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Convergent types for shared memory
Get PDFDissertação de mestrado em Computer ScienceIt is well-known that consistency in shared memory concurrent programming comes with
the price of degrading performance and scalability. Some of the existing solutions to this
problem end up with high-level complexity and are not programmer friendly.
We present a simple and well-defined approach to obtain relevant results for shared memory
environments through relaxing synchronization. For that, we will look into Mergeable
Data Types, data structures analogous to Conflict-Free Replicated Data Types but designed to
perform in shared memory.
CRDTs were the first formal approach engaging a solid theoretical study about eventual
consistency on distributed systems, answering the CAP Theorem problem and providing
high-availability. With CRDTs, updates are unsynchronized, and replicas eventually converge
to a correct common state. However, CRDTs are not designed to perform in shared
memory. In large-scale distributed systems the merge cost is negligible when compared to
network mediated synchronization. Therefore, we have migrated the concept by developing
the already existent Mergeable Data Types through formally defining a programming
model that we named Global-Local View. Furthermore, we have created a portfolio of MDTs
and demonstrated that in the appropriated scenarios we can largely benefit from the model.É bem sabido que para garantir coerência em programas concorrentes num ambiente de
memória partilhada sacrifica-se performance e escalabilidade. Alguns dos métodos existentes
para garantirem resultados significativos introduzem uma elevada complexidade e
não são práticos.
O nosso objetivo é o de garantir uma abordagem simples e bem definida de alcançar
resultados notáveis em ambientes de memória partilhada, quando comparados com os
métodos existentes, relaxando a coerência. Para tal, vamos analisar o conceito de Mergeable
Data Type, estruturas análogas aos Conflict-Free Replicated Data Types mas concebidas para
memória partilhada.
CRDTs foram a primeira abordagem a desenvolver um estudo formal sobre eventual consistency,
respondendo ao problema descrito no CAP Theorem e garantindo elevada disponibilidade.
Com CRDTs os updates não são síncronos e as réplicas convergem eventualmente
para um estado correto e comum. No entanto, não foram concebidos para atuar
em memória partilhada. Em sistemas distribuídos de larga escala o custo da operação
de merge é negligenciável quando comparado com a sincronização global. Portanto, migramos
o conceito desenvolvendo os já existentes Mergeable Data Type através da criação
de uma formalização de um modelo de programação ao qual chamamos de Global-Local
View. Além do mais, criamos um portfolio de MDTs e demonstramos que nos cenários
apropriados podemos beneficiar largamente do modelo
Konsistenz, Isolation und Unwiderruflichkeit in Software Transactional Memory
No full textSoftware transactional memory (STM) is a promising paradigm for the development of concurrent software. It coordinates the potentially conflicting effects of concurrent threads on shared memory by running their critical regions isolated from each other in transactions in an “all-or-nothing” manner. When encountering a conflicting access to shared memory, a conflict resolution strategy decides which transaction has to revert its changes and is restarted.
However, this automatic coordination is sometimes too restrictive: non-revertible operations such as I/O operations are disallowed in a transaction and some transactions fail for minor conflicts that could easily be resolved. In addition, most STM schemes focus on shared-memory architectures where costly memory updates impede scalability.
This thesis tackles these limitations by exploring extensions to the standard STM schemes. It discusses two novel STM algorithms which broaden the scope for applicability of STM and provide insights into the strength and limitations of transactional programming.
Twilight STM proposes to extend STM with non-revertible actions and inconsis- tency repair. It separates a transaction into a functional transactional phase, and an imperative irrevocable phase, which supports a safe embedding of I/O operations as well as a repair facility to resolve minor conflicts. In contrast to other implementations of irrevocable transactions, twilight code may run concurrently with other transactions including their twilight code without data races. Twilight STM further allows the implementation of application-specific conflict resolution strategies. To analyze their influence on the semantics of the transactions, a formal framework for investigating consistency and isolation properties is developed and applied to different repair operations.
Decent STM transfers the STM paradigm to a distributed setting. It is a fully decentralized object-based STM algorithm with versioning. It relies on mostly immutable data structures, which are well-suited for replication and migration. A randomized consensus protocol guarantees consistency of shared memory. Transactions may proceed tentatively before consensus has been reached. Object versioning ensures that transactions read from a consistent memory snapshot, and the consensus protocol determines which transactions can merge in their effects and which transactions need to restart. Hence, delayed communication, e.g. caused by retransmissions in the transport layer, can only affect performance, not consistency.
