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When a Simple Concurrent Program Defies Intuition

2025-01-18

A seemingly simple concurrent program, involving two processes incrementing a variable 'n' ten times each, yielded a surprising result when analyzed with a model checker. Intuitively, the final value of 'n' should be between 10 and 20. However, an extreme interleaving of the processes resulted in 'n' being 2. While a Go program attempting to reproduce this behavior failed, highlighting the rarity of such extreme interleavings in practice, the example underscores the complexities and counter-intuitive nature of concurrent programming.

Visualizing Concurrency: A Guide to Understanding Program State Space

2024-12-20

Concurrent programming is notoriously complex due to the difficulty of enumerating all possible states. This article uses visualization to explain how to understand the mechanics of concurrent program execution. It begins by introducing the concept of program state, which is a combination of variable values and instruction location, and then demonstrates the transition process of program states and the generation of state space using a simple C-like program example. The article then introduces concurrent programs, and, using two concurrently executing programs, P and Q, it explains how to represent the state of a concurrent program and the construction of the state space. Finally, the article explores how to use the model checking tool SPIN and the LTL language to verify the correctness of concurrent programs, highlighting the important role of model checking in ensuring the correctness of concurrent programs.