Autonomic State Management for Optimistic Simulation Platforms

We present the design and implementation of an autonomic state manager (ASM) tailored for integration within optimistic parallel discrete event simulation (PDES) environments based on the C programming language and the executable and linkable format (ELF), and developed for execution on x8664 architectures. With ASM, the state of any logical process (LP), namely the individual (concurrent) simulation unit being part of the simulation model, is allowed to be scattered on dynamically allocated memory chunks managed via standard API (e.g., malloc/free). Also, the application programmer is not required to provide any serialization/deserialization module in order to take a checkpoint of the LP state, or to restore it in case a causality error occurs during the optimistic run, or to provide indications on which portions of the state are updated by event processing, so to allow incremental checkpointing. All these tasks are handled by ASM in a fully transparent manner via (A) runtime identification (with chunk-level granularity) of the memory map associated with the LP state, and (B) runtime tracking of the memory updates occurring within chunks belonging to the dynamic memory map. The co-existence of the incremental and non-incremental log/restore modes is achieved via dual versions of the same application code, transparently generated by ASM via compile/link time facilities. Also, the dynamic selection of the best suited log/restore mode is actuated by ASM on the basis of an innovative modeling/optimization approach which takes into account stability of each operating mode with respect to variations of the model/environmental execution parameters

Using Finite Forkable DEVS for Decision-Making Based on Time Measured with Uncertainty

International audienceThe time-line in Discrete Event Simulation (DES) is a sequence of events defined in a numerable subset of R +. When it comes from an experimental measurement, the timing of these events has a limited precision. This precision is usually well-known and documented for each instruments and procedures used for collecting experimental datas. Therefore, these instruments and procedures produce measurement results expressed using values each associated with an uncertainty quantification, given by uncertainty intervals. Tools have been developed in Continuous Systems modeling for deriving the uncertainty intervals of the final results corresponding to the propagation of the uncertainty intervals being evaluated. These tools cannot be used in DES as they are defined, and no alternative tools that would apply to DES have been developed yet. In this paper, we propose simulation algorithms, based on the Discrete Event System Specification (DEVS) formalism, that can be used to simulate and obtain every possible output and state trajecto-ries of simulations that receive input values with uncertainty quantification. Then, we present a subclass of DEVS models , called Finite Forkable DEVS (FF-DEVS), that can be simulated by the proposed algorithms. This subclass ensures that the simulation is forking only a finite number of processes for each simulation step. Finally, we discuss the simulation of a traffic light model and show the trajectories obtained when it is subject to input uncertainty

Collaborative research: ITR: global multi-scale kinetic simulations of the earth's magnetosphere using parallel discrete event simulation

Issued as final reportNational Science Foundation (U.S.

Enhancing the performance of HLA-based simulation systems via software diversity and active replication

In this paper we explore active replication based on software diversity for improving the responsiveness of simulation systems. Our proposal is framed by the High-Level-Architecture (HLA), namely the emerging standard for interoperability of simulation packages, and results in the design and implementation of an Active Replication Management Layer (ARML), which supports the execution of multiple software diversity-based replicas of a same simulator in a totally transparent manner. Beyond presenting the replication framework and the design/implementation of ARML, we also report the results of an experimental evaluation on a case study, quantifying the benefits from our proposal in terms of execution speed. © 2006 IEEE

Efficient And Scalable Evaluation Of Continuous, Spatio-temporal Queries In Mobile Computing Environments

