324 research outputs found
Loo.py: From Fortran to performance via transformation and substitution rules
A large amount of numerically-oriented code is written and is being written
in legacy languages. Much of this code could, in principle, make good use of
data-parallel throughput-oriented computer architectures. Loo.py, a
transformation-based programming system targeted at GPUs and general
data-parallel architectures, provides a mechanism for user-controlled
transformation of array programs. This transformation capability is designed to
not just apply to programs written specifically for Loo.py, but also those
imported from other languages such as Fortran. It eases the trade-off between
achieving high performance, portability, and programmability by allowing the
user to apply a large and growing family of transformations to an input
program. These transformations are expressed in and used from Python and may be
applied from a variety of settings, including a pragma-like manner from other
languages.Comment: ARRAY 2015 - 2nd ACM SIGPLAN International Workshop on Libraries,
Languages and Compilers for Array Programming (ARRAY 2015
A 380 GHz SIS receiver using Nb/AlO(x)/Nb junctions for a radioastronomical balloon-borne experiment: PRONAOS
The superheterodyne detection technique used for the spectrometer instrument of the PRONAOS project will provide a very high spectral resolution (delta nu/nu = 10(exp -6)). The most critical components are those located at the front-end of the receiver: their contribution dominates the total noise of the receiver. Therefore, it is important to perform accurate studies for specific components, such as mixers and multipliers working in the submillimeter wave range. Difficulties in generating enough local oscillator (LO) power at high frequencies make SIS mixers very desirable for operation above 300 GHz. The low LO power requirements and the low noise temperature of these mixers are the primary reason for building an SIS receiver. This paper reports the successful fabrication of small (less than or equal to 1 sq micron) Nb/Al-O(x)/Nb junctions and arrays with excellent I-V characteristics and very good reliability, resulting in a low noise receiver performance measured in the 368/380 GHz frequency range
An experimental testbed for NEAT to demonstrate micro-pixel accuracy
NEAT is an astrometric mission proposed to ESA with the objectives of
detecting Earth-like exoplanets in the habitable zone of nearby solar-type
stars. In NEAT, one fundamental aspect is the capability to measure stellar
centroids at the precision of 5e-6 pixel. Current state-of-the-art methods for
centroid estimation have reached a precision of about 4e-5 pixel at Nyquist
sampling. Simulations showed that a precision of 2 micro-pixels can be reached,
if intra and inter pixel quantum efficiency variations are calibrated and
corrected for by a metrology system. The European part of the NEAT consortium
is designing and building a testbed in vacuum in order to achieve 5e-6 pixel
precision for the centroid estimation. The goal is to provide a proof of
concept for the precision requirement of the NEAT spacecraft. In this paper we
give the basic relations and trade-offs that come into play for the design of a
centroid testbed and its metrology system. We detail the different conditions
necessary to reach the targeted precision, present the characteristics of our
current design and describe the present status of the demonstration.Comment: SPIE proceeding
More Legal Transformations for Locality
Distinguished paper award Commercial link : http://www.springerlink.de ALCHEMY/http://www.springer.comProgram transformations are one of the most valuable compiler techniques to improve data locality. However, restructuring compilers have a hard time coping with data dependences. A typical solution is to focus on program parts where the dependences are simple enough to enable any transformation. For more complex problems is only addressed the question of checking whether a transformation is legal or not. In this paper we propose to go further. Starting from a transformation with no guarantee on legality, we show how we can correct it for dependence satisfaction with no consequence on its locality properties. Generating code having the best locality is a direct application of this result
Complexity of Bradley-Manna-Sipma Lexicographic Ranking Functions
In this paper we turn the spotlight on a class of lexicographic ranking
functions introduced by Bradley, Manna and Sipma in a seminal CAV 2005 paper,
and establish for the first time the complexity of some problems involving the
inference of such functions for linear-constraint loops (without precondition).
