The Computational Structure of Progress Conditions

Abstract. Understanding the effect of different progress conditions on the com-putability of distributed systems is an important and exciting research direction. For a system with n processes, we define exponentially many new progress con-ditions and explore their properties and strength. We cover all the known, sym-metric and asymmetric, progress conditions and many new interesting conditions. Together with our technical results, the new definitions provide a deeper under-standing of synchronization and concurrency

On the Computational Power of Shared Objects

Abstract. We propose a new classification for evaluating the strength of shared objects. The classification is based on finding, for each object of type o, the strongest progress condition for which it is possible to solve consensus for any number of processes, using any number of objects of type o and atomic registers. We use the strongest progress condition to associate with each object a number call the power number of that object. Objects with higher power numbers are considered stronger. Then, we define the power hierarchy which is an infinite hi-erarchy of objects such that the objects at level i of the hierarchy are exactly those objects with power number i. Comparing our classification with the traditional one which is based on fixing the progress condition (namely, wait-freedom) and finding the largest number of processes for which consensus is solvable, reveals interesting results. Our equivalence and extended universality results, provide a deeper understanding of the nature of the relative computational power of shared objects

The Role of Subtitles in Language Teaching

Da più di venti anni i sottotitoli sono stati proposti come un potenziale aiuto per studenti e insegnanti di lingue. La ricerca, anche se un po’ a macchia di leopardo, suggerisce che la sottotitolazione possa aumentare il lessico, aiutare l’acquisizione di termini nuovi e facilitare la comprensione. Anche la produzione propria di sottotitoli dalla parte degli studenti è stata promossa come lavoro di gruppo in sintonia con le idee di approcci comunicativi e task-based learning. Il presente articolo tenta di mettere in questione la prassi attuale dell’utilizzo dei sottotitoli nell’insegnamento delle lingue, sottolineando l’importanza di sviluppare una base teorica e metodologica chiara per l’uso di questa risorsa, potenzialmente significativa per scoprire una lingua straniera in tutti i suoi crismi, e per trovare strategie efficaci dell’apprendimento. Sottotitoli sia intralinguistici che interlinguistici sono presi in considerazione. La possibilità di promuovere la consapevolezza linguistica attraverso l’analisi dei brevi testi presenti sullo schermo e l’uso di videoclip nelle classe viene esaminata. Riconoscere la complessità dell’uso simultaneo dei canali aurali, visuali e testuali ci porta alla proposta di sfruttare i sottotitoli non solo come un esercizio o una preparazione vocazionale, ma anche come un mezzo per aiutare gli studenti a capire meglio la comunicazione multi-modale e raffinare le loro capacità di pensiero critico. Key words: subtitles; translation; task-based learning; narrative (sottotitoli; traduzione; task-based learning; narrazione

On-line adaptive parallel prefix computation

International audienceWe consider parallel prefix computation on processors of different and possibly changing speeds. Extending previous works on identical processors, we provide a lower bound for this problem. We introduce a new adaptive algorithm which is based on the on-line recursive coupling of an optimal sequential algorithm and a parallel one, non-optimal but recursive and fine-grain. The coupling relies on a work-stealing scheduling. Its theoretical performance is analysed on p processors of different and changing speeds. It is close to the lower bound both on identical processors and close to the lower bound for processors of changing speeds. Experiments performed on an eight-processor machine confirms this theoretical result

The Weakest Failure Detectors to Boost Obstruction-Freedom

This paper determines necessary and sufficient conditions to implement wait-free and non-blocking contention managers in a shared memory system. The necessary conditions hold even when universal objects (like compare-and-swap) or random oracles are available, whereas the sufficient ones assume only registers. We show that failure detector P is the weakest to convert any obstruction-free algorithm into a wait-free one, and Omega*, a new failure detector which we introduce in this paper, and which is strictly weaker than P but strictly stronger than Omega, is the weakest to convert any obstruction-free algorithm into a non-blocking one