Janus II: a new generation application-driven computer for spin-system simulations

This paper describes the architecture, the development and the implementation of Janus II, a new generation application-driven number cruncher optimized for Monte Carlo simulations of spin systems (mainly spin glasses). This domain of computational physics is a recognized grand challenge of high-performance computing: the resources necessary to study in detail theoretical models that can make contact with experimental data are by far beyond those available using commodity computer systems. On the other hand, several specific features of the associated algorithms suggest that unconventional computer architectures, which can be implemented with available electronics technologies, may lead to order of magnitude increases in performance, reducing to acceptable values on human scales the time needed to carry out simulation campaigns that would take centuries on commercially available machines. Janus II is one such machine, recently developed and commissioned, that builds upon and improves on the successful JANUS machine, which has been used for physics since 2008 and is still in operation today. This paper describes in detail the motivations behind the project, the computational requirements, the architecture and the implementation of this new machine and compares its expected performances with those of currently available commercial systems.Comment: 28 pages, 6 figure

Monte Carlo Simulations of Spin Glasses on Cell Broadband Engine

Several large-scale computational scientific problems require high-end computing systems to be solved. In the recent years, design of multi-core architectures delivers on a single chip tens or hundreds Gflops of peak computing performance, with high power dissipation efficiency, and it makes available computational power previously available only on high-end multi-processor systems. The aim of this Ph.D. thesis is to study the capability of multi-core processors for scientific programming, analyzing sustained performance, issues related to multicore programming, data distribution, synchronization, in order to define a set of guideline rules to optimize scientific applications for this class of architectures. As an example of multi-core processor, we consider the Cell Broadband Engine (CBE), developed by Sony, IBM and Toshiba. The CBE is one of the most powerful multi-core CPU current available, integrating eight cores and delivering a peak performance of 200 Gflops in single precision and 100 Gflops in double precision. As case of study, we analyze the performances of CBE for Monte Carlo simulations of the Edwards-Anderson Spin Glass model, a paradigm in theoretical and condensed matter physics, used to describe complex systems characterized by phase transitions (such as the para-ferro transition in magnets) or model “frustrated” dynamics. We descrive several strategies for the distribution of data set among on-chip and off-chip memories and propose analytic models to find out the balance between computational and memory access time as a function of both algorithmic and architectural parameters. We use the analytic models to set the parameters of the algorithm, like for example size of data structures and scheduling of operations, to optimize execution of Monte Carlo spin glass simulations on the CBE architecture

JANUS: an FPGA-based System for High Performance Scientific Computing

This paper describes JANUS, a modular massively parallel and reconfigurable FPGA-based computing system. Each JANUS module has a computational core and a host. The computational core is a 4x4 array of FPGA-based processing elements with nearest-neighbor data links. Processors are also directly connected to an I/O node attached to the JANUS host, a conventional PC. JANUS is tailored for, but not limited to, the requirements of a class of hard scientific applications characterized by regular code structure, unconventional data manipulation instructions and not too large data-base size. We discuss the architecture of this configurable machine, and focus on its use on Monte Carlo simulations of statistical mechanics. On this class of application JANUS achieves impressive performances: in some cases one JANUS processing element outperfoms high-end PCs by a factor ~ 1000. We also discuss the role of JANUS on other classes of scientific applications.Comment: 11 pages, 6 figures. Improved version, largely rewritten, submitted to Computing in Science & Engineerin

Simulating spin models on GPU

Over the last couple of years it has been realized that the vast computational power of graphics processing units (GPUs) could be harvested for purposes other than the video game industry. This power, which at least nominally exceeds that of current CPUs by large factors, results from the relative simplicity of the GPU architectures as compared to CPUs, combined with a large number of parallel processing units on a single chip. To benefit from this setup for general computing purposes, the problems at hand need to be prepared in a way to profit from the inherent parallelism and hierarchical structure of memory accesses. In this contribution I discuss the performance potential for simulating spin models, such as the Ising model, on GPU as compared to conventional simulations on CPU.Comment: 5 pages, 4 figures, elsarticl

Performance potential for simulating spin models on GPU

Graphics processing units (GPUs) are recently being used to an increasing degree for general computational purposes. This development is motivated by their theoretical peak performance, which significantly exceeds that of broadly available CPUs. For practical purposes, however, it is far from clear how much of this theoretical performance can be realized in actual scientific applications. As is discussed here for the case of studying classical spin models of statistical mechanics by Monte Carlo simulations, only an explicit tailoring of the involved algorithms to the specific architecture under consideration allows to harvest the computational power of GPU systems. A number of examples, ranging from Metropolis simulations of ferromagnetic Ising models, over continuous Heisenberg and disordered spin-glass systems to parallel-tempering simulations are discussed. Significant speed-ups by factors of up to 1000 compared to serial CPU code as well as previous GPU implementations are observed.Comment: 28 pages, 15 figures, 2 tables, version as publishe

