Commutative Languages and their Composition by Consensual Methods

Commutative languages with the semilinear property (SLIP) can be naturally recognized by real-time NLOG-SPACE multi-counter machines. We show that unions and concatenations of such languages can be similarly recognized, relying on -- and further developing, our recent results on the family of consensually regular (CREG) languages. A CREG language is defined by a regular language on the alphabet that includes the terminal alphabet and its marked copy. New conditions, for ensuring that the union or concatenation of CREG languages is closed, are presented and applied to the commutative SLIP languages. The paper contributes to the knowledge of the CREG family, and introduces novel techniques for language composition, based on arithmetic congruences that act as language signatures. Open problems are listed.Comment: In Proceedings AFL 2014, arXiv:1405.527

Exploiting commutativity to reduce the cost of updates to shared data in cache-coherent systems

We present Coup, a technique to lower the cost of updates to shared data in cache-coherent systems. Coup exploits the insight that many update operations, such as additions and bitwise logical operations, are commutative: they produce the same final result regardless of the order they are performed in. Coup allows multiple private caches to simultaneously hold update-only permission to the same cache line. Caches with update-only permission can locally buffer and coalesce updates to the line, but cannot satisfy read requests. Upon a read request, Coup reduces the partial updates buffered in private caches to produce the final value. Coup integrates seamlessly into existing coherence protocols, requires inexpensive hardware, and does not affect the memory consistency model. We apply Coup to speed up single-word updates to shared data. On a simulated 128-core, 8-socket system, Coup accelerates state-of-the-art implementations of update-heavy algorithms by up to 2.4×.Center for Future Architectures ResearchNational Science Foundation (U.S.) (CAREER-1452994)Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science (Grier Presidential Fellowship)Microelectronics Advanced Research CorporationUnited States. Defense Advanced Research Projects Agenc

Transformational maintenance by reuse of design histories

This thesis provides theory and procedures for modifying software artifacts implemented by a formal transformation process. Installing modifications requires knowing not only what transformations were applied (a derivation history) to construct the artifact, but also why the application sequence ensures that the artifact meets its specification. The derivation history and the justification are collectively called a design history. A Design Maintenance System (DMS), when provided with a formal change called a maintenance delta, revises a design history to guide construction of a new artifact. A DMS can be used to integrate a stream of deltas into a history, providing implementations as a side effect, leading to an incremental-evolution model for software construction.We provide a broadly applicable formal model of transformation systems in which specifications are performance predicates, subsuming the functional specifications which are traditional for transformation systems. Such performance predicates provide vocabulary used in the design history to describe the effect of applying sets of transformations.A nonprocedural, performance-goal-oriented Transformation Control Language (TCL) is defined to control navigation of the design space for a transformation system. Recording the execution of a TCL metaprogram directly provides a design history.A complete classification of, and representation for, the set of possible maintenance deltas is given in terms of the inputs defined by the transformation system model. Such deltas include not only specification changes, but also changes to implementation support technologies. Delta integration procedures for revising derivation histories given functional or support technology deltas are provided, based on rearranging the order of transformations in the design space. Building on these operations, integration procedures that revise the design history for each type of delta are described. An agenda-oriented TCL execution process dovetails smoothly with the integration procedures.Our DMS is compared to a number of other maintenance systems. By using an explicit delta and verified commutativity, our DMS often reuses transformations correctly when others fail

LOGIC TEACHING IN THE 21ST CENTURY

We are much better equipped to let the facts reveal themselves to us instead of blinding ourselves to them or stubbornly trying to force them into preconceived molds. We no longer embarrass ourselves in front of our students, for example, by insisting that “Some Xs are Y” means the same as “Some X is Y”, and lamely adding “for purposes of logic” whenever there is pushback. Logic teaching in this century can exploit the new spirit of objectivity, humility, clarity, observationalism, contextualism, and pluralism. Besides the new spirit there have been quiet developments in logic and its history and philosophy that could radically improve logic teaching. This lecture expands points which apply equally well in first, second, and third courses, i.e. in “critical thinking”, “deductive logic”, and “symbolic logic”

Decomposition and Descriptional Complexity of Shuffle on Words and Finite Languages

