1,721,032 research outputs found
Abortable and Query-abortable Types and Their Efficient Implementation
Get PDFWe introduce abortable and query-abortable object types intended for implementation in asynchronous shared-memory systems with low contention. Implementations of such types behave like ordinary objects when accessed sequentially, but may abort operations when accessed concurrently. An aborted operation may or may not take effect, i.e., cause a state transition, and it returns no indication of which possibility occurred. Since this uncertainty can be problematic, a query-abortable type supports a QUERY operation that each process can use to determine its last non-QUERY operation on the object that caused a state transition, and the response associated with this state transition.
Our research is closely related to obstruction-free implementations (introduced by Herlihy, Luchangco and Moir) and responsive obstruction-free implementations (introduced by Attiya, Guerraoui and Kouznetsov). Like abortable and query-abortable types, these implementations may exhibit degraded behaviour in the face of contention. We show that abortable registers--registers strictly weaker than safe registers--can be used to obtain obstruction-free and responsive obstruction-free implementations for any type.
We present universal constructions for abortable and query-abortable types that are novel and efficient in the number of registers used. Specifically, they are based on a simple timestamping mechanism for detecting concurrent executions, and, in systems with n processes, use only n abortable registers or only O(n^2) single-reader, single-writer abortable registers. The timestamping mechanism we introduce is based on the inc&read counter type and appears to be interesting in its own right. As a generalization, we study the k-inc&read counter types, for k>0.
We also identify a potential problem with correctness properties based on step contention: with such properties, the composition of correct object implementations may result in an implementation that is not correct. In other words, implementations defined in terms of step contention are not always composable. To avoid this problem, we introduce a property based on interval contention, namely non-triviality, to define the correct behaviour of abortable and query-abortable object implementations.Ph
Abortable and Query-abortable Types and Their Efficient Implementation
No full textWe introduce abortable and query-abortable object types intended for implementation in asynchronous shared-memory systems with low contention. Implementations of such types behave like ordinary objects when accessed sequentially, but may abort operations when accessed concurrently. An aborted operation may or may not take effect, i.e., cause a state transition, and it returns no indication of which possibility occurred. Since this uncertainty can be problematic, a query-abortable type supports a QUERY operation that each process can use to determine its last non-QUERY operation on the object that caused a state transition, and the response associated with this state transition.
Our research is closely related to obstruction-free implementations (introduced by Herlihy, Luchangco and Moir) and responsive obstruction-free implementations (introduced by Attiya, Guerraoui and Kouznetsov). Like abortable and query-abortable types, these implementations may exhibit degraded behaviour in the face of contention. We show that abortable registers--registers strictly weaker than safe registers--can be used to obtain obstruction-free and responsive obstruction-free implementations for any type.
We present universal constructions for abortable and query-abortable types that are novel and efficient in the number of registers used. Specifically, they are based on a simple timestamping mechanism for detecting concurrent executions, and, in systems with n processes, use only n abortable registers or only O(n^2) single-reader, single-writer abortable registers. The timestamping mechanism we introduce is based on the inc&read counter type and appears to be interesting in its own right. As a generalization, we study the k-inc&read counter types, for k>0.
We also identify a potential problem with correctness properties based on step contention: with such properties, the composition of correct object implementations may result in an implementation that is not correct. In other words, implementations defined in terms of step contention are not always composable. To avoid this problem, we introduce a property based on interval contention, namely non-triviality, to define the correct behaviour of abortable and query-abortable object implementations.Ph
Classifications of Asynchronous Shared Memory Objects
No full textSince the early days of the shared memory model for distributed computing, researchers have sought a simple and precise characterization of an object’s ability to implement other objects in a wait-free manner.
The first candidate for such a characterization was the consensus number of an object [24]. But a characterization based on consensus numbers is not precise: there are pairs of objects with the same consensus number that are not equivalent (i.e., one object cannot be wait-free implemented by instances of the other and registers) [2, 42].
A more recent candidate for such a characterization is the set agreement power of an object [15]. In this thesis, we show that this characterization is not precise even for the case of deterministic objects: there are pairs of deterministic objects with the same set agreement power that are not equivalent.
Next, we show that there are uncountably many objects with distinct set agreement power. Consequently, a precise characterization of an object’s ability to implement other objects in a wait-free manner must classify objects into uncountably many cells. This suggests that there is no simple and precise characterization of objects, since the cells of an uncountable classification cannot be labeled using simple integers (as in Herlihy’s hierarchy) or even finite sequences of integers.
Finally, we introduce a generalization of consensus called bounded disagreement, which differs from set agreement in that it restricts the number of disagreeing processes rather than the number of disagreeing values. More precisely, the l-bounded disagreement task for n processes has the following requirement: there is a value v such that at most l processes (the disagreers) decide values that are not v. We then show that for every integer n ≥ 2, there is a bounded disagreement object that has consensus number n, but is not equivalent to the n-consensus object. Prior to this work, the only objects known to have this unusual characteristic for n ≥ 2 (which shows that the characterization of objects via consensus numbers is not precise) were artificial objects crafted solely for the purpose of exhibiting this behaviour [2, 42].Ph.D
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
Classifications of Asynchronous Shared Memory Objects
Get PDFSince the early days of the shared memory model for distributed computing, researchers have sought a simple and precise characterization of an object’s ability to implement other objects in a wait-free manner.
The first candidate for such a characterization was the consensus number of an object [24]. But a characterization based on consensus numbers is not precise: there are pairs of objects with the same consensus number that are not equivalent (i.e., one object cannot be wait-free implemented by instances of the other and registers) [2, 42].
A more recent candidate for such a characterization is the set agreement power of an object [15]. In this thesis, we show that this characterization is not precise even for the case of deterministic objects: there are pairs of deterministic objects with the same set agreement power that are not equivalent.
Next, we show that there are uncountably many objects with distinct set agreement power. Consequently, a precise characterization of an object’s ability to implement other objects in a wait-free manner must classify objects into uncountably many cells. This suggests that there is no simple and precise characterization of objects, since the cells of an uncountable classification cannot be labeled using simple integers (as in Herlihy’s hierarchy) or even finite sequences of integers.
Finally, we introduce a generalization of consensus called bounded disagreement, which differs from set agreement in that it restricts the number of disagreeing processes rather than the number of disagreeing values. More precisely, the l-bounded disagreement task for n processes has the following requirement: there is a value v such that at most l processes (the disagreers) decide values that are not v. We then show that for every integer n ≥ 2, there is a bounded disagreement object that has consensus number n, but is not equivalent to the n-consensus object. Prior to this work, the only objects known to have this unusual characteristic for n ≥ 2 (which shows that the characterization of objects via consensus numbers is not precise) were artificial objects crafted solely for the purpose of exhibiting this behaviour [2, 42].Ph.D
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
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
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