25,783 research outputs found
Efficient Irregular Wavefront Propagation Algorithms on Hybrid CPU-GPU Machines
In this paper, we address the problem of efficient execution of a computation
pattern, referred to here as the irregular wavefront propagation pattern
(IWPP), on hybrid systems with multiple CPUs and GPUs. The IWPP is common in
several image processing operations. In the IWPP, data elements in the
wavefront propagate waves to their neighboring elements on a grid if a
propagation condition is satisfied. Elements receiving the propagated waves
become part of the wavefront. This pattern results in irregular data accesses
and computations. We develop and evaluate strategies for efficient computation
and propagation of wavefronts using a multi-level queue structure. This queue
structure improves the utilization of fast memories in a GPU and reduces
synchronization overheads. We also develop a tile-based parallelization
strategy to support execution on multiple CPUs and GPUs. We evaluate our
approaches on a state-of-the-art GPU accelerated machine (equipped with 3 GPUs
and 2 multicore CPUs) using the IWPP implementations of two widely used image
processing operations: morphological reconstruction and euclidean distance
transform. Our results show significant performance improvements on GPUs. The
use of multiple CPUs and GPUs cooperatively attains speedups of 50x and 85x
with respect to single core CPU executions for morphological reconstruction and
euclidean distance transform, respectively.Comment: 37 pages, 16 figure
Relativity as Support for Presentism: A Modest Evidential Argument
Presentism is roughly the view that only the present exists. This view requires an absolute simultaneity relation. The special theory of relativity, however, is highly successful and does not account for absolute simultaneity. This is widely regarded as an evidential threat to presentism. In what follows, I propose a modest evidential argument in support of presentism on the basis of the physical evidence itself. A weak relativity postulate is shown to follow from a weak light-speed postulate. The weak light-speed postulate, in turn, is shown to be more probable on presentism than on its main rival doctrine, eternalism. Specifically, when one accounts for possible worlds in which the space-time metric is Euclidean (+,+,+,+) rather than Lorentzian (-,+,+,+), the empirical evidence turns out to be more probable on presentism than on eternalism. If successful, this argument provides modest evidential support for presentism and against eternalism. However, the support is drawn from an unexpected source: the physical evidence itself
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Alternative conventions and geometry for special relativity
This paper argues that Einstein’s conventionalist definition of time is sufficient for, but not necessary to the geometric modelling of Special Relativity. A different convention allows that any time interval t, can be measured by dc, the distance travelled from an origin by the spherical wave-front of a light pulse, c. This means that the relationships represented by the hyperbolic geometry of Minkowski can also be represented by circular function geometry (CFG), where the spherical surface of c provides both a fourth set t, of frame-dependent co-ordinate points and a parameter s, for measuring intervals that are invariant between reference frames. However, sine values under the circle range from 1-0, rather than 1-∞. This does not allow that for a reference frame velocity ≈ c, any interval length ≈ ∞. Furthermore, since CFG does not subdivide space-time into past and future zones, it excludes the possibility of backwards time travel for signal velocities > c
Advanced relativistic VLBI model for geodesy
Our present relativistic part of the geodetic VLBI model for Earthbound
antennas is a consensus model which is considered as a standard for processing
high-precision VLBI observations. It was created as a compromise between a
variety of relativistic VLBI models proposed by different authors as documented
in the IERS Conventions 2010. The accuracy of the consensus model is in the
picosecond range for the group delay but this is not sufficient for current
geodetic pur- poses. This paper provides a fully documented derivation of a new
relativistic model having an accuracy substantially higher than one picosecond
and based upon a well accepted formalism of relativistic celestial mechanics,
astrometry and geodesy. Our new model fully confirms the consensus model at the
picosecond level and in several respects goes to a great extent beyond it. More
specifically, terms related to the acceleration of the geocenter are considered
and kept in the model, the gravitational time-delay due to a massive body
(planet, Sun, etc.) with arbitrary mass and spin-multipole moments is derived
taking into account the motion of the body, and a new formalism for the
time-delay problem of radio sources located at finite distance from VLBI
stations is presented. Thus, the paper presents a substantially elaborated
theoretical justification of the consensus model and its significant extension
that allows researchers to make concrete estimates of the magnitude of residual
terms of this model for any conceivable configuration of the source of light,
massive bodies, and VLBI stations. The largest terms in the relativistic time
delay which can affect the current VLBI observations are from the quadrupole
and the angular momentum of the gravitating bodies that are known from the
literature. These terms should be included in the new geodetic VLBI model for
improving its consistency.Comment: 37 pages, 4 figure
Radiative Contributions to the Effective Action of Self-Interacting Scalar Field on a Manifold with Boundary
The effect of quantum corrections to a conformally invariant field theory for
a self-interacting scalar field on a curved manifold with boundary is
considered. The analysis is most easily performed in a space of constant
curvature the boundary of which is characterised by constant extrinsic
curvature. An extension of the spherical formulation in the presence of a
boundary is attained through use of the method of images. Contrary to the
consolidated vanishing effect in maximally symmetric space-times the
contribution of the massless "tadpole" diagram no longer vanishes in
dimensional regularisation. As a result, conformal invariance is broken due to
boundary-related vacuum contributions. The evaluation of one-loop contributions
to the two-point function suggests an extension, in the presence of matter
couplings, of the simultaneous volume and boundary renormalisation in the
effective action.Comment: 14 pages, 1 figure. Additional references and minor elucidating
remarks added. To appear in Classical and Quantum Gravit
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