Directive dielectric designs for high efficiency photovoltaics

Solar energy is, together with wind, one of the main sources of renewable energy for the future, mostly coming from of solar cells based on photovoltaics. The technique has rapidly developed over the past decades, resulting in large efficiency increases and tremendous price reduction. But further improvement is needed to meet the demands for the energy transition. In this thesis we present several concepts and designs to achieve this, all based on directive light emission from dielectric nanophotonic structures. In chapter 2 we explain why not only light absorption, but also light emission in an important parameter for efficient photovoltaics. In the following chapters we present different designs that can lead to the described efficiency increases. In chapter 3 and 4 we work with a highly optimized nanophotonic microlens. We first show how this structure can give record directivity, and subsequently combine it with the new wonder material for photovoltaics, mixed halide perovskite, to create a self-optimizing system with even higher directivity. This self-optimization is exploited further in chapter 5 to make solar concentrator that exhibits self-tracking and diffuse light utilization. In the final two chapters we investigate luminescent solar concentrators that make use of directional light emitters and show a great potential for efficiency increase. Overall we show a variety of directive dielectric designs that each can lead to novel device applications

Novel up-conversion concentrating photovoltaic concepts

This thesis summarises a set of experiments towards the integration of concentrating optics into up-conversion photovoltaics. Up-conversion in rare earths has been investigated here. This optical process is non-linear therefore a high solar irradiance is required. High solar irradiance is achievable by solar concentration. Two concentrating approaches were investigated in this thesis: The first approach involved the concentration of the incident solar irradiance into optical fibres. An optical system with spherical lenses and dielectric tapers was designed accordingly. A solar concentration of 2000 suns was realised at the end of a single optical fibre. In addition to the total solar concentration, the spectral dependence was characterised to account for the effect of chromatic aberrations. Then, the solar concentration could be transferred into rare earth-doped fibres. For this reason, a series of experiments on double-clad erbium-doped silicate fibres was carried out. Although up-conversion in this type of fibre is minimised, the measured power dependence agrees with up-conversion via excited state absorption. In the second approach, concentrating optics were integrated in up-conversion solar cells. The role of the optics was to couple the photons transmitted by the solar cell to the rare earth up-converter. Therefore, imaging and non-imaging optics were investigated, with the latter exhibiting ideal coupling characteristics; concentration and high transmission of the incident irradiance, but also efficient collection of the up-converted emission. Out of the non-imaging optics, the dielectric compound parabolic concentrator fulfilled these characteristics, indicating its novel use in up-conversion solar cells. Two erbium-doped up-converters were utilised in this approach, beta-phase hexagonal sodium yttrium tetrafluoride (β-NaYF4:25%Er3+) and barium diyttrium octafluoride (BaY2F8:30%Er3+). The latter performed best, with an external quantum efficiency (EQE) of 2.07% under 1493 nm illumination, while the former exhibited an EQE of 1.80% under 1523 nm illumination both at an irradiance of 0.02 W/cm2. This corresponds to a relative conversion efficiency of 0.199% and 0.163% under sub-band-gap illumination, respectively, for a solar cell of 17.6% under standard AM1.5G conditions. These values are among the highest in literature for up-conversion solar cells and show the potential of the concentrating concept that can be important for future directions of photovoltaics.Engineering and Physical Sciences Research Council (EPSRC)European Community's Seventh Framework Program (FP7/2007-2013

Research and Technology, 1998

This report selectively summarizes the NASA Lewis Research Center's research and technology accomplishments for the fiscal year 1998. It comprises 134 short articles submitted by the staff scientists and engineers. The report is organized into five major sections: Aeronautics, Research and Technology, Space, Engineering and Technical Services, and Commercial Technology. A table of contents and an author index have been developed to assist readers in finding articles of special interest. This report is not intended to he a comprehensive summary of all the research and technology work done over the past fiscal year. Most of the work is reported in Lewis-published technical reports, journal articles, and presentations prepared by Lewis staff and contractors. In addition, university grants have enabled faculty members and graduate students to engage in sponsored research that is reported at technical meetings or in journal articles. For each article in this report, a Lewis contact person has been identified, and where possible, reference documents are listed so that additional information can be easily obtained. The diversity of topics attests to the breadth of research and technology being pursued and to the skill mix of the staff that makes it possible. At the time of publication, NASA Lewis was undergoing a name change to the NASA John H. Glenn Research Center at Lewis Field

Report / Institute für Physik

The 2016 Report of the Physics Institutes of the Universität Leipzig presents a hopefully interesting overview of our research activities in the past year. It is also testimony of our scientific interaction with colleagues and partners worldwide. We are grateful to our guests for enriching our academic year with their contributions in the colloquium and within our work groups