Virtual Time II: Storage Management in Distributed Simulation

The main contribution of this paper is the Cancelback Protocol, an extension of the Time Warp mechanism that handles storage management. It includes both fossil collection, the recovery of storage for messages and states that can never again influence the computation, and cancelback, the recovery of storage assigned to messages and states at times so far in the future that their memory would be better used for more immediate purposes. It guarantees that Time Warp is optimal in its storage requirements when run in shared memory, i.e. Time Warp will successfully complete a simulation using no more space than it would take to execute the same simulation with the sequential event list algorithm. This is better by a factor of two than the only previously published result. Without this protocol (or equivalent) Time Warp’s behavior can be unstable; hence it should be considered an essential part of Time Warp mechanism, rather than simply a refinement. In addition we also prove that asynchronous conservative algorithms, including all of the Chandy-Misra-Bryant (CMB) mechanisms, are not optimal; they cannot neces

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