Freeze after writing

Abstract

Deterministic-by-construction parallel programming models offer the advantages of parallel speedup while avoiding the nondeterministic, hard-to-reproduce bugs that plague fully concurrent code. A principled approach to deterministic-by-construction parallel programming with shared state is offered by LVars: shared memory locations whose semantics are defined in terms of an applicationspecific lattice. Writes to an LVar take the least upper bound of the old and new values with respect to the lattice, while reads from an LVar can observe only that its contents have crossed a specified threshold in the lattice. Although it guarantees determinism, this interface is quite limited.We extend LVars in two ways. First, we add the ability to "freeze" and then read the contents of an LVar directly. Second, we add the ability to attach event handlers to an LVar, triggering a callback when the LVar's value changes. Together, handlers and freezing enable an expressive and useful style of parallel programming. We prove that in a language where communication takes place through these extended LVars, programs are at worst quasideterministic: on every run, they either produce the same answer or raise an error. We demonstrate the viability of our approach by implementing a library for Haskell supporting a variety of LVarbased data structures, together with a case study that illustrates the programming model and yields promising parallel speedup.

Quantifying and Explaining Immutability in Scala

“…1 The plugin can be enabled when building Scala projects using the sbt or Maven build tools. The immutability analysis is implemented using Reactive Async [4] which extends LVars [6], lattice-based variables, with cyclic dependency resolution. For each template definition we maintain a "cell" that keeps track of the immutability property of the template.…”

Concurrency-aware object-oriented programming with roles

“…I-structures, or IVars, have recently appeared in other languages, for example, in Haskell [Marlow et al 2011] or, as part of the Concurrent Collections system [Budimlić et al 2010], in C++ and Java. Recently, LVars have been proposed as a generalization of IVars [Kuper and Newton 2013;Kuper et al 2014b]. LVars allow multiple writes to a particular variable, as long as its state is monotonically increasing, with respect to a user-defined lattice.…”