6 research outputs found
Contributions à l'amélioration de l'extensibilité de simulations parallèles de plasmas turbulents
Energy needs around the world still increase despite the resources needed to produce fossil energy drain off year after year. An alternative way to produce energy is by nuclear fusion through magnetic confinement. Mastering this reaction is a challenge and represents an active field of the current research. In order to improve our understanding of the phenomena which occur during a fusion reaction, experiment and simulation are both put to use. The performed experiments, thanks to Tokamaks, allow some experimental reading. The process of experimental measurements is of great complexity and requires the use of the most advanced available technologies. Currently, these measurements do not give access to all scales of time and space of physical phenomenon. Numerical simulation permits the exploration of these scales which are still unreachable through experiment. An extreme computing power is mandatory to perform realistic simulations. The use of High Performance Computing (HPC) is necessary to access simulation of realistic cases. This requirement means the use of large computers, also known as supercomputers. The works realized through this thesis focuses on the optimization of the Gysela code which simulates a plasma turbulence. Optimization of a scientific application concerns mainly one of the three following points : (i ) the simulation of larger meshes, (ii ) the reduction of computing time and (iii ) the enhancement of the computation accuracy. The first part of this manuscript presents the contributions relative to simulation of larger mesh. Alike many simulation codes, getting more realistic simulations is often analogous to refine the meshes. The finer the mesh the larger the memory consumption. Moreover, during these last few years, the supercomputers had trend to provide less and less memory per computer core. For these reasons, we have developed a library, the libMTM (Modeling and Tracing Memory), dedicated to study precisely the memory consumption of parallel softwares. The libMTM tools allowed us to reduce the memory consumption of Gysela and to study its scalability. As far as we know, there is no other tool which provides equivalent features which allow the memoryscalability study. The second part of the manuscript presents the works relative to the optimization of the computation time and the improvement of accuracy of the gyroaverage operator. This operator represents a corner stone of the gyrokinetic model which is used by the Gysela application. The improvement of accuracy emanates from a change in the computing method : a scheme based on a 2D Hermite interpolation substitutes the Padé approximation. Although the new version of the gyroaverage operator is more accurate, it is also more expensive in computation time than the former one. In order to keep the simulation in reasonable time, diferent optimizations have been performed on the new computing method to get it competitive. Finally, we have developed a MPI parallelized version of the new gyroaverage operator. The good scalability of this new gyroaverage computer will allow, eventually, a reduction of MPI communication costs which are penalizing in Gysela.Les besoins en énergie dans le monde sont croissants alors que les ressources nécessaires pour la production d'énergie fossile s'épuisent d'année en année. Un des moyens alternatifs pour produire de l'énergie est la fusion nucléaire par confinement magnétique. La maîtrise de cette réaction est un défi et constitue un domaine actif de recherche. Pour améliorer notre connaissance des phénomènes qui interviennent lors de la réaction de fusion, deux approches sont mises en oeuvre : l'expérience et la simulation. Les expérience réalisées grâce aux Tokamaks permettent de prendre des mesures. Ceci nécessite l'utilisation des technologiques les plus avancées. Actuellement, ces mesures ne permettent pas d'accéder à toutes échelles de temps et d'espace des phénomènes physiques. La simulation numérique permet d'explorer ces échelles encore inaccessibles par l'expérience. Les ressources matérielles qui permettent d'effectuer des simulations réalistes sont conséquentes. L'usage du calcul haute performance (High Performance Computing HPC) est nécessaire pour avoir accès à ces simulations. Ceci se traduit par l'exploitation de grandes machines de calcul aussi appelées supercalculateurs. Les travaux réalisés dans cette thèse portent sur l'optimisation de l'application Gysela qui est un code de simulation de turbulence de plasma. L'optimisation d'un code de calcul scientifique vise classiquement l'un des trois points suivants : (i ) la simulation de plus grand domaine de calcul, (ii ) la réduction du temps de calcul et (iii ) l'amélioration de la précision des calculs. La première partie de ce manuscrit présente les contributions concernant la simulation de plus grand domaine. Comme beaucoup de codes de simulation, l'amélioration de la précision de la simulation est souvent synonyme de raffinement du maillage. Plus un maillage est fin, plus la consommation mémoire est grande. De plus, durant ces dernières années, les supercalculateurs ont eu tendance à disposer de moins en moins de mémoire par coeur de calcul. Pour ces raisons, nous avons développé une bibliothèque, la libMTM (Modeling and Tracing Memory), dédiée à l'étude précise de la consommation mémoire d'applications parallèles. Les outils de la libMTM ont permis de réduire la consommation mémoire de Gysela et d'étudier sa scalabilité. À l'heure actuelle, nous ne connaissons pas d'autre outil qui propose de fonctionnalités équivalentes permettant une étude précise de la scalabilité mémoire. La deuxième partie de ce manuscrit présente les travaux concernant l'optimisation du temps d'exécution et l'amélioration de la précision de l'opérateur de gyromoyenne. Cet opérateur est fondamental dans le modèle gyromagnétique qui est utilisé par l'application Gysela. L'amélioration de la précision vient d'un changement de la méthode de calcul : un schéma basé sur une interpolation de type Hermite vient remplacer l'approximation de Padé. Il s'avère que cette nouvelle version de l'opérateur est plus précise mais aussi plus coûteuse en terme de temps de calcul que l'opérateur existant. Afin que les temps de simulation restent raisonnables, différentes optimisations ont été réalisées sur la nouvelle méthode de calcul pour la rendre très compétitive. Nous avons aussi développé une version parallélisée en MPI du nouvel opérateur de gyromoyenne. La bonne scalabilité de cet opérateur de gyromoyenne permettra, à terme, de réduire des coûts en communication qui sont pénalisants dans une application parallèle comme Gysela
