Time Warp's 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.
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 mechanismsThe 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 f o r a shared memory Tame 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.
One of the greatest challenges in making optimistic synchronization techniques such as Time Warp practical tools is making state saving efficient and easy to use, State saving is necessary so that when optimistic execution is found to be out of order, rollback can be
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