Reconciling Distributed and Shared Hash Tables Approaches for Parallel State Space Construction
Résumé
We propose an algorithm for parallel state space construction based on an original concurrent data structure, called a localization table, that aims at better space and temporal balance. Our proposal is close in spirit to algorithms based on distributed hash tables, with the distinction that states are dynamically assigned to processors; i.e. we do not rely on an a-priori static partition of the state space. In our solution, every process keeps a share of the global state space. Data distribution and coordination between processes is made through the localization table, that is a lockless, thread-safe data structure that approximates the set of states being processed. The localization table is used to dynamically assign newly discovered states and can be queried to return the identity of the processor that own a given state. With this approach, we are able to consolidate a network of local hash tables into an (abstract) distributed one without sacrificing memory affinity - data that are "logically connected" and physically close to each others - and without incurring performance costs associated to the use of locks to ensure data consistency. We evaluate the performance of our algorithm on different benchmarks and compare these results with other solutions proposed in the literature and with existing verification tools. At a more general level, the usefulness of localization tables goes beyond the domain of formal verification, since it provides an efficient substitute for concurrent hash maps. For instance, our experiments show that our solution performed very well against an industrial strength, lockless hash table (taken from the Intel-TBB).
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