arXiv subjects

Gabrielle Allen

Publications and source records attributed to Gabrielle Allen.

27 records · Page 2Linked to original sources

arXiv2007

Cactus Framework: Black Holes to Gamma Ray Bursts

Gamma Ray Bursts (GRBs) are intense narrowly-beamed flashes of gamma-rays of cosmological origin. They are among the most scientifically interesting astrophysical systems, and the riddle concerning their central engines and emission mechanisms is one of the most complex and challenging problems of astrophysics today. In this article we outline our petascale approach to the GRB problem and discuss the computational toolkits and numerical codes that are currently in use and that will be scaled up to run on emerging petaflop scale computing platforms in the near future. Petascale computing will require additional ingredients over conventional parallelism. We consider some of the challenges which will be caused by future petascale architectures, and discuss our plans for the future development of the Cactus framework and its applications to meet these challenges in order to profit from these new architectures.

cs.DC↗

arXiv2007

An Extensible Timing Infrastructure for Adaptive Large-scale Applications

Real-time access to accurate and reliable timing information is necessary to profile scientific applications, and crucial as simulations become increasingly complex, adaptive, and large-scale. The Cactus Framework provides flexible and extensible capabilities for timing information through a well designed infrastructure and timing API. Applications built with Cactus automatically gain access to built-in timers, such as gettimeofday and getrusage, system-specific hardware clocks, and high-level interfaces such as PAPI. We describe the Cactus timer interface, its motivation, and its implementation. We then demonstrate how this timing information can be used by an example scientific application to profile itself, and to dynamically adapt itself to a changing environment at run time.

cs.PF↗

arXiv2003

Toward standard testbeds for numerical relativity

In recent years, many different numerical evolution schemes for Einstein's equations have been proposed to address stability and accuracy problems that have plagued the numerical relativity community for decades. Some of these approaches have been tested on different spacetimes, and conclusions have been drawn based on these tests. However, differences in results originate from many sources, including not only formulations of the equations, but also gauges, boundary conditions, numerical methods, and so on. We propose to build up a suite of standardized testbeds for comparing approaches to the numerical evolution of Einstein's equations that are designed to both probe their strengths and weaknesses and to separate out different effects, and their causes, seen in the results. We discuss general design principles of suitable testbeds, and we present an initial round of simple tests with periodic boundary conditions. This is a pivotal first step toward building a suite of testbeds to serve the numerical relativists and researchers from related fields who wish to assess the capabilities of numerical relativity codes. We present some examples of how these tests can be quite effective in revealing various limitations of different approaches, and illustrating their differences. The tests are presently limited to vacuum spacetimes, can be run on modest computational resources, and can be used with many different approaches used in the relativity community.

gr-qc↗

arXiv2001

The Cactus Worm: Experiments with Dynamic Resource Discovery and Allocation in a Grid Environment

The ability to harness heterogeneous, dynamically available "Grid" resources is attractive to typically resource-starved computational scientists and engineers, as in principle it can increase, by significant factors, the number of cycles that can be delivered to applications. However, new adaptive application structures and dynamic runtime system mechanisms are required if we are to operate effectively in Grid environments. In order to explore some of these issues in a practical setting, we are developing an experimental framework, called Cactus, that incorporates both adaptive application structures for dealing with changing resource characteristics and adaptive resource selection mechanisms that allow applications to change their resource allocations (e.g., via migration) when performance falls outside specified limits. We describe here the adaptive resource selection mechanisms and describe how they are used to achieve automatic application migration to "better" resources following performance degradation. Our results provide insights into the architectural structures required to support adaptive resource selection. In addition, we suggest that this "Cactus Worm" is an interesting challenge problem for Grid computing.

cs.DC↗

arXiv2000

Towards an understanding of the stability properties of the 3+1 evolution equations in general relativity