We show that finding such a function, if one exists, can be done in polynomial
time in a way which is sound and complete when the variables range over the
rationals (or reals). We show that when variables range over the integers, the
problem is harder -- deciding the existence of a ranking function is
coNP-complete. Next, we study the problem of minimizing the number of
components in the ranking function (a.k.a. the dimension). This number is
interesting in contexts like computing iteration bounds and loop
parallelization. Surprisingly, and unlike the situation for some other classes
of lexicographic ranking functions, we find that even deciding whether a
two-component ranking function exists is harder than the unrestricted problem:
NP-complete over the rationals and -complete over the integers.Comment: Technical report for a corresponding CAV'15 pape
ExTrA: Exoplanets in Transit and their Atmospheres
The ExTrA facility, located at La Silla observatory, will consist of a
near-infrared multi-object spectrograph fed by three 60-cm telescopes. ExTrA
will add the spectroscopic resolution to the traditional differential
photometry method. This shall enable the fine correction of color-dependent
systematics that would otherwise hinder ground-based observations. With both
this novel method and an infrared-enabled efficiency, ExTrA aims to find
transiting telluric planets orbiting in the habitable zone of bright nearby M
dwarfs. It shall have the versatility to do so by running its own independent
survey and also by concurrently following-up on the space candidates unveiled
by K2 and TESS. The exoplanets detected by ExTrA will be amenable to
atmospheric characterisation with VLTs, JWST, and ELTs and could give our first
peek into an exo-life laboratory.Comment: 15 pages, 11 figures, SPIE 201
On Multiphase-Linear Ranking Functions
Multiphase ranking functions () were proposed as a means
to prove the termination of a loop in which the computation progresses through
a number of "phases", and the progress of each phase is described by a
different linear ranking function. Our work provides new insights regarding
such functions for loops described by a conjunction of linear constraints
(single-path loops). We provide a complete polynomial-time solution to the
problem of existence and of synthesis of of bounded depth
(number of phases), when variables range over rational or real numbers; a
complete solution for the (harder) case that variables are integer, with a
matching lower-bound proof, showing that the problem is coNP-complete; and a
new theorem which bounds the number of iterations for loops with
. Surprisingly, the bound is linear, even when the
variables involved change in non-linear way. We also consider a type of
lexicographic ranking functions, , more expressive than types
of lexicographic functions for which complete solutions have been given so far.
We prove that for the above type of loops, lexicographic functions can be
reduced to , and thus the questions of complexity of
detection and synthesis, and of resulting iteration bounds, are also answered
for this class.Comment: typos correcte
First experimental results of very high accuracy centroiding measurements for the neat astrometric mission
NEAT is an astrometric mission proposed to ESA with the objectives of
detecting Earth-like exoplanets in the habitable zone of nearby solar-type
stars. NEAT requires the capability to measure stellar centroids at the
precision of 5e-6 pixel. Current state-of-the-art methods for centroid
estimation have reached a precision of about 2e-5 pixel at two times Nyquist
sampling, this was shown at the JPL by the VESTA experiment. A metrology system
was used to calibrate intra and inter pixel quantum efficiency variations in
order to correct pixelation errors. The European part of the NEAT consortium is
building a testbed in vacuum in order to achieve 5e-6 pixel precision for the
centroid estimation. The goal is to provide a proof of concept for the
precision requirement of the NEAT spacecraft. In this paper we present the
metrology and the pseudo stellar sources sub-systems, we present a performance
model and an error budget of the experiment and we report the present status of
the demonstration. Finally we also present our first results: the experiment
had its first light in July 2013 and a first set of data was taken in air. The
analysis of this first set of data showed that we can already measure the pixel
positions with an accuracy of about 1e-4 pixel.Comment: SPIE conference proceeding
A detector interferometric calibration experiment for high precision astrometry
Context: Exoplanet science has made staggering progress in the last two
decades, due to the relentless exploration of new detection methods and
refinement of existing ones. Yet astrometry offers a unique and untapped
potential of discovery of habitable-zone low-mass planets around all the
solar-like stars of the solar neighborhood. To fulfill this goal, astrometry
must be paired with high precision calibration of the detector.
Aims: We present a way to calibrate a detector for high accuracy astrometry.
An experimental testbed combining an astrometric simulator and an
interferometric calibration system is used to validate both the hardware needed
for the calibration and the signal processing methods. The objective is an
accuracy of 5e-6 pixel on the location of a Nyquist sampled polychromatic point
spread function.
Methods: The interferometric calibration system produced modulated Young
fringes on the detector. The Young fringes were parametrized as products of
time and space dependent functions, based on various pixel parameters. The
minimization of func- tion parameters was done iteratively, until convergence
was obtained, revealing the pixel information needed for the calibration of
astrometric measurements.
Results: The calibration system yielded the pixel positions to an accuracy
estimated at 4e-4 pixel. After including the pixel position information, an
astrometric accuracy of 6e-5 pixel was obtained, for a PSF motion over more
than five pixels. In the static mode (small jitter motion of less than 1e-3
pixel), a photon noise limited precision of 3e-5 pixel was reached
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