Both STM algorithms have been implemented in functional, object-oriented, and imperative languages, on multi-core and distributed architectures. This comprehensive study points out the need for an enhanced STM interface for more flexibility and higher programming convenience. Benchmarks featuring various workloads show the scalability and competitiveness with state-of-the-art systems.Software transactional memory (STM) ist ein Programmiermodell zur Entwicklung nebenläufiger Programme. In diesem Modell werden potentiell konfligierende Datenzugriffe von Threads verhindert, indem kritischen Programmabschnitte voneinander isoliert in Form von Transaktionen ausgeführt werden. Tritt ein Zugriffskonflikt auf, wird er zur Laufzeit aufgelöst, indem Transaktionen ihre Änderungen rückgängig machen und die Berechnung erneut ausgeführt werden. Das Laufzeitsystem garantiert außerdem, dass die Modifikationen der transaktional verwalteten Daten konsistent und aus der Sicht der anderen Transaktionen atomar durchgeführt werden.
Eine solche transparente Konfliktstrategie ist bisweilen zu restriktiv: Transaktionen oft auf Grund von Konflikten abgebrochen, die einfach aufgelöst und kompensiert werden könnten. Sie erfordert außerdem, dass unumkehrbare Operationen, wie I/O, nicht innerhalb von Transaktionen ausgeführt werden. Außerdem ist das klassische STM-Modell auf Shared-Memory-Architekturen beschränkt, bei denen die Konsistenz der Daten in den lokalen Caches der Prozessoren durch Speicherbarrieren erzwungen werden.
Die vorliegende Dissertation beschreibt zwei neuartige Algorithmen, die zum einen das klassische STM um adaptive Konfliktstrategien und die Einbettung von I/O erweitert, zum anderen STM auch in verteilten System zur Anwendung bringt.
Twilight STM erlaubt die Einbettung unumkehrbarer Operationen sowie flexibler Konfliktbehandlungsstrategien für Transaktionen. Es teilt eine Transaktion in eine funktionale, transaktionale Phase, sowie eine imperative, irreversible Phase auf. Im Gegensatz zu anderen STM-Erweiterungen für irreversible Transaktionen können Twilight Transaktionen vollständig parallel ohne konfligierende Datenzugriffe ablaufen. Desweiteren erlaubt Twilight STM die Implementierung von anwendungsspezifischen Konfliktstrategien. Um deren Semantik zu analysieren, stellt die Dissertation ein formales System zur Verfügung und wendet es auf verschiedene Strategien an.
Decent STM transferiert STM in eine verteilte Ausführungsumgebung. Es ist ein dezentraler, objekt-orientierter STM-Algorithmus, der auf Datenversionierung aufbaut. Da Versionen nach ihrem Transfer in das dezentrale Speichersystem unveränderlich sind, können sie einfach repliziert und migriert werden. Ein randomisiertes Konsensusprotokoll garantiert die Konsistenz bei transaktionalem Datenzugriff. Darüberhinaus können Transaktionen optimistisch bereits vor Erreichen eines Konsensus ihre Berechnungen starten. Verzögerungen im Datentransfer beeinträchtigen daher lediglich die Geschwindigkeit des Programms, nicht jedoch die Konsistenz der Daten.
Die STM-Algorithmen sind in funktionalen, objekt-orientierten und imperativen Programmiersprachen, für Multi-Core-Architekturen und verteilte Systeme implementiert. Die vorliegende Dissertation bietet einen Überblick über die praktische Anwendbarkeit von STM und unterstreicht den Bedarf einer Erweiterung des Basismodells. Verschiedene Testprogramme belegen die Skalierbarkeit der Systeme und zeigen die Wettbewerbsfähigkeit in Bezug auf andere STM-Systeme
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Explicit weakening
Get PDFI present a novel formulation of substitution, where facts about substitution that previously required tens or hundreds of lines to justify in a proof assistant now follow immediately - they can be justified by writing the four letters "refl". The paper is an executable literate Agda script, and source of the paper is available as an artifact in the file Weaken.lagda.md. Not all consequences of the pandemic have been awful. For the last three years, I've had the great pleasure of meeting with Peter Thiemann and Jeremy Siek for a couple of hours every week, via Zoom, exploring topics including core calculi, gradual typing, and formalisation in Agda. The work reported here arose from those discussions, and is dedicated to Peter on the occasion of his 60th birthday
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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