A variety of research exists for the processing of continuous queries in large, mobile environments. Each method tries, in its own way, to address the computational bottleneck of constantly processing so many queries. For this research, we present a two-pronged approach at addressing this problem. Firstly, we introduce an efficient and scalable system for monitoring traditional, continuous queries by leveraging the parallel processing capability of the Graphics Processing Unit. We examine a naive CPU-based solution for continuous range-monitoring queries, and we then extend this system using the GPU. Additionally, with mobile communication devices becoming commodity, location-based services will become ubiquitous. To cope with the very high intensity of location-based queries, we propose a view oriented approach of the location database, thereby reducing computation costs by exploiting computation sharing amongst queries requiring the same view. Our studies show that by exploiting the parallel processing power of the GPU, we are able to significantly scale the number of mobile objects, while maintaining an acceptable level of performance. Our second approach was to view this research problem as one belonging to the domain of data streams. Several works have convincingly argued that the two research fields of spatiotemporal data streams and the management of moving objects can naturally come together. [IlMI10, ChFr03, MoXA04] For example, the output of a GPS receiver, monitoring the position of a mobile object, is viewed as a data stream of location updates. This data stream of location updates, along with those from the plausibly many other mobile objects, is received at a centralized server, which processes the streams upon arrival, effectively updating the answers to the currently active queries in real time. iv For this second approach, we present GEDS, a scalable, Graphics Processing Unit (GPU)-based framework for the evaluation of continuous spatio-temporal queries over spatiotemporal data streams. Specifically, GEDS employs the computation sharing and parallel processing paradigms to deliver scalability in the evaluation of continuous, spatio-temporal range queries and continuous, spatio-temporal kNN queries. The GEDS framework utilizes the parallel processing capability of the GPU, a stream processor by trade, to handle the computation required in this application. Experimental evaluation shows promising performance and shows the scalability and efficacy of GEDS in spatio-temporal data streaming environments. Additional performance studies demonstrate that, even in light of the costs associated with memory transfers, the parallel processing power provided by GEDS clearly counters and outweighs any associated costs. Finally, in an effort to move beyond the analysis of specific algorithms over the GEDS framework, we take a broader approach in our analysis of GPU computing. What algorithms are appropriate for the GPU? What types of applications can benefit from the parallel and stream processing power of the GPU? And can we identify a class of algorithms that are best suited for GPU computing? To answer these questions, we develop an abstract performance model, detailing the relationship between the CPU and the GPU. From this model, we are able to extrapolate a list of attributes common to successful GPU-based applications, thereby providing insight into which algorithms and applications are best suited for the GPU and also providing an estimated theoretical speedup for said GPU-based application