We investigate various questions related to the shuffle operation on words and finite languages. First we investigate a special variant of the shuffle decomposition problem for regular languages, namely, when the given regular language is the shuffle of finite languages. The shuffle decomposition into finite languages is, in general not unique. Thatis,therearelanguagesL^,L2,L3,L4withLiluL2= £3luT4but{L\,L2}^ {I/3, L4}. However, if all four languages are singletons (with at least two combined letters), it follows by a result of Berstel and Boasson [6], that the solution is unique; that is {L\,L2} = {L3,L4}. We extend this result to show that if L\ and L2 are arbitrary finite sets and Lz and Z-4 are singletons (with at least two letters in each), the solution is unique. This is as strong as it can be, since we provide examples showing that the solution can be non-unique already when (1) both L\ and L2 are singleton sets over different unary alphabets; or (2) L\ contains two words and L2 is singleton. We furthermore investigate the size of shuffle automata for words. It was shown by Campeanu, K. Salomaa and Yu in [11] that the minimal shuffle automaton of two regular languages requires 2mn states in the worst case (where the minimal automata of the two component languages had m and n states, respectively). It was also recently shown that there exist words u and v such that the minimal shuffle iii DFA for u and v requires an exponential number of states. We study the size of shuffle DFAs for restricted cases of words, namely when the words u and v are both periods of a common underlying word. We show that, when the underlying word obeys certain conditions, then the size of the minimal shuffle DFA for u and v is at most quadratic. Moreover we provide an efficient algorithm, which decides for a given DFA A and two words u and v, whether u lu u C L(A)

Verifying and Enforcing Application Constraints in Antidote SQL

Geo-replicated storage systems are currently a fundamental piece in the development of large-scale applications where users are distributed across the world. To meet the high requirements regarding la- tency and availability of these applications, these database systems are forced to use weak consistency mechanisms. However, under these consistency models, there is no guarantee that the invariants are preserved, which can jeopardise the correctness of applications. The most obvious alternative to solve this problem would be to use strong consistency, but this would place a large burden on the system. Since neither of these options was feasible, many systems have been developed to preserve the invariants of the applications without sacrificing low latency and high availability. These systems, based on the analysis of operations, make it possible to increase the guarantees of weak consistency by introducing consistency at the level of operations that are potentially dangerous to the invariant. Antidote SQL is a database system that, by combining strong with weak consistency mechanisms, attempts to guarantee the preservation of invariants at the data level. In this way, and after defining the concurrency semantics for the application, any operation can be performed without coordination and without the risk of violating the invariant. However, this approach has some limitations, namely the fact that it is not trivial for developers to define appropriate concurrency semantics. In this document, we propose a methodology for the verification and validation of defined prop- erties, such as invariants, for applications using Antidote SQL. The proposed methodology uses a high-level programming language with automatic verification features called VeriFx and provides guidelines for programmers who wish to implement and verify their own systems and specifications using this tool.Os sistemas de armazenamento geo-replicados são atualmente uma peça fundamental no desenvolvi- mento de aplicações de grande escala em que os utilizadores se encontram espalhados pelo mundo. Com o objetivo de satisfazer os elevados requisitos em relação à latência e à disponibilidade destas aplicações, estes sistemas de bases de dados vêem-se obrigados a recorrer a mecanismos de consistên- cia fracos. No entanto, sob estes modelos de consistência não existe qualquer tipo de garantia de que os invariantes são preservados, o que pode colocar em causa a correção das aplicações. A alternativa mais óbvia para resolver este problema passaria por utilizar consistência forte, no entanto esta incutiria uma grande sobrecarga no sistema. Sendo que nenhuma destas opções é viável, muitos sistemas foram desenvolvidos no sentido de preservar os invariantes das aplicações, sem contudo, abdicar de baixas latências e alta disponibilidade. Estes sistemas, baseados na análise das operações, permitem aumentar as garantias de consistência fraca com a introdução de consistência ao nível das operações potencialmente perigosas para o invari- ante. O Antidote SQL é um sistema de base de dados que através da combinação de mecanismos de consistência fortes com mecanismos de consistência fracos tenta garantir a preservação dos invariantes ao nível dos dados. Desta forma, e depois de definidas as semânticas de concorrência para a aplicação, qualquer operação pode ser executada sem coordenação e sem perigo de quebra do invariante. No entanto esta abordagem apresenta algumas limitações nomeadamente o facto de não ser trivial para os programadores definirem as semânticas de concorrência adequadas. Neste documento propomos uma metodologia para a verificação e validação de propriedades defi- nidas, como os invariantes, para aplicações que usam o Antidote SQL. A metodologia proposta utiliza uma linguagem de programação de alto nível com capacidade de verificação automática designada por VeriFx, e fornece as diretrizes a seguir para que o programador consiga implementar e verificar os seus próprios sistemas e especificações, utilizando a ferramenta