Fair cooperative multithreading: typing termination in a higher-order concurrent imperative language

• Authors: Gérard Boudol
• Venue: CONCUR'07 Proceedings of the 18th international conference on Concurrency Theory
• Year: 2007

We propose a new operational model for shared variable concurrency, in the context of a concurrent, higher-order imperative language à la ML. In our model the scheduling of threads is cooperative, and a non-terminating process suspends itself on each recursive call. A property to ensure in such a model is fairness, that is, any thread should yield the scheduler after some finite computation. To this end, we follow and adapt the classical method for proving termination in typed formalisms, namely the realizability technique. There is a specific difficulty with higher-order state, which is that one cannot define a realizability interpretation simply by induction on types, because applying a function may have side-effects at types not smaller than the type of the function. Moreover, such higher-order side-effects may give rise to computations that diverge without resorting to explicit recursion. We overcome these difficulties by introducing a type and effect system for our language that enforces a stratification of the memory. The stratification prevents the circularities in the memory that may cause divergence, and allows us to define a realizability interpretation of the types and effects, which we then use to prove the intended termination property.