Simulations that run at multiple levels of resolution often encounter consistency problems because of insufficient correlation between the attributes at multiple levels of the same entity. Inconsistency may occur despite the existence of valid models at each resolution level. Cross-Resolution Modeling (CRM) attempts to build effective multiresolution simulations. The traditional approach to CRM-aggregation-disaggregation-causes chain disaggregation and puts an unacceptable burden on resources. We present four fundamental observations that would help guide future approaches to CRM. These observations form the basis of an approach we propose that involves the design of Multiple Resolution Entities (MREs). MREs are the foundation of a design that incorporates maintaining internal consistency. We also propose maintenance of core attributes as an approach to maintaining internal consistency within an MRE.

We introduce a new class of synchronization protocols for parallel discrete event simulation, those based on near-perfect state information (NPSI). NPSI protocols are adaptive, dynamically controlling the rate at which processes constituting a parallel simulation proceed, with the goal of completing a simulation efficiently. We show by analysis that a class of adaptive protocols (that includes NPSI and several others) can both arbitrarily outperform and be arbitrarily outperformed by the Time Warp synchronization protocol. This mixed result both substantiates the promising results we and other adaptive protocol designers have observed, and cautions those who might assume that any adaptive protocol will always be better than any nonadaptive one. We establish in an experimental study that a particular NPSI protocol, the Elastic Time Algorithm, outperforms Time Warp, both temporally and spatially, on every workload tested. Although significant options remain with respect to the design of ETA, the work presented here establishes the class of NPSI protocols as a very promising approach.

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GVT computation has been a significant problem in Time Warp implementations since it affects the completion time of the simulation. We present a new method for computing GVT using inexpensive hardware designed to support synchronization in parallel computations. The main contribution of this paper is this new GVT algorithm. Unlike previous methods, the computation of GVT using our method does not interfere with the simulation. For the LP's, the cost of obtaining GVT is reduced to insignificance. Preliminary studies indicate that our method provides very accurate GVT.

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