Formal verification and empirical analysis of rollback relaxation

Abstract

Time Warp is the most common mechanism used for implementing optimistically synchronized Parallel Discrete Event Simulation (PDES). Rollback relaxation is an optimization to Time Warp that reduces the space and time requirements of rollback. Rollback relaxation is applicable to simulation systems that contain memoryless components (i.e., components whose output at any instant of time is determined completely by its inputs at that time). For such components, a complete rollback is not necessary for the correct completion of simulation. Instead, on the receipt of a straggler message, a rollback relaxed processes merely aligns the input set to send new, and validate already sent, output messages. This optimization has been implemented and has experimentally proven to enhance the performance of Time Warp simulations. However, no formal proof of the correctness of rollback relaxation exists (although correctness proofs of Time Warp do). In this paper, we formally specify and verify the correctness of rollback relaxation. The problem is specified using the Prototype Verification System (PVS) Specification Language and proved using the PVS Prover.