Efficient data structures for Time Warp simulation queues

Abstract

Abstract Optimistic simulation is besides the conservative methods the most promising approach to distributed discrete-event simulation (DDEVS). In the optimistic Time Warp strategy, every sub-simulator has to handle in addition to its main event queue (input queue) two other queues, holding information about sent messages and past states, respectively. We present efficient data structures and algorithms for the three queue types in Time Warp simulation. It shows that for two queue types the simple doubly linked list (DLL) approach suffices, for the input queue however, significant speed-up can be achieved by enhancing a data structure originally developed for sequential simulation. We show an improvement from an O(n) to an O(1) average case behavior for some operations by moving from the DLL to the new data structure without increasing the average case and worst case order of the other operations. Empirical results show that the new data structure yields substantial faster operation for enqueuing items into the list, which — regardless of the overhead introduced by the new data structure to the other queue operations — results in queue access acceleration for typical Time Warp simulations even for medium input queue sizes of about 100. For large queue sizes of about 10000 the new data structure yields more than 15 times faster overall queue operation performance.