arXiv subjects

Ludovic Henrio

Publications and source records attributed to Ludovic Henrio.

15 recordsLinked to original sources

arXiv2023

Provably Fair Cooperative Scheduling

The context of this work is cooperative scheduling, a concurrency paradigm, where task execution is not arbitrarily preempted. Instead, language constructs exist that let a task voluntarily yield the right to execute to another task. The inquiry is the design of provably fair schedulers and suitable notions of fairness for cooperative scheduling languages. To the best of our knowledge, this problem has not been addressed so far. Our approach is to study fairness independently from syntactic constructs or environments, purely from the point of view of the semantics of programming languages, i.e., we consider fairness criteria using the formal definition of a program execution. We develop our concepts for classic structural operational semantics (SOS) as well as for the recent locally abstract, globally concrete (LAGC) semantics. The latter is a highly modular approach to semantics ensuring the separation of concerns between local statement evaluation and scheduling decisions. The new knowledge contributed by our work is threefold: first, we show that a new fairness notion, called quiescent fairness, is needed to characterize fairness adequately in the context of cooperative scheduling; second, we define a provably fair scheduler for cooperative scheduling languages; third, a qualitative comparison between the SOS and LAGC versions yields that the latter, while taking higher initial effort, is more amenable to proving fairness and scales better under language extensions than SOS. The grounding of our work is a detailed formal proof of quiescent fairness for the scheduler defined in LAGC semantics. The importance of our work is that it provides a formal foundation for the implementation of fair schedulers for cooperative scheduling languages, an increasingly popular paradigm (for example: akka/Scala, JavaScript, async Rust). Being based solely on semantics, our ideas are widely applicable. Further, our work makes clear that the standard notion of fairness in concurrent languages needs to be adapted for cooperative scheduling and, more generally, for any language that combines atomic execution sequences with some form of preemption.

cs.PL↗

arXiv2022

Choice Trees: Representing Nondeterministic, Recursive, and Impure Programs in Coq

This paper introduces Choice Trees (ctrees), a monad for modeling nondeterministic, recursive, and impure programs in Coq. Inspired by Xia et al.'s itrees, this novel data structure embeds computations into coinductive trees with three kind of nodes: external events, and two variants of nondeterministic branching. This apparent redundancy allows us to provide shallow embedding of denotational models with internal choice in the style of ccs, while recovering an inductive LTS view of the computation. CTrees inherit a vast collection of bisimulation and refinement tools, with respect to which we establish a rich equational theory. We connect ctrees to the itrees infrastructure by showing how a monad morphism embedding the former into the latter permits to use ctrees to implement nondeterministic effects. We demonstrate the utility of ctrees by using them to model concurrency semantics in two case studies: ccs and cooperative multithreading.

cs.PL↗

arXiv2022

A Survey on Parallelism and Determinism

Parallelism is often required for performance. In these situations an excess of non-determinism is harmful as it means the program can have several different behaviours or even different results. Even in domains such as high-performance computing where parallelism is crucial for performance, the computed value should be deterministic. Unfortunately, non-determinism in programs also allows dynamic scheduling of tasks, reacting to the first task that succeeds, cancelling tasks that cannot lead to a result, etc. Non-determinism is thus both a desired asset or an undesired property depending on the situation. In practice, it is often necessary to limit non-determinism and to identify precisely the sources of non-determinism in order to control what parts of a program are deterministic or not. This survey takes the perspective of programming languages, and studies how programming models can ensure the determinism of parallel programs. This survey studies not only deterministic languages but also programming models that prevent one particularly demanding source of non-determinism: data races. Our objective is to compare existing solutions to the following questions: How programming languages can help programmers write programs that run in a parallel manner without visible non-determinism? What programming paradigms ensure this kind of properties? We study these questions and discuss the merits and limitations of different approaches.

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arXiv2022

LAGC Semantics of Concurrent Programming Languages

Formal, mathematically rigorous programming language semantics are the essential prerequisite for the design of logics and calculi that permit automated reasoning about concurrent programs. We propose a novel modular semantics designed to align smoothly with program logics used in deductive verification and formal specification of concurrent programs. Our semantics separates local evaluation of expressions and statements performed in an abstract, symbolic environment from their composition into global computations, at which point they are concretised. This makes incremental addition of new language concepts possible, without the need to revise the framework. The basis is a generalisation of the notion of a program trace as a sequence of evolving states that we enrich with event descriptors and trailing continuation markers. This allows to postpone scheduling constraints from the level of local evaluation to the global composition stage, where well-formedness predicates over the event structure declaratively characterise a wide range of concurrency models. We also illustrate how a sound program logic and calculus can be defined for this semantics.

