Sequential and Distributed On-the-Fly Computation of Weak Tau-Confluence

Radu Mateescu and Anton Wijs

Science of Computer Programming 77(10-11):1075-1094, 2012.


The notion of τ-confluence provides a form of partial order reduction of Labelled Transition Systems (Ltss), by enabling to identify the τ-transitions whose execution does not alter the observable behaviour of the system. Several forms of τ-confluence adequate with branching bisimulation were studied in the literature, ranging from strong to weak forms according to the length of τ-transition sequences considered. Weak τ-confluence is more complex to compute than strong τ-confluence, but provides better Lts reductions. In this paper, we aim at devising an efficient detection of weak τ-confluent transitions during an on-the-fly exploration of Ltss. With this purpose, we define and prove new encodings of several weak τ-confluence variants using alternation-free boolean equation systems (Bess), and we apply efficient local Bes resolution algorithms to perform the detection. The resulting reduction module, developed within the Cadp toolbox using the generic Open/Cæsar environment for Lts exploration, was tested in both a sequential and a distributed setting on numerous examples of large Ltss underpinning communication protocols and distributed systems. These experiments assessed the efficiency of the reduction and enabled us to identify the best variants of weak τ-confluence that are useful in practice.

38 pages