On Randomization versus Synchronization

We expose limitations of randomization in asynchronous distributed systems by presenting two new impossibility results for paradigmatic problems. The first result states that mutual exclusion cannot be solved starvation free with probability 1 in the absence of (strongly) fair synchronization. That is, for example, an agent waiting to access a shared variable eventually accesses the variable. The second result states that a crash-tolerant version of the dining philosophers problem cannot be solved starvation free with probability 1 even if fair synchronization is assumed. In both cases, randomization does not (sufficiently) increase the ability of the underlying model to synchronize independent objects

Real-time cheat-free gaming on the basis of time-stamp service

A cheat-proof protocol for real-time gaming is proposed under the assumption that time-stamp servers issue serially numbered time stamps honestly and are available near every player, i.e., they exist everywhere in the Internet. With this protocol, each player sends its action to the other player and also sends its hash to the nearest time-stamp server. The time-stamp server sends the signed hash with the time and a serial number back to the player. The actions are checked to verify that they are compatible with the hashes, and the signed hashes are checked to verify that they have the correct time and the serial numbers are contiguous. The only latency in this protocol is the travel time of the packet from one player to another. In comparison with other existing protocols, we confirm that the proposed protocol is as fast as and more secure than the fair synchronization protocol, the fastest existing protoco

Injection of a Body into a Geodesic: Lessons Learnt from the LISA Pathfinder Case

Launch lock and release mechanisms constitute a common space business, however, some science missions due to very challenging functional and performance requirements need the development and testing of dedicated systems. In the LISA Pathfinder mission, a gold-coated 2-kg test mass must be injected into a nearly pure geodesic trajectory with a minimal residual velocity with respect to the spacecraft. This task is performed by the Grabbing Positioning and Release Mechanism, which has been tested on-ground to provide the required qualification. In this paper, we describe the test method that analyzes the main contributions to the mechanism performance and focuses on the critical parameters affecting the residual test mass velocity at the injection into the geodesic trajectory. The test results are also presented and discussed

Laws for Communicating Parallel Processes

Key Words and Phrases: parallel processes, parallel or asynchronous computations, partial orders of events, Actor theory. CR Categories: 5.21, 5.24, 5.26. This report describes research done at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Support for the laboratory's artificial intelligence research is provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-75-C-0522.This paper presents some laws that must be satisfied by computations involving communicating parallel processes. The laws are stated in the context of the actor theory, a model for distributed parallel computation, and take the form of stating plausible restrictions on the histories of parallel computations to make them physically realizable. The laws are justified by appeal to physical intuition and are to be regarded as falsifiable assertions about the kinds of computations that occur in nature rather than as proven theorems in mathematics. The laws are used to analyze the mechanisms by which multiple processes can communicate to work effectively together to solve difficult problems. Since the causal relations among the events in a parallel computation do not specify a total order on events, the actor model generalizes the notion of computation from a sequence of states to a partial order of events. The interpretation of unordered events in this partial order is that they proceed concurrently. The utility of partial orders is demonstrated by using them to express our laws for distributed computation.MIT Artificial Intelligence Laboratory Department of Defense Advanced Research Projects Agenc

Recognition of Customary Court: Perspective of Judicial Power and Special Autonomy of Papua

The excellence of customary court for indigenous peoples of Papua as a peace justice institution which is one of the specific rights of Special Autonomy of Papua and it regulated in Article 50 paragraph (2) juncto Article 51 paragraph (1), and Article 43 paragraph (1) of Act No. 21 of 2001 in the field of executive. The recognition of customary court of Papua referred to as “traditional rights” in accordance with Article 18B paragraph (2) juncto Article 24 paragraph (3) and Article 28I paragraph (3) of the 1945 Constitution, its relevance to Article 35 paragraph (3) letter d and Article 58 No. 48 of 2009 and in Article 1 No. (5) of Act No. 49 of 2009 is not synchronized for indigenous peoples of Papuan that perform the function of customary court in the Judicial Power system in the field of judicative informally. The object of this study is related to the primacy of the recognition of customary justice: perspective of judicial power and special autonomy of Papua by using normative juridical method. The results indicate the weakness of the recognition of customary court of Papua against: 1) the institutional of customary court, 2) authority and 3) the decision of customary court over the case or the customary dispute and the principle of ne bis in idem in the function of Judicial Power. Its implementation raises the conflict of norms on the Acts of Judicial Power and the Special Autonomy of Papua. For future, the customary court of Papua needs to be a synchronization of the legal basis of the relationship of authority recognition in the Act of Judicial Power and the Special Autonomy of Papua, in order to fulfill a sense of legal certainty and justice for indigenous people of Papua as multicultural and customary law as the living law