Probing the accretion physics of Sagittarius A*

Das Galaktische Zentrum und das darin befindende massereiche Schwarze Loch Sagitarrius A* (Sgr A*) stellt einen der exotischsten Orte des Universums dar, welcher der Menschheit bekannt ist. In dieser Dissertation untersuche ich zwei verschiedene Aspekte des Galaktischen Zentrum: den Akkretionsfluss in der direkten Umgebung von Sgr A*, sowie die Verteilung der jungen Sterne, die sich in der unmittelbaren Nachbarschaft des Schwarzen Loches befinden. Die in dieser Disseration vorgestellte Arbeit hat zu drei neuartigen Beobachtungen der spektralen Energieverteilung (englisch: Spectral Energy Distribution, SED) von Sgr A* geführt, welche ich in den ersten drei Kapitel vorstelle. Im letzten Kapitel der Thesis stelle ich meine Resultate zur Population von jungen Sterne im Galaktischen Zentrum vor. Das erste Kapitel handelt von dem gleichzeitigen Nachweis von Sgr A* in zwei Ferninfrarotbeobachtungsbändern bei Wellenlängen von 160 μm und 100 μm. Dies sind die ersten Beobachtungen von Sgr A* in diesem Wellenlängenbereich und wurden mit der PACS Kamera on-board des Weltraumteleskops Herschel aufgezeichnet. Die Messung wurden mit Hilfe einer maßgeschneiderten Datenreduktion ermöglicht, die eine differentielle Flussmessungen im Ferninfroten mit einem bisher unerreichten Rauschpegel erlaubt. Dies führt zum ersten Nachweis des variablen Flusses mit einer Signifikanz von 4.5σ bei 160 μm und 1.6σ bei 100 μm. Die Entdeckung des variablen Flusses bestätigt, dass die SED im Sub-mm-Bereich ihr Maximum erreicht und ermöglicht die Bestimmung der Elektronendichte, der Magnetfeldstärke und der Elektronentemperatur im Akkretionsfluss. In Kombination mit modernen ALMA-Beobachtungen von Sgr A* deuten diese Ergebnisse auf niedrigere Sub-mm-Flüsse hin als bis dato angenommen wurde. Die Messergebnisse erfordern aus diesem Grund höhere Elektronentemperaturen im Akkretionsfluss. Dies deutet darauf hin, dass der Akkretionsfluss im Sub-mm- und teilweise auch im mm-Bereich optisch dünn ist. Im zweiten Kapitel nutze ich die ersten drei Jahre der interferometrischen GRAVITY-Beobachtungen, welche am Very Large Telescope Interferometer durchgefuehrt wurden, um die Flussverteilung von Sgr A* im Nahinfraroten zu untersuchen. Aus den GRAVITY-Daten erstelle ich die erste kohärente Flussmessung von Sgr A*, die 2019 mit dem neuartigen Dual-Beam-Beobachtungsmodus beobachtet wurde. Zusätzlich, verwende ich Lichtkurven aus den Jahren 2017 und 2018, die bereits in der Literatur veröffentlicht wurden. Aufgrund der sehr hohen räumlichen Auflösung von GRAVITY wird diese Sgr A*-Flussmessung nicht durch das Licht von nahegelegenen Sternen gestört, was ähnliche auf adaptive Optik gestützte Studien in der Vergangenheit stark einschränkte. Außerdem konnte ich das Licht des Akkretionsflusses von Sgr A* zu jedem Messzeitpunkt nachweisen, eine Neuerung gegenüber den vorherrgehenden Studien, in welchen nur hellere Zustände von Sgr A* beobachtet werden konnten. Infolgedessen bin ich in der Lage, die erste rein empirische und nicht konfusionslimitierte Flussverteilung von Sgr A* zu erstellen und die 2.2 μm-Flussquantile zu messen. Durch den Vergleich mehrerer statistischer Modelle der Flussverteilung kann ich nachweisen, dass die Flussverteilung logarithmisch rechtsschief ist und nur schlecht durch eine Lognormalverteilung beschrieben wird. Im Gegensatz dazu ist die Flussverteilung gut durch ein Zweikomponentenmodell beschrieben: eine Log-Normalverteilung zur Beschreibung der Ruheemission in Kombination mit einer zweiten Komponente, die einem Potenzgesetz folgt. In diesem Szenario werden die hellen Nahinfrarot- und Röntgenflares in lokalisierten und aufgeheizten Zonen des Akkretionsstroms erzeugt, die sich von der variablen Ruheemission unterscheiden. Das dritte Kapitel in dieser Dissertation untersucht die Eigenschaften eines solchen Flares. Ich berichte über den Nachweis eines simultanen hellen Nahinfrarot- und eines moderaten Röntgenflare. Hierbei verwende ich die Kontrollkamera von GRAVITY, um H-Band-Beobachtungen gleichzeitig zu den interferometrischen K-Band-Beobachtung zu erstellen. Desweiteren kombiniere ich diese beiden Nahinfrarot-Lichtkurven mit gleichzeitigen Beobachtungen durch die Weltraumteleskope Spitzer, Chandra und NuSTAR. Mit Hilfe der so gewonnen Flussmessung modelliere ich die Emissionsregion im Flare-Szenario. Ich berechne die SED des Flares unter Berücksichtigung der Synchrotron- und Synchrotron-Selbst-Compton-Emission. Dies erlaubt mir, die Eigenschaften der für die Emission verantwortlichen Elektronenpopulation abzuleiten. Hierbei stelle ich fest, dass die mäßige Röntgenemission entweder sehr hohe Teilchendichten erfordert oder eine Teilchenverteilung erfordert, die bei Lorentz-Faktoren, die dem Röntgenband entsprechen, abgeschnitten ist. Für das letzte Kapitel der Disseration analysiere ich SINFONI Archivdaten der zentralen ∼ 30 ′′ × 30 ′′ Bogensekunden des Galaktischen Zentrums. Diese Analyse führt zum bis dato größten spektroskopischen Katalog dieser Region. Durch die Kombination dieser Daten konnte ich über 2800 Sterne in jung und alt klassifizieren. Über 200 junge Sterne konnten spektroskpisch identifiziert werden. Für 35 dieser junge Sterne konnte eine vollständige Lösung der Orbitgleichungen gefunden werden. Für die anderen 166 Sterne sind nur fünf von sechs Phasenraumkoordinaten bekannt. Ich stelle eine neue, und statistisch formale, Methode vor, welche die Bestimmung der Posteriorverteilung der Phasenraumkoordinaten erlaubt. Diese neue Methode erlaubt es mir, die Posteriorverteilung der Orbitelemente zu bestimmen und die Posteriorverteilung des Drehmoments der