GENE-3D - ein globaler gyrokinetischer Turbulenzcode für Stellaratoren und gestörte Tokamaks
This thesis describes the development and application of GENE-3D, a global gyrokinetic turbulence HPC code for stellarators. The gyrokinetic equations as well as their implementation and the use of field-aligned coordinates in non-axisymmetric geometries are discussed. GENE-3D is benchmarked for validity and performance. Different geometries of Wendelstein 7-X are investigated for their influence on turbulent properties. Also the influence of the machine size on linear growth rates is studied.Diese Arbeit beschreibt die Entwicklung und Anwendung von GENE-3D, ein globaler gyrokinetischer Turbulenzcode für Stellaratoren. Die gyrokinetischen Gleichungen sowie deren Implementierung und das am Feld ausgerichtete Koordinatensystem werden für nicht-axisymmetrische Geometrien vorgestellt. GENE-3D wird auf Korrektheit getestet.Der Einfluß unterschiedlicher Wendelstein 7-X Geometrien auf den turbulenten Transport und der Einfluß der Maschinengröße auf die linearen Anwachsraten wird untersucht
Toward memory scalability of GYSELA code for extreme scale computers
International audienceGyrokinetic simulations lead to huge computational needs. Up to now, the Semi-Lagrangian code GYSELA performed large simulations using up to 65k cores. To understand more accurately the nature of plasma turbulence, finer resolutions are necessary, which make GYSELA a good candidate to exploit the computational power of future extreme scale machines. Among the Exascale challenges, the less memory per core is one of the most critical issues. This paper deals with memory management in order to reduce the memory peak and presents a general method to understand the memory behavior of an application when dealing with very large meshes. This enables us to extrapolate the behavior of GYSELA for expected capabilities of extreme scale machines
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Collisional and Electromagnetic Physics in Gyrokinetic Models
One of the most challenging problems facing plasma physicists today involves themodeling of plasma turbulence and transport in magnetic confinement experiments.The most successful model to this end so far is the reduced gyrokinetic model. Such amodel cannot be solved analytically, but can be used to simulate the plasma behaviorand transport with the help of present-day supercomputers. This has lead to the developmentof many different codes which simulate the plasma using the gyrokinetic modelin various ways. These models have achieved a large amount of success in describingthe core of the plasma for conventional tokamak devices. However, numerous difficultieshave been encountered when applying these models to more extreme parameterregimes, such as the edge and scrape-off layer of the tokamak, and high plasma devices,such as spherical tokamaks. The development and application of the gyrokineticmodel (specifically with the gyrokinetic code, GENE) to these more extreme parameterranges shall be the focus of this thesis.One of the main accomplishments during this thesis project is the development ofa more advanced collision operator suitable for studying the low temperature plasmaedge. The previous collision operator implemented in the code was found to artificiallycreate free energy at high collisionality, leading to numerical instabilities when oneattempted to model the plasma edge. This made such an analysis infeasible. Thenewly implemented collision operator conserves particles, momentum, and energy tomachine precision, and is guaranteed to dissipate free energy, even in a nonisothermalscenario. Additional finite Larmor radius correction terms have also been implementedin the local code, and the global code version of the collision operator has been adaptedfor use with an advanced block-structured grid scheme, allowing for more affordablecollisional simulations.The GENE code, along with the newly implemented collision operator developedin this thesis, has been applied to study plasma turbulence and transport in the edge(tor = 0:9) of an L-mode magnetic confinement discharge of ASDEX Upgrade. Ithas been found that the primary microinstabilities at that radial position are electrondrift waves destabilized by collisions and electromagnetic effects. At low toroidal modenumbers, ion temperature gradient driven modes and microtearing modes also seem toexist. In nonlinear simulations with the nominal experimental parameters, the simulatedelectron heat flux was four times higher than the experimental reconstruction,and the simulated ion heat flux was twice as high. However, both the ion and electronsimulated heat flux could be brought into agreement with experimental values by loweringthe input logarithmic electron temperature gradient by 40%. It was also foundthat the cross-phases between the electrostatic potential and the moments agreed wellfor the part of the binormal spectrum where the dominant transport occurred, and wasfairly poor at larger scales where minimal transport occurred.Finally, a new scheme for evaluating the electromagnetic fields has been developedto address the instabilities occurring in nonlinear local and global gyrokinetic simulationsat high plasma . This new scheme is based on evaluating the electromagneticinduction explicitly, and constructing the gyrokinetic equation based on the original distribution,rather than the modified distribution which implicitly takes into account theinduction. This new scheme removes the artificial instability occurring in global simulations,enabling the study of high scenarios with GENE. The new electromagneticscheme can also be generalized to a full-f implementation, however, it would requireupdating the field matrix every time-step to avoid the cancellation problem. The newscheme (including the parallel nonlinearity) does not remove the local instability, suggestingthat that instability (caused by magnetic field perturbations shorting out zonalflows) is part of the physics of the local model