Preventing Atomicity Violations with Contracts

Software developers are expected to protect concurrent accesses to shared regions of memory with some mutual exclusion primitive that ensures atomicity properties to a sequence of program statements. This approach prevents data races but may fail to provide all necessary correctness properties.The composition of correlated atomic operations without further synchronization may cause atomicity violations. Atomic violations may be avoided by grouping the correlated atomic regions in a single larger atomic scope. Concurrent programs are particularly prone to atomicity violations when they use services provided by third party packages or modules, since the programmer may fail to identify which services are correlated. In this paper we propose to use contracts for concurrency, where the developer of a module writes a set of contract terms that specify which methods are correlated and must be executed in the same atomic scope. These contracts are then used to verify the correctness of the main program with respect to the usage of the module(s). If a contract is well defined and complete, and the main program respects it, then the program is safe from atomicity violations with respect to that module. We also propose a static analysis based methodology to verify contracts for concurrency that we applied to some real-world software packages. The bug we found in Tomcat 6.0 was immediately acknowledged and corrected by its development team

Instability and new phases of higher-dimensional rotating black holes

It has been conjectured that higher-dimensional rotating black holes become unstable at a sufficiently large value of the rotation, and that new black holes with pinched horizons appear at the threshold of the instability. We search numerically, and find, the stationary axisymmetric perturbations of Myers-Perry black holes with a single spin that mark the onset of the instability and the appearance of the new black hole phases. We also find new ultraspinning Gregory-Laflamme instabilities of rotating black strings and branes.Comment: 5 pages, 5 figures. The instability of the black hole is argued to appear at the second zero mode. The first zero mode is not associated to a new branch of black hole solution

Biomechanics of a pedestrian accident reconstruction

Pedestrians represent about 20% of the overall fatalities in Europe's road traffic accidents, as well as in Portugal. An in-depth investigation, accident reconstruction, is mandatory in order to analyze with care fundamental aspects of an accident. Detailed information about the injuries and injury causation mechanisms are with great use for it. Accident reconstruction computational techniques using pedestrian biomechanical models are used to evaluate the accident conditions that lead to major pedestrian injuries, such as the vehicle's speed and the impact location. For biomechanical injury criterions, the AIS (Abbreviated Injury Scale), the HIC (Head Injury Criterion) and some other injury criterions are used. Pedestrian's height, weight, gender, age, vehicle geometry (or category) and travelling speed, significantly influence the pedestrian injuries in the event of a road collision. The use of injury scales and biomechanical criterions in the in-depth accident investigation of road accidents, such as AIS, can improve significantly the quality of the reconstruction process. The computer simulation of vehicle-to-pedestrian collisions is fundamental for the study of the dynamics of these road accidents. Where did the pedestrian is when the vehicle hits him? Normally, there is no simple and clear answer. The pedestrian, if survives, states that he uses the crossway, the driver of the vehicle states otherwise, that the location of the pedestrian was much later than the pedestrian crossing. This work presents an accident where only the use of accident reconstruction specialized software, PC-Crash, conjugated with all available information allowed the determination of the vehicle's speed and the Location of the pedestrian.info:eu-repo/semantics/publishedVersio

Application of binary packing for starch separation by hydrodynamic chromatography

Columns packed with commercial glass beads of 5 and 19 microns average diameter and binary mixtures with finest fraction of 5 micron (30 % volume fraction of the mixture) were used to analyse starch by hydrodynamic chromatography (HDC). Experiments were carried out at 3 and 15 ºC. The observed resolution increased with the application of binary packing as compared with single-size packing. The best results were obtained at starch’s amylopectin and amylose separation with a glass beads mixture (5 + 19 micron) at 3ºC. In what concerns amylopectin and amylose separation, a lower pressure drop were obtained for the mixed binary packing when compared with the packing containing uniform 5 micron glass beads. For the Hylon VII starch RRT were 0.777 and 0.964 for amylopectin (AP) and amylose (AM), respectively, while for the Tapioca starch the obtained RRTs were 0.799 and 0.923. Application of unbound glass beads as column packing might reduce equipment and running costs in preparative scale separations.Fundação para a Ciência e a Tecnologia (FCT) .FEDER - projecto POCI_EQU_58337/200