We study the stability properties of the standard ADM formulation of the 3+1 evolution equations of general relativity through linear perturbations of flat spacetime. We focus attention on modes with zero speed of propagation and conjecture that they are responsible for instabilities encountered in numerical evolutions of the ADM formulation. These zero speed modes are of two kinds: pure gauge modes and constraint violating modes. We show how the decoupling of the gauge by a conformal rescaling can eliminate the problem with the gauge modes. The zero speed constraint violating modes can be dealt with by using the momentum constraints to give them a finite speed of propagation. This analysis sheds some light on the question of why some recent reformulations of the 3+1 evolution equations have better stability properties than the standard ADM formulation.

gr-qc↗

arXiv1999

Gravitational Collapse of Gravitational Waves in 3D Numerical Relativity

We demonstrate that evolutions of three-dimensional, strongly non-linear gravitational waves can be followed in numerical relativity, hence allowing many interesting studies of both fundamental and observational consequences. We study the evolution of time-symmetric, axisymmetric {\it and} non-axisymmetric Brill waves, including waves so strong that they collapse to form black holes under their own self-gravity. The critical amplitude for black hole formation is determined. The gravitational waves emitted in the black hole formation process are compared to those emitted in the head-on collision of two Misner black holes.

gr-qc↗

arXiv1998

Black Hole Spectroscopy: Determining Waveforms from 3D Excited Black Holes

We present the first results for Cauchy nonlinear evolution of 3D, nonaxisymmetric distorted black holes. We focus on the extraction and verification of 3D waveforms determined by numerical relativity. We show that the black hole evolution can be accurately followed through the ringdown period, and comparing with a recently developed perturbative evolution technique, we show that many waveforms in the black hole spectrum of modes, such as l=2 and l=4, including weakly excited nonaxisymmetric modes with m not zero, can be accurately evolved and extracted from the full nonlinear numerical evolution. We also identify new physics contained in higher modes, due to nonlinear effects. The implications for simulations related to gravitational wave astronomy are discussed.

gr-qc↗

arXiv1998

Evolution of Distorted Black Holes: A Perturbative Approach

We consider a series of distorted black hole initial data sets, and develop techniques to evolve them using the linearized equations of motion for the gravitational wave perturbations on a Schwarzschild background. We apply this to 2D and 3D distorted black hole spacetimes. In 2D, waveforms for different modes of the radiation are presented, comparing full nonlinear evolutions for different axisymmetric l-modes with perturbative evolutions. We show how axisymmetric black hole codes solving the full, nonlinear Einstein equations are capable of very accurate evolutions, and also how these techniques aid in studying nonlinear effects. In 3D we show how the initial data for the perturbation equations can be computed, and we compare with analytic solutions given from a perturbative expansion of the initial value problem. In addition to exploring the physics of these distorted black hole data sets, in particular allowing an exploration of linear, nonlinear, and mode mixing effects, this approach provides an important testbed for any fully nonlinear numerical code designed to evolve black hole spacetimes in 2D or 3D.

gr-qc↗

arXiv1997

Gravitational waves from pulsating stars: Evolving the perturbation equations for a relativistic star

We consider the perturbations of a relativistic star as an initial-value problem. Having discussed the formulation of the problem (the perturbation equations and the appropriate boundary conditions at the centre and the surface of the star) in detail we evolve the equations numerically from several different sets of initial data. In all the considered cases we find that the resulting gravitational waves carry the signature of several of the star's pulsation modes. Typically, the fluid $f$-mode, the first two $p$-modes and the slowest damped gravitational $w$-mode are present in the signal. This indicates that the pulsation modes may be an interesting source for detectable gravitational waves from colliding neutron stars or supernovae. We also survey the literature and find several indications of mode presence in numerical simulations of rotating core collapse and coalescing neutron stars. If such mode-signals can be detected by future gravitational-wave antennae one can hope to infer detailed information about neutron stars. Since a perturbation evolution should adequately describe the late time behaviour of a dynamically excited neutron star, the present work can also be used as a bench-mark test for future fully nonlinear simulations.

gr-qc↗