protocol for synchronizing parallel discrete event simulations. To achieve performance in a multiuser network of workstation (NOW) environment, Time Warp must continue to operate efficiently in the presence of external workloads caused by other users, processor heterogeneity, and irregular internal workloads caused by the simulation model. However, these performance problems can cause a Time Warp program to become grossly unbalanced, resulting in slower execution. The key observation asserted in this article is that each of these performance problems, while different in source, has a similar manifestation. For a Time Warp program to be balanced, the amount of wall clock time necessary to advance an LP one unit of simulation time should be about the same for all LPs. Using this observation, we devise a single algorithm that mitigates these performance problems and enables the "background" execution of Time Warp programs on heterogeneous distributed computing platforms in the presence of external as well as irregular internal workloads.

An approach for federating parallel simulators

Abstract: This paper investigates issues concerning federations of sequential and/or parallel simulators. An approach is proposed for creating federated simulations by defining a global conceptual model of the entire simulation, and then mapping individual entities of the conceptual model to implementations within individual federates. Proxy entities are defined as a means for linking entities that are mapped to different federates. Using this approach, an implementation of a federation of optimistic simulators is examined. Issues concerning the adaptation of optimistic simulators to a federated system are discussed. The performance of the federated system utilizing runtime infrastructure (RTI) software executing on a shared memory multiprocessor (SMP) is compared with a native (non-federated) SMP-based optimistic parallel simulator. It is demonstrated that a well designed federated simulation system can yield performance comparable to a native, parallel simulation engine, but important implementation issues must be properly addressed.

Stateless routing in network simulations

Abstract: The memory resources required by network simulations can grow quadratically with the size of the simulated network. In simulations that use routing tables at each node to perform per-hop packet forwarding, the storage required for the routing tables is O(N/sup 2/), where N is the number of simulated network nodes in the topology. Additionally, the CPU time required in the simulation environment to compute and populate these routing tables can be excessive and can dominate the overall simulation time. We propose a new routing technique, known as Neighbor-Index Vectors, or NIx-Vectors, which eliminates both the storage required for the routing tables and the CPU time required to compute them. We show experimental results using NIx-Vector routing in the popular network simulator ns (S. McCanne and S. Floyd, 1997). With our technique, we achieve a near order of magnitude increase in the maximum size of a simulated network running ns on a single workstation. Further, we demonstrate an increase of two orders of magnitude in topology size (networks as large as 250000 nodes) by using this technique and running the simulation in parallel on a network of workstations.

Pre-sampling as an approach for exploiting temporal uncertainty

Abstract: In this paper we describe an approach to exploit temporal uncertainty in parallel and distributed simulation by utilizing time intervals rather than precise time stamps. Unlike previously published work that propose new message ordering semantics, our approach is based on conservative, time stamp order execution and enhancing the lookahead of the simulation by pre-drawing random numbers from a distribution that models temporal uncertainty. The advantages of this approach are that it allows time intervals to be exploited using a conventional Time Stamp Order (TSO) delivery mechanism, and it offers the modeler greater statistical control over the assigned time stamps. An implementation of this approach is described and initial performance measurements are presented.

Repeatability in real-time distributed simulation executions

Abstract: Real-time distributed simulations, such as on-line gaming or military training simulations are normally considered to be non-deterministic. Analysis of these simulations is therefore difficult depending solely on logging and runtime observations. This paper explores an approach for removing one major source of non-determinism in these simulations, thereby allowing repeatable executions. Specifically, we use a synchronization protocol to ensure repeatable delivery of messages. Through limited instrumentation of the simulation code, we maintain a virtual time clock, by which message delivery is governed. The additional overhead imposed by the scheme is shown to be reasonable, although additional reductions in this overhead are anticipated. The results are demonstrated in the context of a simple combat model, whose only source of non-determinism is communications latency. The simulation is shown to be made repeatable, and the perturbation on the execution compared to the non-repeatable execution small. The paper is one step in bridging the gap between the traditional PDES perspective and real-time simulation world.

Network aware time management and event distribution

Abstract: In this paper we discuss new synchronization algorithms for Parallel and Distributed Discrete Event Simulations (PDES) which exploit the capabilities and behavior of the underlying communications network. Previous work in this areas has assumed the network to be a Black Box which provides a one-to-one, reliable and in-order message passing paradigm. In our work, we utilize the Broadcast capability of the ubiquitous Ethernet for synchronization computations, and both unreliable and reliable protocols for message passing, to achieve more efficient communications between the participating systems.We describe two new algorithms for computation of a distributed snapshot of global reduction operations on monotonically increasing values. The algorithms require O(N) messages (where N is the number of systems participating in the snapshot) in the normal case. We specifically target the use of this algorithm for distributed discrete event simulations to determine a global lower bound on time-stamp (LBTS), but expect the algorithm has applicability outside the simulation community.