Randomized Two-Process Wait-Free Test-and-Set

We present the first explicit, and currently simplest, randomized algorithm for 2-process wait-free test-and-set. It is implemented with two 4-valued single writer single reader atomic variables. A test-and-set takes at most 11 expected elementary steps, while a reset takes exactly 1 elementary step. Based on a finite-state analysis, the proofs of correctness and expected length are compressed into one table.Comment: 9 pages, 4 figures, LaTeX source; Submitte

Data management for cloud supported cooperative driving

Tese de mestrado, Engenharia Informática (Arquitetura, Sistemas e Redes de Computadores) Universidade de Lisboa, Faculdade de Ciências, 2020The increasing number of technologies inserted into vehicles, allowed the common user to have access to a broad number of utilities that allows driving to be easier, safer and more economical. ABS, GPS, Bluetooth and onboard computer are some of the technologies associated with a recent vehicle. On more experimental ones there is obstacle detection, automatic braking and self-driving technologies, which can be supported by a wireless network connection to further improve their capabilities. That connection allows the transformation of each independent vehicle into nodes in an ad-hoc network. The current challenge is to connect all those vehicles and be able to provide the data needed for their correct functioning in a timely manner. That is the challenge this dissertation will seek to analyse: the possibility to create a reliable vehicular information system for cooperative driving based on the cloud. Cloud-based storage can support an ever changing number of vehicles while still satisfying scalability requirements and maintaining ease of access without the need to maintain a physical infrastructure, as that responsibility is laid upon the provider. To understand which service is the best to host the vehicular information system it was analyzed three services from Amazon Web Services (AWS): S3, EC2 and DynamoDB. Ease of utility, latency, scalability and cost were the main requirements tested as they are the most important aspects for a real-time vehicular information system for autonomous vehicles. After deciding which cloud service would be the most appropriate to implement the vehicular information system, two client models were created that fulfilled a set of requirements. They were based in an already existing algorithm named Two-Step Full Replication which utilizes a group of Key-Value Stores services from various clouds to simulate a shared-memory based on multi-writer, multi-reader (MWMR) registers. This algorithm tolerates Byzantine faults by using Byzantine quorum techniques and integrity and authenticity checks. It was defined and implemented the necessary changes on the algorithm to create usable a client for a vehicular information system. The first model called ”Atomic Snapshot Client”, uses the modified Two-Step Full Replication interface with the Atomic Snapshot algorithm. This model guarantees that the read of the system (snapshot) is done atomically without being adulterated by concurrent writes, sacrificing execution latency. The second model is a faster version of the first one with the objective of obtaining faster responses from the system without overly sacrificing data consistency, which is called ”Fast Snapshot Client”. The main change from the first one is the reduction of the guarantees of the atomic registers to regular ones making the reads (scan) and writes (update) simpler and faster, although removing the atomic snapshot feature. With the analysis of the data collected from experiments performed with this model it was possible to observe a relation between the increase of the scan latency time and the total time spent on the execution of the read and write operations on an application with various clients. To solve this problem a simple garbage collector was implemented, which cleans each register when the number of outdated writes that it contains goes over a specified threshold. This solution, although simple, proved to be effective to reduce each scan time. Finally, a vehicular information system based on the AWS S3 service was implemented. It is composed by two types of clients based on the Fast Snapshot Client, named vehicular client and calculator client. The two types of client work together, where the vehicular clients trade information with the calculator. The calculator client scans the registers of the vehicle clients and writes on its registers the processed data for each vehicular client. The vehicle clients need to write all the relevant data they gather and read the register of their respective calculator client and act according to the data read. Each of the clients was tested separately and analysed in order to discuss the viability of this system in a real-world application as well as possible changes to further improve it

Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models

We use 3D numerical MHD simulations to follow the evolution of cold, turbulent, gaseous systems with parameters representing GMC conditions. We study three cloud simulations with varying mean magnetic fields, but identical initial velocity fields. We show that turbulent energy is reduced by a factor two after 0.4-0.8 flow crossing times (2-4 Myr), and that the magnetically supercritical cloud models collapse after ~6 Myr, while the subcritical cloud does not collapse. We compare density, velocity, and magnetic field structure in three sets of snapshots with matched Mach numbers. The volume and column densities are both log-normally distributed, with mean volume density a factor 3-6 times the unperturbed value, but mean column density only a factor 1.1-1.4 times the unperturbed value. We use a binning algorithm to investigate the dependence of kinetic quantities on spatial scale for regions of column density contrast (ROCs). The average velocity dispersion for the ROCs is only weakly correlated with scale, similar to the mean size-linewidth relation for clumps within GMCs. ROCs are often superpositions of spatially unconnected regions that cannot easily be separated using velocity information; the same difficulty may affect observed GMC clumps. We analyze magnetic field structure, and show that in the high density regime, total magnetic field strengths increase with density with logarithmic slope 1/3 -2/3. Mean line-of-sight magnetic field strengths vary widely across a projected cloud, and do not correlate with column density. We compute simulated interstellar polarization maps at varying orientations, and determine that the Chandrasekhar-Fermi formula multiplied by a factor ~0.5 yields a good estimate of the plane-of sky magnetic field strength provided the dispersion in polarization angles is < 25 degrees.Comment: 56 pages, 25 figures; Ap.J., accepte

Notes on Theory of Distributed Systems

Notes for the Yale course CPSC 465/565 Theory of Distributed Systems

Coupling SPH and thermochemical models of planets: Methodology and example of a Mars-sized body

Giant impacts have been suggested to explain various characteristics of terrestrial planets and their moons. However, so far in most models only the immediate effects of the collisions have been considered, while the long-term interior evolution of the impacted planets was not studied. Here we present a new approach, combining 3-D shock physics collision calculations with 3-D thermochemical interior evolution models. We apply the combined methods to a demonstration example of a giant impact on a Mars-sized body, using typical collisional parameters from previous studies. While the material parameters (equation of state, rheology model) used in the impact simulations can have some effect on the long-term evolution, we find that the impact angle is the most crucial parameter for the resulting spatial distribution of the newly formed crust. The results indicate that a dichotomous crustal pattern can form after a head-on collision, while this is not the case when considering a more likely grazing collision. Our results underline that end-to-end 3-D calculations of the entire process are required to study in the future the effects of large-scale impacts on the evolution of planetary interiors.Comment: 29 pages, 10 figures, accepted for publication in Icaru

GEANT4 : a simulation toolkit

Abstract Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics. PACS: 07.05.Tp; 13; 2