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arXiv2021

Compositional equivalences based on Open pNets

Establishing equivalences between programs or systems is crucial both for verifying correctness of programs, by establishing that two implementations are equivalent, and for justifying optimisations and program transformations, by establishing that a modified program is equivalent to the source one. There exist several equivalence relations for programs, and bisimulations are among the most versatile of these equivalences. Among bisimulation relations one distinguishes strong bisimulation, that requires that each action of a program is simulated by a single action of the equivalent program, a weak bisimulation that is a coarser relation, allowing some of the actions to be invisible or internal moves, and thus not simulated by the equivalent program. pNet is a generalisation of automata that model open systems. They feature variables and hierarchical composition. Open pNets are pNets with holes, i.e. placeholders inside the hierarchical structure that can be filled later by sub-systems. This article defines bisimulation relations for the comparison of systems specified as pNets. We first define a strong bisimulation for open pNets. We then define an equivalence relation similar to the classical weak bisimulation, and study its properties. Among these properties we are interested in compositionality: if two systems are proven equivalent they will be undistinguishable by their context, and they will also be undistinguishable when their holes are filled with equivalent systems. We identify sufficient conditions on the automata to ensure compositionality of strong and weak bisimulation. The article is illustrated with a transport protocol running example; it shows the characteristics of our formalism and our bisimulation relations.

cs.LO↗

arXiv2021

An Optimised Flow for Futures: From Theory to Practice

A future is an entity representing the result of an ongoing computation. A synchronisation with a "get" operation blocks the caller until the computation is over, to return the corresponding value. When a computation in charge of fulfilling a future delegates part of its processing to another task, mainstream languages return nested futures, and several "get" operations are needed to retrieve the computed value (we call such futures "control-flow futures"). Several approaches were proposed to tackle this issues: the "forward" construct, that allows the programmer to make delegation explicit and avoid nested futures, and "data-flow explicit futures" which natively collapse nested futures into plain futures. This paper supports the claim that data-flow explicit futures form a powerful set of language primitives, on top of which other approaches can be built. We prove the equivalence, in the context of data-flow explicit futures, between the "forward" construct and classical "return" from functions. The proof relies on a branching bisimulation between a program using "forward" and its "return" counterpart. This result allows language designers to consider "forward" as an optimisation directive rather than as a language primitive. Following the principles of the Godot system, we provide a library implementation of control-flow futures, based on data-flow explicit futures implemented in the compiler. This small library supports the claim that the implementation of classical futures based on data-flow ones is easier than the opposite. Our benchmarks show the viability of the approach from a performance point of view.

cs.PL↗

arXiv2021

On Reachability in Parameterized Phaser Programs

We address the problem of statically checking safety properties (such as assertions or deadlocks) for parameterized phaser programs. Phasers embody a non-trivial and modern synchronization construct used to orchestrate executions of parallel tasks. This generic construct supports dynamic parallelism with runtime registrations and deregistrations of spawned tasks. It generalizes many synchronization patterns such as collective and point-to-point schemes. For instance, phasers can enforce barriers or producer-consumer synchronization patterns among all or subsets of the running tasks. We consider in this work programs that may generate arbitrarily many tasks and phasers. We study different formulations of the verification problem and propose an exact procedure that is guaranteed to terminate for some reachability problems even in the presence of unbounded phases and arbitrarily many spawned tasks. In addition, we prove undecidability results for several problems on which our procedure cannot be guaranteed to terminate.

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arXiv2019

Leveraging access mode declarations in a model for memory consistency in heterogeneous systems

On a system that exposes disjoint memory spaces to the software, a program has to address memory consistency issues and perform data transfers so that it always accesses valid data. Several approaches exist to ensure the consistency of the memory accessed. Here we are interested in the verification of a declarative approach where each component of a computation is annotated with an access mode declaring which part of the memory is read or written by the component. The programming framework uses the component annotations to guarantee the validity of the memory accesses. This is the mechanism used in VectorPU, a C++ library for programming CPU-GPU heterogeneous systems. This article proves the correctness of the software cache-coherence mechanism used in VectorPU. Beyond the scope of VectorPU, this article provides a simple and effective formalisation of memory consistency mechanisms based on the explicit declaration of the effect of each component on each memory space. The formalism we propose also takes into account arrays for which a single validity status is stored for the whole array; additional mechanisms for dealing with overlapping arrays are also studied.

cs.DC↗

arXiv2019

Proceedings 12th Interaction and Concurrency Experience

This volume contains the proceedings of ICE'19, the 12th Interaction and Concurrency Experience, which was held in Copenhagen, Denmark on the 20th and 21st of June 2019, as a satellite event of DisCoTec'19. The ICE workshop series features a distinguishing review and selection procedure, allowing PC members to interact anonymously with authors. As in the past 11 editions, this interaction considerably improved the accuracy of the feedback from the reviewers and the quality of accepted papers, and offered the basis for lively discussion during the workshop. The 2019 edition of ICE included double blind reviewing of original research papers, in order to increase fairness and avoid bias in reviewing. Each paper was reviewed by three PC members, and altogether 9 papers were accepted for publication - plus 2 oral presentations which are not part of this volume. We were proud to host 4 invited talks, by Dilian Gurov, Fritz Henglein, Sophia Knight, and Hernán Melgratti. The abstracts of these talks are included in this volume together with the regular papers. Final versions of the contributions, taking into account the discussion at the workshop, are included.

