Wed 19 Jan 2022 16:40 - 17:05 at Salon I - Algorithmic Verification 2 Chair(s): Qirun Zhang

Thread pooling is a common programming idiom in which a fixed set of worker threads are maintained to execute tasks concurrently. The workers pick tasks and execute them; executing a task can lead to more new tasks. Each task is sequential, possibly recursive, code and tasks communicate over shared memory. We consider the safety verification problem for thread-pooled programs. We parameterize the problem with two parameters: the size of the thread pool as well as the number of context switches for each task. The size of the thread pool determines the number of workers running concurrently. The number of context switches determines how many times a worker can be swapped out while executing a single task—like many verification problems for multithreaded recursive programs, the context bounding is important for decidability.

We show that the safety verification problem for thread-pooled, context-bounded programs is EXPSPACE-complete, even if the size of the thread pool and the context bound are given in binary. Our main result, the EXPSPACE upper bound, is derived using a sequence of new succinct encoding techniques of independent language-theoretic interest. In particular, we show a polynomial-time construction of downward closures of languages accepted by succinct pushdown automata as doubly succinct nondeterministic finite automata. Since there are explicit doubly exponential lower bounds on the size of nondeterministic finite automata accepting the downward closure, our result shows these automata can be compressed. We show that thread pooling significantly reduces computational power: in contrast, if only the context bound is provided in binary, but there is no thread pooling, the safety verification problem becomes 3EXPSPACE-complete. Given the high complexity lower bounds of related problems involving binary parameters, the relatively low complexity of safety verification with thread-pooling comes as a surprise.

Wed 19 Jan

Displayed time zone: Eastern Time (US & Canada) change

16:40 - 17:30
Algorithmic Verification 2POPL at Salon I
Chair(s): Qirun Zhang Georgia Institute of Technology
Research paper
Context-Bounded Verification of Thread PoolsRemote
DOI Media Attached
Research paper
What’s Decidable about Linear Loops?InPerson
Toghrul Karimov MPI-SWS, Engel Lefaucheux MPI-SWS, Joël Ouaknine MPI-SWS, David Purser MPI-SWS, Anton Varonka MPI-SWS, Markus A. Whiteland MPI-SWS, James Worrell University of Oxford
DOI Pre-print Media Attached