Sequential and distributed on-the-fly computation of weak tau-confluence

The notion of ττ-confluence provides a form of partial order reduction of Labelled Transition Systems (Ltss), by enabling to identify the ττ-transitions, whose execution does not alter the observable behaviour of the system. Several forms of ττ-confluence adequate with branching bisimulation were studied in the literature, ranging from strong to weak forms according to the length of ττ-transition sequences considered. Weak ττ-confluence is more complex to compute than strong ττ-confluence, but provides better Lts reductions. In this paper, we aim at devising an efficient detection of weak ττ-confluent transitions during an on-the-fly exploration of Ltss. With this purpose, we define and prove new encodings of several weak ττ-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 ττ-confluence that are useful in practice.

► Study of several forms of weak tau-confluence reductions on labelled transition systems.
► Definition and proof of new encodings of these reductions using alternation-free Boolean equation systems.
► Implementation and experiments of these reductions using the CADP verification toolbox.