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arXiv2019

Who is to Blame? Runtime Verification of Distributed Objects with Active Monitors

Since distributed software systems are ubiquitous, their correct functioning is crucially important. Static verification is possible in principle, but requires high expertise and effort which is not feasible in many eco-systems. Runtime verification can serve as a lean alternative, where monitoring mechanisms are automatically generated from property specifications, to check compliance at runtime. This paper contributes a practical solution for powerful and flexible runtime verification of distributed, object-oriented applications, via a combination of the runtime verification tool Larva and the active object framework ProActive. Even if Larva supports in itself only the generation of local, sequential monitors, we empower Larva for distributed monitoring by connecting monitors with active objects, turning them into active, communicating monitors. We discuss how this allows for a variety of monitoring architectures. Further, we show how property specifications, and thereby the generated monitors, provide a model that splits the blame between the local object and its environment. While Larva itself focuses on monitoring of control-oriented properties, we use the Larva front-end StaRVOOrS to also capture data-oriented (pre/post) properties in the distributed monitoring. We demonstrate this approach to distributed runtime verification with a case study, a distributed key/value store.

cs.SE↗

arXiv2017

Proceedings 10th Interaction and Concurrency Experience

This volume contains the proceedings of ICE 2017, the 10th Interaction and Concurrency Experience, which was held in Neuchâtel, Switzerland on the 21st and 22nd of June 2017 as a satellite event of DisCoTec'17. The ICE procedure for paper selection allows PC members to interact, anonymously, with authors. During the review phase, each submitted paper is published on a discussion forum whose access is restricted to the authors and to all the PC members not declaring a conflict of interest. The PC members post comments and questions that the authors reply to. For the first time, the 2017 edition of ICE included a double blind reviewing of original research papers, in order to increase fairness and avoid bias in reviewing. The gender balance of accepted papers was also more even than in past years. Each paper was reviewed by three PC members, and altogether five papers were accepted for publication (the workshop also featured five brief announcements which are not part of this volume). We were proud to host three invited talks, by Christian Cachin, Marieke Huismann, and Paweł Sobocinski. The abstracts of these talks are included in this volume together with the regular papers.

cs.LO↗

arXiv2017

Multiactive objects and their applications

In order to tackle the development of concurrent and distributed systems, the active object programming model provides a high-level abstraction to program concurrent behaviours. There exists already a variety of active object frameworks targeted at a large range of application domains: modelling, verification, efficient execution. However, among these frameworks, very few consider a multi-threaded execution of active objects. Introducing controlled parallelism within active objects enables overcoming some of their limitations. In this paper, we present a complete framework around the multi-active object programming model. We present it through ProActive, the Java library that offers multi-active objects, and through MultiASP, the programming language that allows the formalisation of our developments. We then show how to compile an active object language with cooperative multi-threading into multi-active objects. This paper also presents different use cases and the development support to illustrate the practical usability of our language. Formalisation of our work provides the programmer with guarantees on the behaviour of the multi-active object programming model and of the compiler.

cs.PL↗

arXiv2016

Proceedings 9th Interaction and Concurrency Experience

This volume contains the proceedings of ICE 2016, the 9th Interaction and Concurrency Experience, which was held in Heraklion, Greece on the 8th and 9th of June 2016 as a satellite event of DisCoTec 2016. The ICE procedure for paper selection allows PC members to interact, anonymously, with authors. During the review phase, each submitted paper is published on a discussion forum whose access is restricted to the authors and to all the PC members not declaring a conflict of interest. The PC members post comments and questions that the authors reply to. For the first time, the 2016 edition of ICE included a feature targeting review transparency: reviews of accepted papers were made public on the workshop website and workshop participants in particular were able to access them during the workshop. Each paper was reviewed by three PC members, and altogether nine papers were accepted for publication (the workshop also featured three brief announcements which are not part of this volume). We were proud to host two invited talks, by Alexandra Silva and Uwe Nestmann. The abstracts of these two talks are included in this volume together with the regular papers.

cs.PL↗

arXiv2015

Verifying the correct composition of distributed components: Formalisation and Tool

This article provides formal definitions characterizing well-formed composition of components in order to guarantee their safe deployment and execution. Our work focuses on the structural aspects of component composition; it puts together most of the concepts common to many component models, but never formalized as a whole. Our formalization characterizes correct component architectures made of functional and non-functional aspects, both structured as component assemblies. Interceptor chains can be used for a safe and controlled interaction between the two aspects. Our well-formed components guarantee a set of properties ensuring that the deployed component system has a correct architecture and can run safely. Finally, those definitions constitute the formal basis for our Eclipse-based environment for the development and specification of component-based applications.

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arXiv2010

Behavioural Models for Group Communications

Group communication is becoming a more and more popular infrastructure for efficient distributed applications. It consists in representing locally a group of remote objects as a single object accessed in a single step; communications are then broadcasted to all members. This paper provides models for automatic verification of group-based applications, typically for detecting deadlocks or checking message ordering. We show how to encode group communication, together with different forms of synchronisation for group results. The proposed models are parametric such that, for example, different group sizes or group members could be experimented with the minimum modification of the original model.

cs.DC↗