55,343 research outputs found
Processor Allocation for Optimistic Parallelization of Irregular Programs
Optimistic parallelization is a promising approach for the parallelization of
irregular algorithms: potentially interfering tasks are launched dynamically,
and the runtime system detects conflicts between concurrent activities,
aborting and rolling back conflicting tasks. However, parallelism in irregular
algorithms is very complex. In a regular algorithm like dense matrix
multiplication, the amount of parallelism can usually be expressed as a
function of the problem size, so it is reasonably straightforward to determine
how many processors should be allocated to execute a regular algorithm of a
certain size (this is called the processor allocation problem). In contrast,
parallelism in irregular algorithms can be a function of input parameters, and
the amount of parallelism can vary dramatically during the execution of the
irregular algorithm. Therefore, the processor allocation problem for irregular
algorithms is very difficult.
In this paper, we describe the first systematic strategy for addressing this
problem. Our approach is based on a construct called the conflict graph, which
(i) provides insight into the amount of parallelism that can be extracted from
an irregular algorithm, and (ii) can be used to address the processor
allocation problem for irregular algorithms. We show that this problem is
related to a generalization of the unfriendly seating problem and, by extending
Tur\'an's theorem, we obtain a worst-case class of problems for optimistic
parallelization, which we use to derive a lower bound on the exploitable
parallelism. Finally, using some theoretically derived properties and some
experimental facts, we design a quick and stable control strategy for solving
the processor allocation problem heuristically.Comment: 12 pages, 3 figures, extended version of SPAA 2011 brief announcemen
Modeling the mechanics of amorphous solids at different length and time scales
We review the recent literature on the simulation of the structure and
deformation of amorphous glasses, including oxide and metallic glasses. We
consider simulations at different length and time scales. At the nanometer
scale, we review studies based on atomistic simulations, with a particular
emphasis on the role of the potential energy landscape and of the temperature.
At the micrometer scale, we present the different mesoscopic models of
amorphous plasticity and show the relation between shear banding and the type
of disorder and correlations (e.g. elastic) included in the models. At the
macroscopic range, we review the different constitutive laws used in finite
element simulations. We end the review by a critical discussion on the
opportunities and challenges offered by multiscale modeling and transfer of
information between scales to study amorphous plasticity.Comment: 58 pages, 14 figure
Structure and Dielectric Properties of Amorphous High-kappa Oxides: HfO2, ZrO2 and their alloys
High- metal oxides are a class of materials playing an increasingly
important role in modern device physics and technology. Here we report
theoretical investigations of the properties of structural and lattice
dielectric constants of bulk amorphous metal oxides by a combined approach of
classical molecular dynamics (MD) - for structure evolution, and quantum
mechanical first principles density function theory (DFT) - for electronic
structure analysis. Using classical MD based on the Born-Mayer-Buckingham
potential function within a melt and quench scheme, amorphous structures of
high- metal oxides HfZrO with different values of the
concentration , are generated. The coordination numbers and the radial
distribution functions of the structures are in good agreement with the
corresponding experimental data. We then calculate the lattice dielectric
constants of the materials from quantum mechanical first principles, and the
values averaged over an ensemble of samples agree well with the available
experimental data, and are very close to the dielectric constants of their
cubic form.Comment: 5 pages, 4 figure
Glass and Jamming Transitions: From Exact Results to Finite-Dimensional Descriptions
Despite decades of work, gaining a first-principle understanding of amorphous
materials remains an extremely challenging problem. However, recent theoretical
breakthroughs have led to the formulation of an exact solution in the
mean-field limit of infinite spatial dimension, and numerical simulations have
remarkably confirmed the dimensional robustness of some of the predictions.
This review describes these latest advances. More specifically, we consider the
dynamical and thermodynamic descriptions of hard spheres around the dynamical,
Gardner and jamming transitions. Comparing mean-field predictions with the
finite-dimensional simulations, we identify robust aspects of the description
and uncover its more sensitive features. We conclude with a brief overview of
ongoing research.Comment: 5 figures, 26 page
Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique
A detailed description of the activation-relaxation technique (ART) is
presented. This method defines events in the configurational energy landscape
of disordered materials, such as a-Si, glasses and polymers, in a two-step
process: first, a configuration is activated from a local minimum to a nearby
saddle-point; next, the configuration is relaxed to a new minimum; this allows
for jumps over energy barriers much higher than what can be reached with
standard techniques. Such events can serve as basic steps in equilibrium and
kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure
A Phase Transition in a Quenched Amorphous Ferromagnet
Quenched thermodynamic states of an amorphous ferromagnet are studied. The
magnet is a countable collection of point particles chaotically distributed
over , . Each particle bears a real-valued spin with
symmetric a priori distribution; the spin-spin interaction is pair-wise and
attractive. Two spins are supposed to interact if they are neighbors in the
graph defined by a homogeneous Poisson point process. For this model, we prove
that with probability one: (a) quenched thermodynamic states exist; (b) they
are multiple if the particle density (i.e., the intensity of the underlying
point process) and the inverse temperature are big enough; (c) there exist
multiple quenched thermodynamic states which depend on the realizations of the
underlying point process in a measurable way
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