43,484 research outputs found
Predicting Intermediate Storage Performance for Workflow Applications
Configuring a storage system to better serve an application is a challenging
task complicated by a multidimensional, discrete configuration space and the
high cost of space exploration (e.g., by running the application with different
storage configurations). To enable selecting the best configuration in a
reasonable time, we design an end-to-end performance prediction mechanism that
estimates the turn-around time of an application using storage system under a
given configuration. This approach focuses on a generic object-based storage
system design, supports exploring the impact of optimizations targeting
workflow applications (e.g., various data placement schemes) in addition to
other, more traditional, configuration knobs (e.g., stripe size or replication
level), and models the system operation at data-chunk and control message
level.
This paper presents our experience to date with designing and using this
prediction mechanism. We evaluate this mechanism using micro- as well as
synthetic benchmarks mimicking real workflow applications, and a real
application.. A preliminary evaluation shows that we are on a good track to
meet our objectives: it can scale to model a workflow application run on an
entire cluster while offering an over 200x speedup factor (normalized by
resource) compared to running the actual application, and can achieve, in the
limited number of scenarios we study, a prediction accuracy that enables
identifying the best storage system configuration
Gemini spectra of 12000K white dwarf stars
We report signal-to-noise ratio SNR ~ 100 optical spectra for four DA white
dwarf stars acquired with the GMOS spectrograph of the 8m Gemini north
telescope. These stars have 18<g<19 and are around Teff ~ 12000 K, were the
hydrogen lines are close to maximum. Our purpose is to test if the effective
temperatures and surface gravities derived from the relatively low
signal-to-noise ratio ( ~ 21) optical spectra acquired by the Sloan
Digital Sky Survey through model atmosphere fitting are trustworthy. Our
spectra range from 3800A to 6000A, therefore including H beta to H9. The H8
line was only marginally present in the SDSS spectra, but is crucial to
determine the gravity. When we compare the values published by Kleinman et al.
(2004) and Eisenstein et al. (2006) with our line-profile (LPT) fits, the
average differences are: Delta Teff ~ 320 K, systematically lower in SDSS, and
Delta log g ~ 0.24 dex, systematically larger in SDSS. The correlation between
gravity and effective temperature can only be broken at wavelengths bluer than
3800 A. The uncertainties in Teff are 60% larger, and in log g larger by a
factor of 4, than the Kleinman et al. (2004) and Eisenstein et al. (2006)
internal uncertainties.Comment: 11 pages and 8 figure
Graphene-based spin-pumping transistor
We demonstrate with a fully quantum-mechanical approach that graphene can
function as gate-controllable transistors for pumped spin currents, i.e., a
stream of angular momentum induced by the precession of adjacent
magnetizations, which exists in the absence of net charge currents.
Furthermore, we propose as a proof of concept how these spin currents can be
modulated by an electrostatic gate. Because our proposal involves nano-sized
systems that function with very high speeds and in the absence of any applied
bias, it is potentially useful for the development of transistors capable of
combining large processing speeds, enhanced integration and extremely low power
consumption
Graphene as a non-magnetic spin-current lens
In spintronics, the ability to transport magnetic information often depends
on the existence of a spin current traveling between two different magnetic
objects acting as source and probe. A large fraction of this information never
reaches the probe and is lost because the spin current tends to travel
omni-directionally. We propose that a curved boundary between a gated and a
non-gated region within graphene acts as an ideal lens for spin currents
despite being entirely of non-magnetic nature. We show as a proof of concept
that such lenses can be utilized to redirect the spin current that travels away
from a source onto a focus region where a magnetic probe is located, saving a
considerable fraction of the magnetic information that would be otherwise lost.Comment: 9 pages, 3 figure
Modelling a Particle Detector in Field Theory
Particle detector models allow to give an operational definition to the
particle content of a given quantum state of a field theory. The commonly
adopted Unruh-DeWitt type of detector is known to undergo temporary transitions
to excited states even when at rest and in the Minkowski vacuum. We argue that
real detectors do not feature this property, as the configuration "detector in
its ground state + vacuum of the field" is generally a stable bound state of
the underlying fundamental theory (e.g. the ground state-hydrogen atom in a
suitable QED with electrons and protons) in the non-accelerated case. As a
concrete example, we study a local relativistic field theory where a stable
particle can capture a light quantum and form a quasi-stable state. As
expected, to such a stable particle correspond energy eigenstates of the full
theory, as is shown explicitly by using a dressed particle formalism at first
order in perturbation theory. We derive an effective model of detector (at
rest) where the stable particle and the quasi-stable configurations correspond
to the two internal levels, "ground" and "excited", of the detector.Comment: 13 pages, references added, final versio
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