jungen Sterne zu bestimmen. Damit kann ich zeigen, dass mindestens vier verschiedene kinematische Strukturen im Galaktischen Zentrum statistisch signifkant sind. Ich bestätige die Präsenz der bekannten verdrehten Sternenscheibe, die sich im Uhrzeigersinn dreht, und der Sternenscheibe im Gegenuhrzeigersinn. Desweiteren kann ich eine neue Sternenscheibe im Galaktischen Zentrum nachweisen. Diese reichhaltige dynamische Struktur ist konsistent mit einer lokalen Bildung der jungen Sterne. Ich favorisiere die Entstehung der jungen Sternen nach Kollision zweier Molekülgaswolken.The Galactic Center, and the massive black hole Sagittarius A* (Sgr A*) therein, represent one of the most exotic places known to mankind. In this thesis, I present two aspects of the Galactic Center: the accretion flow in the direct proximity of the massive Black Hole and the distribution of young stars in its neighbourhood. The thesis has led to three novel observations of Sgr A*’s spectral energy distribution (SED), which I present in the first three chapters of the thesis. In the last chapter of the thesis, I present my results on the young star population found in the Galactic Center. In the first chapter, I report on the simultaneous detection of Sgr A* in two far-infrared observation bands at 160 μm and 100 μm. These are the first observations of Sgr A* in this wavelength regime obtained using the PACS camera on-board the Herschel space-telescope. The measurements are enabled by a custom-tailored data reduction pipeline, which allow far-infrared differential flux measurements in the Galactic Center at an unprecedented noise level. This led to the detection of variable flux at a significance level of 4.5σ at 160 μm and 1.6σ at 100 μm. The detection of variable flux confirms the turn-over of the SED in the sub-mm, and constrains the electron density, magnetic field strength and electron temperature. The results, in combination with modern ALMA observations of Sgr A*, imply lower than previously measured sub-mm fluxes of Sgr A* which require higher electron temperatures. This implies that the accretion flow is optically thin in the sub-mm, and parts of the mm regime. In the second chapter, to study the flux distribution of Sgr A*. I derive the first coherent flux measurement of Sgr A* obtained from the novel dual beam observing mode in 2019. Furthermore, I use light curves of the year 2017 and 2018 which were published in literature before. Due to the very high spatial resolution of GRAVITY Sgr A*’s flux is unconfused from the light of near-by stars, which severely limited similar adaptive optics-assisted studies in the past. This allows, for the first time, to detect Sgr A* at times it is observed with GRAVITY. In consequence, I report the first purely-empirically derived and unconfused flux distribution of Sgr A* and am able to infer the 2.2 μm flux quantiles. I compare several statistical probability distributions to the observed flux distribution. I find that the flux distribution is log-right skewed and only poorly described by a log-normal distribution. The flux distribution is well described by a two-component model: a quiescent log-normal distribution in combination with a powerlaw tail. This manifests the two component consistent of a flaring and quiescence state scenario proposed for Sgr A*. In this scenario, occasional bright near-infrared and X-ray flares are generated in localized and heated zones of the accretion flow, which are distinct from the variable quiescence emission. In the third chapter of this thesis, I study the properties of such a flare. I report the detection of a simultaneous near-infrared bright and moderate X-ray flare. I use the acquisition camera of GRAVITY to derive simultaneous H-band observations alongside the interfero-metric K-band observation. I combine the two near-infrared light curves with simultaneous observations obtained by the Spitzer, Chandra and NuSTAR spacecrafts. With the help of these flux measurements I model the emission region in the flare-scenario and compute the flare’s SED taking into account synchrotron and synchrotron self-Compton emission. This allows me to derive the properties of electron population responsible for the emission. I find that the moderate X-ray emission either requires very high particle densities or a particle distribution which is cut at Lorentz factors corresponding to the X-ray band. In the last chapter of the thesis I present the largest spectroscopic survey of the Galactic Center to date (∼ 30 ′′ ×30 ′′ ). Combining all available SINFONI observations of the Galactic Center allows me to classify over 2800 stars into young and old stars. My work now includes over 230 young stars, for 35 of which full orbital solutions have been determined. For the other 198 young stars only five of six phase space coordinates are known. I present a new, and statistically rigorous method to determine their posterior phase space distribution. This allows to determine the posterior distribution of orbital elements, and, specifically, to determine the ensemble angular momentum direction. Using the new statistical method I show that at least four kinematic structures in the Galactic Center are statistically significant. I confirm the presence of a warp of the clockwise disk, and the presence of a counter-clockwise disk. In addition to the previously introduced, but disputed kinematic features, I show that a third disk of young stars is present in the Galactic Center. This rich dynamical structure is consistent with an in-situ star formation scenario, and specifically, I favour a star formation event after the collision of two giant molecular clouds