1,721,037 research outputs found
A new technique for a parallel dealiased pseudospectral Navier-Stokes code
No full textA novel aspect of a parallel procedure for the numerical simulation of the solution of the Navier-Stokes equations through the Fourier-Galerkin pseudospectral method is presented. It consists of a dealiased ("3/2" rule) transposition of the data that organizes the computations in the distributed direction in such a way that whenever a Fast Fourier Transform must be calculated, the algorithm will employ data stored solely an the proper memory of the processor which is computing it. This provide for the employment of standard routines for the computations of the Fourier transform. The aliasing removal procedure has been directly inserted into the transposition algorithm. The code is written for distributed memory computers, but not specifically for a peculiar architecture. The use on a variety of machines is allowed by the adoption of the Message Passing Interface library. The portability of the code is demonstrated by the similar performances, in particular the high efficiency, that all the machines tested show up to a number of parallel processors equal to 1/2 the truncation parameter N/2. Explicit time integration is used. The present code organization is relevant to physical and mathematical problems which require a three dimensional spectral treatment
Ab initio molecular dynamics study of the structure of emeraldine base polymers
No full textWe present a Car-Parrinello molecular-dynamics study of the three-dimensional crystalline regions of the emeraldine base II polymer. Our results are compared with the structure obtained from experimental x-ray diffraction spectra and with similar calculations performed on an isolated infinite chain of the emeraldine base polymer. Environmental interchain effects are thus highlighted
Base and salt 3D forms of Emeraldine II polymers by Car-Parrinello molecular dynamics
No full textWe have studied structural and electronic properties of the three-dimensional crystalline regions of Emeraldine II polymers, in the base (EB-II) and salt (ES-II) forms, by means of first principle Car-Parrinello molecular dynamics. We compare the geometrical structures of the polymer chains in the primitive cells of EB-II and ES-II, pointing out the structural effects due to the protonation with HCl of the iminic nitrogens in the EB-II chains, and the effect of the counterions between neighboring chains. We also analyze the HOMO electron density distribution, band structure and density of states of the resulting bipolaronic structure of ES-II, which is energetically stable and maintains semiconductor character
Car-Parrinello molecular dynamics study of electronic and structural properties of neutral polyanilines
No full textWe present an ab initio quantum-mechanical study, based on Car-Parrinello molecular dynamics, of electronic, optical, and structural properties of the three base forms of polyaniline polymers. To avoid finite-size effects encountered in the study of oligomers, we have represented each polymer by a periodic isolated infinite chain. We have provided detailed information on the morphology of each chain in terms of bond lengths, bond angles, torsion angles, twisting and bending of the chain backbone, analyzing also temperature effects. From the ground-state geometry optimization we have observed a definite nonplanarity of the nitrogen atoms which constitute the chain backbone in the emeraldine base polymer. Moreover, by monitoring the time evolution of the geometry of the pernigraniline base polymer, we have evidenced an oscillating quinoid-phenilene character of each chain ring. Electronic band-structure and density of states have been calculated for emeraldine, pernigraniline, and leucoemeraldine base polymers within the Kohn-Sham density-functional theory. We have analyzed the global electronic charge density and the nature of the highest occupied molecular orbital and lowest unoccupied molecular orbital levels and reproduced existing experimental absorption and ultraviolet photoelectron spectra
COUNTDOWN: A Run-Time Library for Performance-Neutral Energy Saving in MPI Applications
Full text linkPower and energy consumption are becoming key challenges for the supercomputers' exascale race. HPC systems' processors waist active power during communication and synchronization among the MPI processes in large-scale HPC applications. However, due to the time scale at which communication happens, transitioning into low-power states while waiting for the completion of each communication may introduce unacceptable overhead. In this article, we present COUNTDOWN, a run-time library for identifying and automatically reducing the power consumption of the CPUs during communication and synchronization. COUNTDOWN saves energy without penalizing the time-to-completion by lowering CPUs power consumption only during idle times for which power state transition overhead is negligible. This is done transparently to the user, without requiring labor-intensive and error-prone application code modifications, nor requiring recompilation of the application. We test our methodology on a production Tier-1 system. For the NAS benchmarks, COUNTDOWN saves between 6 and 50 percent energy, with a time-to-solution penalty lower than 5 percent. In a complete production - Quantum ESPRESSO - for a 3.5K cores run, COUNTDOWN saves 22.36 percent energy, with a performance penalty below 3 percent. Energy saving increases to 37 percent with a performance penalty of 6.38 percent, if the application is executed without communication tuning
HCl-doped conducting Emeraldine polymer studied by ab initio Car-Parrinello molecular dynamic
No full textWe present a Car-Parrinello molecular dynamics study of the highly conducting Emeraldine salt, which definitely settles the controversy between the polaronic and the bipolaronic lattice models present in the literature. Our treatment is fully microscopic and takes into account interchain interactions, individual chain conformation, and the dynamics of the HCl protonation. We have highlighted the peculiar role of the Cl− counterions both for the polymer structure and for the interpretation of its metallic character. Our study indicates that this metallic character is due to the π electrons along each chain with chlorine counterions in polaronic arrangement and that only the Pc2a lattice symmetry provides an x-ray spectrum in complete agreement with the experiments
Thermal Characterization of a Tier0 Datacenter Room in Normal and Thermal Emergency Conditions
No full textDatacenters are at the heart of the AI, Industry 4.0 and cloud revolution. A datacenter contains a large number of computing nodes hosted in a large temperature-controlled room. Due to the increasing total power and power density of computing nodes, the overall datacenter compute capacity is often capped by peak power consumption and temperature bottlenecks. To preserve the homogeneous performance assumption between all the nodes, complex cooling solution are required, but they might not be sufficient. In this work, we analysed and characterised the thermal properties of a Tier0 datacenter deploying advanced hybrid cooling technologies: specifically, we studied the spatial and temporal heterogeneity during production and cooling emergency hazards. This paper gives first quantitative evidence of thermal bottlenecks in real-life production workload, showing the presence of significant spatial thermal heterogeneity which could be exploited by thermal-aware job scheduling and datacenter-room run-time workload adaptation and distribution
COUNTDOWN - A run-time library for application-agnostic energy saving in MPI communication primitives
No full textEnergy and power consumption are prominent issues in today’s supercomputers and are foreseen as a limiting factor of future installations. In scientific computing, a significant amount of power is spent in the communication and synchronization-related idle times among distributed processes participating to the same application. However, due to the time scale at which communication happens, taking advantage of low-power states to reduce power in idle times in the computing resources, may introduce significant overheads. In this paper we present COUNTDOWN, a methodology and a tool for identifying and automatically reducing the frequency of the computing elements in order to save energy during communication and synchronization primitives. COUNTDOWN is able to filter out phases which would detriment the time to solution of the application transparently to the user, without touching the application code nor requiring recompilation of the application. We tested our methodology in a production Tier-0 system, a production application - Quantum ESPRESSO (QE) - with production datasets which can scale up to 3.5K cores. Experimental results show that our methodology saves 22.36% of energy consumption with a performance penalty of 2.88% in real production MPI-based application
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