Tortuosity variation in a low density binary particulate bed

The importance of particle size ratio and particle composition in the properties of a mixed bed is well known. Nevertheless, the dependence of the bed channel tortuosity T on the porosity ε in the form T=1/εn, where n is assumed to be a constant, shows that the value of n depends on the properties of the packed bed. For loose packing, experimental data for binary mixtures of glass beads of a size ratio from 1 up to 53.8 was analysed in terms of porosity, tortuosity and permeability. The packing procedure was performed without intensive compacting methods e.g. vibration, etc. Obtained results show that the parameter n is a function of the volume fraction of large particles xD and, for spherical particles, lies in the range 0.4–0.5. The explanation for this variation is (1) a distortion effect on the small particles arrangement occurring near the large particle surface; (2) in the region of minimum porosity, near contact points of large particles, the occurrence of dead zones that are free of small particles. A relationship accounting for this effect is proposed that may be useful for the analysis of transport phenomena in granular bed filters, chromatographic columns, etc.Fundação para a Ciência e a Tecnologia FCT); FEDE

Effect of real particles packing with large size ratio on porosity and tortuosity of filter bed

The complexity of processes involved in the formation of granular beds results in limited information about permeability κ , which directly rela tes with packing porosity ε and tortuosity T . For a mixed bed of particles significantly different in size, the influence of packing affects permeability. For a better understanding of the underlying relationship between κ , ε , and T in mixed beds of particles significantly different in size, simplified porous media model of binary mixture of spheres were used. Boundary analysis of the binary packing showed that the approach based on the fractional porosity of large and small size particle fractions gives a tool for ε control. This approach allows a new insight into the mixture structure and provides explanation for the different types of the obtained porosity. Binary packing of glass beads with size ratios 13.3, 20, and 26.7 were investigated. As a basic relation for the dependence of T on ε, at different volume fraction xD of large particles in the mixture, the formula T=1/ εⁿ was used. The obtained experimental results show that the parameter n is a function of the packing content xD and may vary in the range of 0.4-0.5. The reason for n variation was explained by the wall effect of the small particles arrangement occurring near the large particles surface. A model accounting for this effect is proposed and may be useful for transport phenomena analysis in granular bed filters.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” (POCTI) - POCTI/EQU/37500/2001.União Europeia (UE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Particulate binary mixtures : dependence of packing porosity on particle size ratio

Binary mixtures with significant size ratios are scarcely studied. Yet, contaminants of chromatographic columns or ion-exchange resins have size ratios of delta < 0.1. Binary mixtures of glass beads with delta = 0.1-0.0375 were used experimentally to measure packing porosity. Simultaneously, a significant number of published data was analyzed. A linear mixing model was adopted to predict the porosity of each particle fraction in the binary mixture. Deviations from the model may be caused by wedging of small particles between the large ones. Large particles may disturb the porous medium properties by inducing a wall effect over the small particles. Wedging analysis led to the conclusion that, for delta < 0.01, its effect is insignificant. The wall effect yields an additional void around the large particles as long as delta > 0.0035. For delta < 0.0035, the small particles form a monosized dense packing and both wedging and wall effects become negligible.Fundação para a Ciência e a Tecnologia (FCT) e FEDER – POCTI/EQU/37500/2001

Ultraspinning instability: the missing link

We study linearized perturbations of Myers-Perry black holes in d=7, with two of the three angular momenta set to be equal, and show that instabilities always appear before extremality. Analogous results are expected for all higher odd d. We determine numerically the stationary perturbations that mark the onset of instability for the modes that preserve the isometries of the background. The onset is continuously connected between the previously studied sectors of solutions with a single angular momentum and solutions with all angular momenta equal. This shows that the near-extremality instabilities are of the same nature as the ultraspinning instability of d>5 singly-spinning solutions, for which the angular momentum is unbounded. Our results raise the question of whether there are any extremal Myers-Perry black holes which are stable in d>5.Comment: 19 pages. 1 figur

Application of binary packing for chromatographic separation

Separation of dextran and polyethylene glycol of different molecular mass was performed using a binary packed column of glass beads (size ratio ~ 10) and a binary packed column formed by kieselguhr-G (for thin layer chromatography, Merck) and glass beads as the large size particulate fraction (size ratio ~ 30). In addition, data on the separation of micro-spheres, bacillus and yeast cells using monosized and binary glass beads columns are presented. Obtained results show the advantages of using binary packed columns formed by fine and coarse particles instead of a monosize packing of fine particles. The importance of pore channels tortuosity effect on the separation of shaped microparticles using a binary packing is demonstrated