Causality representation and cancellation mechanism in time warp simulations

  • Authors:

  • Malolan Chetlur;Philip A. Wilsey

  • Affiliations:

  • Experimental Computing Laboratory, Dept. of ECECS, PO Box 210030, Cincinnati, OH;Experimental Computing Laboratory, Dept. of ECECS, PO Box 210030, Cincinnati, OH

  • Venue:
  • Proceedings of the fifteenth workshop on Parallel and distributed simulation
  • Year:
  • 2001

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Abstract

The Time Warp synchronization protocol allows causality errors and then recovers from them with the assistance of a cancellation mechanism. Cancellation can cause the rollback of several other simulation objects that may trigger a cascading rollback situation where the rollback cycles back to the original simulation object. These cycles of rollback can cause the simulation to enter a unstable (or thrashing) state where little real forward simulation progress is achieved. To address this problem, knowledge of causal relations between events can be used during cancellation to avoid cascading rollbacks and to initiate early recovery operations from causality errors. In this paper, we describe a logical time representation for Time Warp simulations that is used to disseminate causality information. The new timestamp representation, called Total Clocks, has two components: (i) a virtual time component, and (ii) a vector of event counters similar to Vector clocks. The virtual time component provides a one dimensional global simulation time, and the vector of event counters records event processing rates by the simulation objects. This time representation allows us to disseminate causality information during event execution that can be used to allow early recovery during cancellation. We propose a cancellation mechanism using Total Clocks that avoids cascading rollbacks in Time Warp simulations that have FIFO communication channels.