Probabilistic adaptive direct optimism control in Time Warp

Abstract

In a distributed memory environment the communication overhead of Time Warp as induced by the rollback procedure due to “overoptimistic” progression of the simulation is the dominating performance factor. To limit optimism to an extent that can be justified from the inherent model parallelism, an optimism control mechanism is proposed, which by maintaining a history record of virtual time differences from the time stamps carried by arriving messages, and forecasting the timestamps of forthcoming messages, probabilistically delays the execution of scheduled events to avoid potential rollback and associated communication overhead (antimessages). After investigating statistical forecast methods which express only the central tendency of the arrival process, we demonstrate that arrival processes in the context of Time Warp simulations of timed Petri nets have certain predictable and consistent ARIMA characteristics, which encourage the use of sophisticated and recursive forecast procedures based on those models. Adaptiveness is achieved in two respects: the synchronization behavior of logical processes automatically progressing and conservatively blocking, that is the most adequate for (i) the specific simulation model and (ii) the communication/computation speed characteristics of the underlying execution platform.