Coordination and Computation in distributed intelligent MEMS

International audienceOver the last decades, research on microelectromechanical systems (MEMS) has focused on the engineering process which has led to major advances. Future challenges will consist in adding embedded intelligence to MEMS systems to obtain distributed intelligent MEMS. One intrinsic characteristic of MEMS is their ability to be mass-produced. This, however, poses scalability problems because a significant number of MEMS can be placed in a small volume. Managing this scalability requires paradigm-shifts both in hardware and software parts. Furthermore, the need for actuated synchronization, programming, communication and mobility management raises new challenges in both control and programming. Finally, MEMS are prone to faulty behaviors as they are mechanical systems and they are issued from a batch fabrication process. A new programming paradigm which can meet these challenges is therefore needed. In this article, we present CO2Dim, which stands for Coordination and Computation in Distributed Intelligent MEMS. CO2DIM is a new programming environment which includes a language based on a joint development of programming and control capabilities, a simulator and real hardware

A Look at Basics of Distributed Computing *

International audienceThis paper presents concepts and basics of distributed computing which are important (at least from the author's point of view), and should be known and mastered by Master students and engineers. Those include: (a) a characterization of distributed computing (which is too much often confused with parallel computing); (b) the notion of a synchronous system and its associated notions of a local algorithm and message adversaries; (c) the notion of an asynchronous shared memory system and its associated notions of universality and progress conditions; and (d) the notion of an asynchronous message-passing system with its associated broadcast and agreement abstractions, its impossibility results, and approaches to circumvent them. Hence, the paper can be seen as a guided tour to key elements that constitute basics of distributed computing

Generalized Universality

State machine replication reduces distributed to centralized computing. Any sequential service, modeled by a state machine, can be replicated over any number of processes and made highly available to all of them. At the heart of this fundamental reduction lies the so called universal consensus abstraction, key to providing the illusion of single shared service, despite replication. Yet, as universal as it may be, consensus is just one specific instance of a more general abstraction, k-set consensus where, instead of agreeing on a unique decision, the processes may diverge but at most k different decisions are reached. It is legitimate to ask whether the celebrated state machine replication construct has its analogue with k > 1. If it did not, one could question the aura of distributed computing deserving an underpinning Theory for 1, the unit of multiplication, would be special in a field, distributed computing, that does not arithmetically multiply. This paper presents, two decades after k-set consensus was introduced, the generalization with k > 1 of state machine replication. We show that with k-set consensus, any number of processes can emulate k state machines of which at least one remains highly available. While doing so, we also generalize the very notion of consensus universality. © 2011 Springer-Verlag

Power and limits of distributed computing shared memory models

Due to the advent of multicore machines, shared memory distributed computing models taking into account asynchrony and process crashes are becoming more and more important. This paper visits some of the models for these systems, and analyses their properties from a computability point of view. Among them, the snapshot model and the iterated model are particularly investigated. The paper visits also several approaches that have been proposed to model crash failures. Among them, the wait-free case where any number of processes can crash is fundamental. The paper also considers models where up to t processes can crash, and where the crashes are not independent. The aim of this survey is to help the reader to better understand recent advances on what is known about the power and limits of distributed computing shared memory models and their underlying mathematics.Ce rapport est une introduction au modèles de calcul asynchrone pour les systèmes à mémoire partagée

Chasing the Weakest Failure Detector for k-Set Agreement in Message-passing Systems

This paper continues our quest for the weakest failure detector which allows the k-set agreement problem to be solved in asynchronous message-passing systems prone to any number of process failures. It has two main contributions which (we hope) will be instrumental to complete this quest. The first contribution is a new failure detector (denoted ∏∑x,y). This failure detector has several noteworthy properties. (a) It is stronger than ∑x which has been shown to be necessary. (b) It is equivalent to the pair (∑, Ω) when x = y = 1 (from which it follows that ∏∑1,1 is optimal to solve consensus). (c) It is equivalent to the pair (∑n−1, Ωn−1) when x = y = n − 1 (from which it follows that ∏∑n−1, n−1) is optimal for (n − 1)-set agreement). (d) It is strictly weaker than the pair (∑x,Ωy) (which has been investigated in previous works) for the pairs (x, y) such that 1 < y < x < n. (e) It is operational: the paper presents a ∏∑x,y-based algorithm that solves k-set agreement for k ⩾ xy. The second contribution of the paper is a proof that, for 1 < k < n − 1, the eventual leaders failure detector k (which eventually provides each process with the same set of k process identities, this set including at least one correct process) is not necessary to solve k-set agreement problem. More precisely, the paper shows that the weakest failure detector for k-set agreement and k cannot be compared

The Universe of Symmetry Breaking Tasks

Processes in a concurrent system need to coordinate using a shared memory or a message-passing subsystem in order to solve agreement tasks such as, for example, consensus or set agreement. However, often coordination is needed to “break the symmetry” of processes that are initially in the same state, for example, to get exclusive access to a shared resource, to get distinct names or to elect a leader. This paper introduces and studies the family of generalized symmetry breaking (GSB) tasks, that includes election, renaming and many other symmetry breaking tasks. Differently from agreement tasks, a GSB task is “inputless”, in the sense that processes do not propose values; the task specifies only the symmetry breaking requirement, independently of the system's initial state (where processes differ only on their identifiers). Among many various characterizing the family of GSB tasks, it is shown that (non adaptive) perfect renaming is universal for all GSB tasks

Distributed Universality: Contention-Awareness, Wait-freedom, Object Progress, and Other Properties

A notion of a universal construction suited to distributed computing has been introduced by M. Herlihy in his celebrated paper "Wait-free synchronization" (ACM TOPLAS, 1991). A universal construction is an algorithm that can be used to wait-free implement any object defined by a sequential specification. Herlihy's paper shows that the basic system model, which supports only atomic read/write registers, has to be enriched with consensus objects to allow the design of universal constructions. The generalized notion of a k-universal construction has been recently introduced by Gafni and Guerraoui (CONCUR, 2011). A k-universal construction is an algorithm that can be used to simultaneously implement k objects (instead of just one object), with the guarantee that at least one of the k constructed objects progresses forever. While Herlihy's universal construction relies on atomic registers and consensus objects, a k-universal construction relies on atomic registers and k-simultaneous consensus objects (which are wait-free equivalent to k-set agreement objects in the read/write system model). This paper significantly extends the universality results introduced by Herlihy and Gafni-Guerraoui. In particular, we present a k-universal construction which satisfies the following five desired properties, which are not satisfied by the previous k-universal construction: (1) among the k objects that are constructed, at least l objects (and not just one) are guaranteed to progress forever; (2) the progress condition for processes is wait-freedom, which means that each correct process executes an infinite number of operations on each object that progresses forever; (3) if any of the k constructed objects stops progressing, all its copies (one at each process) stop in the same state; (4) the proposed construction is contention-aware, in the sense that it uses only read/write registers in the absence of contention; and (5) it is generous with respect to the obstruction-freedom progress condition, which means that each process is able to complete any one of its pending operations on the k objects if all the other processes hold still long enough. The proposed construction, which is based on new design principles, is called a (k, l)-universal construction. It uses a natural extension of k-simultaneous consensus objects, called (k, l)-simultaneous consensus objects ((k, l)-SC). Together with atomic registers, (k, l)-SC objects are shown to be necessary and sufficient for building a (k, l)-universal construction, and, in that sense, (k, l)-SC objects are (k, l)-universal.Cet article explore la notion de construction universelle dans les systèmes distribués. Il présente une construction k-universelle wait-free qui s'adapte à la concurrence à partir d'objets k-consensus