Efficient and scalable starvation prevention mechanism for token coherence

[EN] Token Coherence is a cache coherence protocol that simultaneously captures the best attributes of the traditional approximations to coherence: direct communication between processors (like snooping-based protocols) and no reliance on bus-like interconnects (like directory-based protocols). This is possible thanks to a class of unordered requests that usually succeed in resolving the cache misses. The problem of the unordered requests is that they can cause protocol races, which prevent some misses from being resolved. To eliminate races and ensure the completion of the unresolved misses, Token Coherence uses a starvation prevention mechanism named persistent requests. This mechanism is extremely inefficient and, besides, it endangers the scalability of Token Coherence since it requires storage structures (at each node) whose size grows proportionally to the system size. While multiprocessors continue including an increasingly number of nodes, both the performance and scalability of cache coherence protocols will continue to be key aspects. In this work, we propose an alternative starvation prevention mechanism, named priority requests, that outperforms the persistent request one. This mechanism is able to reduce the application runtime more than 20 percent (on average) in a 64-processor system. Furthermore, thanks to the flexibility shown by priority requests, it is possible to drastically minimize its storage requirements, thereby improving the whole scalability of Token Coherence. Although this is achieved at the expense of a slight performance degradation, priority requests still outperform persistent requests significantly.This work was partially supported by the Spanish MEC and MICINN, as well as European Commission FEDER funds, under Grants CSD2006-00046 and TIN2009-14475-C04-01. Antonio Robles is taking a sabbatical granted by the Universidad Politecnica de Valencia for updating his teaching and research activities.Cuesta Sáez, BA.; Robles Martínez, A.; Duato Marín, JF. (2011). Efficient and scalable starvation prevention mechanism for token coherence. IEEE Transactions on Parallel and Distributed Systems. 22(10):1610-1623. doi:10.1109/TPDS.2011.30S16101623221

O ensino de História no Brasil: contextualização e abordagem historiográfica

Th is article aims to outline a structural overview of the teaching of history in Brazil. Th us, the chronological period is from 1837 – the date of the introduction of the teaching of history in highs school through the establishment of Colégio D. Pedro II – until the fi rst years of the 21st century – the height of the institution of the neoliberal political program in the teaching of history. Th e text contextualizes the practice of history teaching in Brazil. Th erefore, it correlates this practice with the various social contexts that underlie the  chronological period covered. It also correlates it with the historiographical approaches whose influences can be seen in the orientation of educational policies concerning the teaching of history in Brazil. Th e article claims that there is a fair synchronization between historiographic trends and the practice of history teaching due to the mediation of the social contexts in which both spheres are inserted.Key words: practice of history teaching, social context, historiographical approach.O presente artigo objetiva esboçar um panorama estrutural acerca do ensino de História no Brasil.  Nesse sentido, o recorte cronológico engloba o período de 1837 – data da introdução do ensino de História na escola secundária mediante a criação do Colégio D. Pedro II – até os primeiros anos do século XXI – auge da instituição do programa político neoliberal no ensino de História. O mote  geral do texto consiste em contextualizar a prática de ensino de História no Brasil. Portanto, tal prática  será correlacionada com os diversos contextos  sociais que perpassam o recorte cronológico abordado e com as abordagens historiográficas cujas influências podem ser observadas na orientação das políticas educacionais concernentes ao ensino de História no Brasil. Em conclusão, o texto defende a existência de uma razoável sincronia entre as tendências historiográficas e a prática de ensino de História em função da mediação dos contextos sociais nos quais ambas as esferas estão inseridas.Palavras-chave: prática de ensino de História, contexto social, abordagem historiográfica

Control contributions to the universal operation of wind turbines

Ante la creciente dependencia energética de los países de la Unión Europea y los informes de contaminación atmosférica, la generación distribuida mediante energías renovables está modificando el sistema eléctrico actualmente basado en el paradigma centralizado. Dentro de las energías renovables con mayor impacto actual se encuentra la energía eólica. Un aspecto importante a mejorar en el marco de la calidad de potencia por parte del operador de la red de transmisión, es la continuidad del suministro. En estas circunstancias se define el concepto de microgrid como un sistema compuesto de al menos una fuente de generación distribuida asociada a cargas locales que pueden intencionalmente desconectarse del sistema de distribución con el objetivo de mejorar la fiabilidad del suministro. Este trabajo introduce la Operación Universal de aerogeneradores, donde éstos pueden trabajar conectados a red eléctrica y desconectarse de ella cuando ocurre un hueco o interrupción del suministro operando en modo isla. Es una aplicación específica del concepto de microgrid a aerogeneradores que evita el uso de sistemas de almacenamiento empleando únicamente las capacidades de almacenamiento y disipación intrínsecas de los aerogeneradores y se centra en contrarrestar interrupciones del suministro eléctrico del orden de unidades de minutos. Este trabajo se centra en abordar la problemática asociada a la Operación Universal de aerogeneradores desde el punto de vista del control de los convertidores de potencia: regulación del balance energético, compartición de la carga y control de la tensión local en modo isla y transiciones suaves entre modos de operación. Además, el sistema debe seguir manteniendo un rendimiento óptimo en modo conectado a red respetando los códigos de red: respuesta en potencia, calidad de potencia y respuesta ante perturbaciones

Design Issues for Peer-to-Peer Massively Multiplayer Online Games.

Massively Multiplayer Online Games (MMOGs) are increasing in both popularity and scale, and while classical Client/Server (C/S) architectures convey some benefits, they suffer from significant technical and commercial drawbacks. This realisation has sparked intensive research interest in adapting MMOGs to Peer-to-Peer (P2P) architectures. This paper articulates a comprehensive set of six design issues to be addressed by P2P MMOGs, namely Interest Management (IM), game event dissemination, Non-Player Character (NPC) host allocation, game state persistency, cheating mitigation and incentive mechanisms. Design alternatives for each issue are systematically compared, and their interrelationships discussed. We further evaluate how well representative P2P MMOG architectures fulfil the design criteria