The goal of partial-order methods is to accelerate the exploration of concurrent systems by examining only a representative subset of all possible runs. The stateful approach builds a transition system with representative runs, while the stateless method simply enumerates them. The stateless approach may be preferable if the transition system is tree-like; otherwise, the stateful method is more effective. In the last decade, optimality has been a guiding principle for developing stateless partial-order reduction algorithms, and without doubt contributed to big progress in the field. In this paper we ask if we can get a similar principle for the stateful approach. We show that in stateful exploration, a polynomially close to optimal partial-order algorithm cannot exist unless P=NP. The result holds even for acyclic programs with just await instructions. This lower bound result justifies systematic study of heuristics for stateful partial-order reduction. We propose a notion of IFS oracle as a useful abstraction. The oracle can be used to get a very simple optimal stateless algorithm, which can then be adapted to a non-optimal stateful algorithm. While in general the oracle problem is NP-hard, we show a simple case where it can be solved in linear time.
@InProceedings{herbreteau_et_al:LIPIcs.CONCUR.2025.22, author = {Herbreteau, Fr\'{e}d\'{e}ric and Larroze-Jardin\'{e}, Sarah and Walukiewicz, Igor}, title = {{Partial-Order Reduction Is Hard}}, booktitle = {36th International Conference on Concurrency Theory (CONCUR 2025)}, pages = {22:1--22:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-389-8}, ISSN = {1868-8969}, year = {2025}, volume = {348}, editor = {Bouyer, Patricia and van de Pol, Jaco}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2025.22}, URN = {urn:nbn:de:0030-drops-239727}, doi = {10.4230/LIPIcs.CONCUR.2025.22}, annote = {Keywords: Formal verification, Concurrent systems, Partial-order reduction, Complexity} }
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