Estimating rollback overhead for optimism control in Time Warp

The main performance pitfall of the Time Warp distributed discrete event simulation (DDES) protocol has been widely recognized to be the overoptimistic progression of event execution into the simulated future. The premature execution of events that eventually have to be "rolled back" due to causality violations induces memory and communication overheads as sources of performance inefficiencies. Optimistic Time Windows and self adaptive mechanisms have been proposed in the literature to control the optimism in Time Warp in order to improve or optimize its execution performance. An adaptive optimism control mechanism based on the observed model parallelism is proposed. Methodologically, logical processes (LPs) monitor the local virtual time (LVT) progression per unit CPU time from the timestamp of arriving messages and establish a cost model for the tradeoff between optimistically progressing and conservatively blocking the simulation engine. Compared to previous approaches, an optimal CPU delay interval is computed from the rollback probability and the overhead induced by the rollback procedure, such that the LP can adapt the synchronization behavior to the amount of optimism that can be justified from the parallelism inherent in the simulation model. Experiments with an implementation on a distributed memory multiprocessor (iPSC/860) show that the protocol is able to automatically adjust the local virtual time progression such that rollback overhead is minimized, and that the original Time Warp protocol can be outperformed.<
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