A survey of parallel execution strategies for transitive closure and logic programs

An important feature of database technology of the nineties is the use of parallelism for speeding up the execution of complex queries. This technology is being tested in several experimental database architectures and a few commercial systems for conventional select-project-join queries. In particular, hash-based fragmentation is used to distribute data to disks under the control of different processors in order to perform selections and joins in parallel. With the development of new query languages, and in particular with the definition of transitive closure queries and of more general logic programming queries, the new dimension of recursion has been added to query processing. Recursive queries are complex; at the same time, their regular structure is particularly suited for parallel execution, and parallelism may give a high efficiency gain. We survey the approaches to parallel execution of recursive queries that have been presented in the recent literature. We observe that research on parallel execution of recursive queries is separated into two distinct subareas, one focused on the transitive closure of Relational Algebra expressions, the other one focused on optimization of more general Datalog queries. Though the subareas seem radically different because of the approach and formalism used, they have many common features. This is not surprising, because most typical Datalog queries can be solved by means of the transitive closure of simple algebraic expressions. We first analyze the relationship between the transitive closure of expressions in Relational Algebra and Datalog programs. We then review sequential methods for evaluating transitive closure, distinguishing iterative and direct methods. We address the parallelization of these methods, by discussing various forms of parallelization. Data fragmentation plays an important role in obtaining parallel execution; we describe hash-based and semantic fragmentation. Finally, we consider Datalog queries, and present general methods for parallel rule execution; we recognize the similarities between these methods and the methods reviewed previously, when the former are applied to linear Datalog queries. We also provide a quantitative analysis that shows the impact of the initial data distribution on the performance of methods

Il giudice "rottamatore" e l'enfant préjudice

Nell’ambito delle problematiche relative alla “nascita indesiderata”, l’Autore analizza i fondamenti dell’azione intentata avverso i medici dai genitori e dai fratelli del bambino nato handicappato, nell’ipotesi di mancata rilevazione, nel corso della gravidanza, delle malformazioni congenite presentate dal feto. Inoltre, è oggetto di studio e di riflessione, anche in un’ottica di comparazione con le soluzioni ideate da giurisprudenza e legislatore francesi, la legittimazione attiva dello stesso bambino, nonché la risarcibilità, in suo favore, del danno conseguente ad una vita segnata dall’handicap

La sedazione palliativa profonda e continua nell'imminenza della morte: le sette inquietudini del diritto.

La sedazione profonda e continua nell’imminenza della morte rappresenta il last resort della palliazione: ne incarna, anzi, il misero fallimento, laddove l’ultima chance di eliminare la sofferenza è quella di annientare la coscienza di chi soffre. Potrebbe persino trattarsi, del resto, di una slow and soft eutanasia, e non solo per gli ésiti umani e relazionali che ne derivano, ma altresì per l’azione diretta eventualmente esercitata sulla durata della sopravvivenza del paziente – esclusa dalla quasi totalità della letteratura scientifica, ma espressamente contemplata, per esempio, dal legislatore francese del 2016. D’altro canto, per autodeterminarsi in merito all’attivazione di simile sedazione, chi sta morendo deve conoscere la verità, anche riguardo alla prognosi: la comunicazione fra medico e paziente non è qui inquinata dallo spauracchio dell’azione legale, ma mai come in quest’àmbito sarà la comunicazione orale, anche dilatata nel tempo, a formare il convincimento dell’assistito, con una condivisione lenta e graduale di informazioni, da una parte, e di preoccupazioni, dall’altra. Per converso, la scelta d’interrompere o di non iniziare trattamenti di sostegno vitale costituisce decisione distinta rispetto a quella riguardante l’attivazione della sedazione palliativa profonda e continua, benché frequentemente a questa associata. Infine, il sintomo refrattario può ben essere un existential distress e la patologia senz’altro irreversibile, ma la morte non imminente: in questo modo, l’individuale desiderio di morire della persona gravissimamente handicappata, che tale condizione non voglia e non possa più sostenere, potrebbe giustappunto rinvenire, in tale sedazione, un mero succedaneo dell’atto eutanasico

Il consenso informato del paziente al trattamento sanitario

Il consenso informato attiene a quel passaggio di nozioni, dal medico al proprio paziente, vòlto a garantire che la manifestazione di volontà espressa dall’ammalato sia frutto del corretto svolgersi, in àmbito sanitario, della sua autodeterminazione. Dall’inadempimento di tale obbligazione, invero, scaturisce una responsabilità contrattuale del professionista autonoma e diversa rispetto a quella conseguente ad un’esecuzione negligente, imperita o imprudente del trattamento medico, con una liquidazione del danno parimenti a sé stante

Efficient implementation of characteristic-based schemes on unstructured triangular grids

Using characteristics to treat advection terms in time-dependent PDEs leads to a class of schemes, e.g., semi-Lagrangian and Lagrange–Galerkin schemes, which preserve stability under large Courant numbers, and may therefore be appealing in many practical situations. Unfortunately, the need of locating the feet of characteristics may cause a serious drop of efficiency in the case of unstructured space grids, and thus prevent the use of large time-step schemes on complex geometries. In this paper, we perform an in-depth analysis of the main recipes available for characteristic location, and propose a technique to improve the efficiency of this phase, using additional information related to the advecting vector field. This results in a clear improvement of execution times in the unstructured case, thus extending the range of applicability of large time-step schemes

A stability with optimality analysis of consensus-based distributed filters for discrete-time linear systems

In this paper we investigate how stability and optimality of consensus-based distributed filters depend on the number of consensus steps in a discrete-time setting for both directed and undirected graphs. By introducing two new algorithms, a simpler one based on dynamic averaging of the estimates and a more complex version where local error covariance matrices are exchanged as well, we are able to derive a complete theoretical analysis. In particular we show that dynamic averaging alone suffices to approximate the optimal centralized estimate if the number of consensus steps is large enough and that the number of consensus steps needed for stability can be computed in a distributed way. These results shed light on the advantages as well as the fundamental limitations shared by all the existing proposals for this class of algorithms in the basic case of linear time-invariant systems, that are relevant for the analysis of more complex situations

Investigation on the effect of the gas-to-metal ratio on powder properties and PBF-LB/M processability

Metal powders for the laser powder bed fusion process are usually produced via gas atomization. However, due to the tight particle size distribution required for this application, the yield of the atomization process is low, resulting in a high-powder cost. In this work, atomization process parameters were varied to increase the gas-to-metal ratio to reduce the particle size distribution produced, and therefore increase the yield of the process. As a result, eight powders were produced starting from scrap AISI 136L material at different gas-to-metal ratio values, and the atomization process yield was successfully increased by 50%. First, the eight powders were characterized in terms of powder size, shape distributions, and flowability. Later, all powders were used to produce tensile specimens. The powders produced at higher yield exhibited a larger number of fine particles but slightly lower circularity, particularly in the coarse fraction. Furthermore, powders produced at a high gas-to-metal ratio demonstrated enhanced flowing properties and higher packing density. Consequently, these powders exhibited superior tensile performance, with ultimate tensile strength (UTS) ranging from 651 to 673 MPa and elongation values between 63 and 66%