Fabian Gomes

SimKit: a high performance logical process simulation class library in C++

SimKit is a C++ class library that is designed for fast discrete event simulation. SimKit presents a simple, elegant logical process view of simulation enabling both sequential and parallel execution without code changes to application models. The sequential executive performs well on a variety of UNIX platforms and facilitates debugging and testing. The parallel executive (WarpKit) is based on the Time Warp paradigm and supports efficient parallel execution on shared memory multiprocessor hardware such as the Silicon Graphics Power Challenge and the Sun SparcServer 1000/2000. This paper describes the design features of the SimKit system. A brief overview of the logical process modeling view commonly used in parallel discrete event simulation (PDES) is presented. The SimKit classes are then introduced followed by a brief tutorial on how to build and simulate object oriented models using SimKit.

State saving for interactive optimistic simulation

Time Warps optimistic scheduling requires the maintenance of simulation state history to support rollback in the event of causality violations. State history, and the ability to rollback the simulation, can provide unique functionality for human-in-the-loop simulation environments. This paper investigates the use of Time Warp to output valid simulation state in a near real-time manner, re-execute portions of the simulation, and interactively probe simulation values to ascertain underlying causes of transient behavior.A shared-memory, multi-threaded interactive simulation architecture is presented and the additional state saving requirements imposed by interactivity are examined. The shortcomings of existing state saving schemes lead us to propose Multiplexed State Saving (MSS). By interleaving checkpointing and incremental state logs MSS provides bounded rollback costs and asynchronous access to prior simulation state. The interaction algorithms and MSS form a scalable, bounded cost component suitable for use in a real-time interactive Time Warp system.

A high fidelity ATM traffic and network simulator

The design of an ATM traffic and network (ATM-TN) simulator which characterizes cell level network behavior is presented. The simulator incorporates three classes of ATM traffic source models: an aggregate ethernet model, an MPEG model and a World Wide Webb transactions model. Six classes of ATM switch architectures are modeled including output buffered, shared memory buffered and cross bar switch models, and then multistage switches which can be built from these three basic models. The ATM-TN simulator can be used to characterize arbitrary ATM networks with dynamic multimedia traffic loads. Call set up and tear down via ATM signaling is implemented in addition to the various types of cell traffic streams generated by voice, video and data. The simulator is built on a simple, efficient simulation language called SimKit which is capable of supporting both fast sequential and parallel execution. Parallel execution is supported using WarpKit, an optimistically synchronized kernel that is aimed at shared memory multiprocessor platforms such as the Silicon Graphics Powerchallenge and Sun Spare 1000 series machines. The paper outlines general requirements for ATM traffic and network simulation, presents an ATM-TN simulator architecture, describes its major components and discusses the major issues associated with cell level ATM modeling and simulation.

Cost of state saving & rollback

Approaches to state saving and rollback for a shared memory, optimistically synchronized, simulation executive are presented. An analysis of copy state saving and incremental state saving is made and these two schemes are compared. Two benchmark programs are then described, one a simple, all overhead, model and one a performance model of a regional Canadian public telephone network. The latter is a large SS7 common channel signalling model that represents a very challenging, practical, test application for parallel simulation. Experimental results are presented which show the necessity and sufficiency of incremental state saving for this application.

A fast asynchronous GVT algorithm for shared memory multiprocessor architectures

The computation of Global Virtual Time is of fundamental importance in Time Warp based Parallel Discrete Event Simulation Systems. Shared memory multiprocessor architectures can support interprocess communication with much smaller overheads than distributed memory systems. This paper presents a new, completely asynchronous, Gvt algorithm which provides very fast and accurate Gvt estimation with significantly lower overhead than previous approaches. The algorithm presented is able to support more efficient memory management, termination, and other global control mechanisms. The Gvt algorithm described enables any Time Warp entity to compute Gvt at any time without slowing down other entities, in particular, those executing on the critical path. Experimental results are presented for a shared memory Time Warp system that employs a two tiered distributed memory management scheme. The proof of the correctness and the accuracy of the algorithm are also presented. Finally, some suggestions on possible further optimization of the implementation are given.

Multiplexed state saving for bounded rollback

Optimistic parallel discrete event simulation (PDES) uses a state history trail to support rollback. State saving strategies range from making a complete copy of a model’s state after each event execution to recording a sequence of each state modification made during event execution. The former is called copy state saving (CSS) and the latter incremental state saving (ISS). Periodic State Saving (PSS) a variant of CSS, saves the entire state but not after every event. This paper presents a scheme for maintaining a multiplexed state history stream that interleaves a PSS mechanism with an optimized ISS mechanism. This muI

Optimizing incremental state-saving and restoration

plexed state saving (MSS) mechanism keeps fgrward execution overhead low while bounding rollback overhead over an arbitrary rollback distance. A key advantage of MSS over PSS is that events are not re-executed during a coasting forward phase. Bounding rollback costs has two significant benefits. First, rollback delay can be controlled reducing rollback cascading and potential rollback thrashing. Second, interactive optimistic simulation with bounded response times to human queries can be supported. Absolute forward execution overheads for CSS and several ISS schemes are presented for two hardware platforms. A rough comparative analysis of MSS versus PSS and ISS is also included.