61 research outputs found
Triple-C: resource-usage prediction for semi-automatic parallelization of groups of dynamic image-processing tasks
With the emergence of dynamic video processing, such as in image analysis, runtime estimation of resource usage would be highly attractive for automatic parallelization and QoS control with shared resources. A possible solution is to characterize the application execution using model descriptions of the resource usage. In this paper, we introduce Triple-C, a prediction model for computation, cache-memory and communication-bandwidth usage with scenario-based Markov chains. As a typical application, we explore a medical imaging function to enhance objects of interest in X-ray angiography sequences. Experimental results show that our method can be successfully applied to describe the resource usage for dynamic image-processing tasks, even if the flow graph dynamically switches between groups of tasks. An average prediction accuracy of 97% is reached with sporadic excursions of the prediction error up to 20-30%. As a case study, we exploit the prediction results for semi-automatic parallelization. Results show that with Triple-C prediction, dynamic processing tasks can be executed in real-time with a constant low latency
Evolution of progenitor stars of Type Ibc supernovae and long gamma-ray bursts
We discuss how rotation and binary interactions may be related to the
diversity of type Ibc supernovae and long gamma-ray bursts. After presenting
recent evolutionary models of massive single and binary stars including
rotation, the Tayler-Spruit dynamo and binary interactions, we argue that the
nature of SNe Ibc progenitors from binary systems may not significantly differ
from that of single star progenitors in terms of rotation, and that most long
GRB progenitors may be produced via the quasi-chemically homogeneous evolution
at sub-solar metallicity. We also briefly discuss the possible role of magnetic
fields generated in the convective core of a massive star for the transport of
angular momentum, which is potentially important for future stellar evolution
models of supernova and GRB progenitors.Comment: 6 pages, 4 figures, to appear in IAU Symp. 250, Massive Stars as
Cosmic Engines, Kauai (HI), 12/2007, ed. F. Bresolin, P. Crowther, & J. Pul
Probing the internal rotation of pre-white dwarf stars with asteroseismology: the case of PG 122+200
We put asteroseismological constraints on the internal rotation profile of
the GW Vir (PG1159-type) star PG 0122+200. To this end we employ a
state-of-the-art asteroseismological model for this star and we assess the
expected frequency splittings induced by rotation adopting a forward approach
in which we compare the theoretical frequency separations with the observed
ones assuming different types of plausible internal rotation profiles. We also
employ two asteroseismological inversion methods for the inversion of the
rotation profile of PG 0122+200. We find evidence for differential rotation in
this star. We demonstrate that the frequency splittings of the rotational
multiplets exhibited by PG 0122+200 are compatible with a rotation profile in
which the central regions are spinning about 2.4 times faster than the stellar
surface.Comment: 8 pages, 6 figures, 2 tables. To be published in MNRA
Decentralized subcontractor scheduling with divisible jobs
Subcontracting allows manufacturer agents to reduce completion times of their jobs and thus obtain savings. This paper addresses the coordination of decentralized scheduling systems with a single subcontractor and several agents having divisible jobs. Assuming complete information, we design parametric pricing schemes that strongly coordinate this decentralized system, i.e., the agents’ choices of subcontracting intervals always result in efficient schedules. The subcontractor’s revenue under the pricing schemes depends on a single parameter which can be chosen to make the revenue as close to the total savings as required. Also, we give a lower bound on the subcontractor’s revenue for any coordinating pricing scheme. Allowing private information about processing times, we prove that the pivotal mechanism is coordinating, i.e., agents are better off by reporting their true processing times, and by participating in the subcontracting. We show that the subcontractor’s maximum revenue with any coordinating mechanism under private information equals the lower bound of that with coordinating pricing schemes under complete information. Finally, we address the asymmetric case where agents obtain savings at different rates per unit reduction in completion times. We show that coordinating pricing schemes do not always exist in this case
Rotating massive main-sequence stars:I. Grids of evolutionary models and isochrones⋆
We present a dense grid of evolutionary tracks and isochrones of rotating massive main-sequence stars. We provide three grids with different initial compositions tailored to compare with early OB stars in the Small and Large Magellanic Clouds and in the Galaxy. Each grid covers masses ranging from 5 to 60 M and initial rotation rates between 0 and about 600 km s−1. To calibrate our models we used the results of the VLT-FLAMES Survey of Massive Stars. We determine the amount of convective overshooting by using the observed drop in rotation rates for stars with surface gravities logg <3.2 to determine the width of the main sequence. We calibrate the efficiency of rotationally induced mixing using the nitrogen abundance determinations for B stars in the Large Magellanic cloud. We describe and provide evolutionary tracks and the evolution of the central and surface abundances. In particular, we discuss the occurrence of quasi-chemically homogeneous evolution, i.e. the severe effects of efficient mixing of the stellar interior found for the most massive fast rotators. We provide a detailed set of isochrones for rotating stars. Rotation as an initial parameter leads to a degeneracy between the age and the mass of massive main sequence stars if determined from its observed location in the Hertzsprung-Russell diagram. We show that the consideration of surface abundances can resolve this degeneracy
Rotational mixing in massive binaries: detached short-period systems
Models of rotating single stars can successfully account for a wide variety
of observed stellar phenomena, such as the surface enhancements of N and He.
However, recent observations have questioned the idea that rotational mixing is
the main process responsible for the surface enhancements, emphasizing the need
for a strong and conclusive test.
We investigate the consequences of rotational mixing for massive
main-sequence stars in short-period binaries. In these systems the tides spin
up the stars to rapid rotation. We use a state-of-the-art stellar evolution
code including the effect of rotational mixing, tides, and magnetic fields. We
discuss the surface abundances expected in massive close binaries (M1~20 solar
masses) and we propose using such systems to test the concept of rotational
mixing. As these short-period binaries often show eclipses, their parameters
can be determined with high accuracy, allowing for a direct comparison with
binary evolution models.
In more massive close systems (M1~50 solar masses, Porb<~2 days) we find that
helium is efficiently mixed throughout the envelope. The star remains blue and
compact during the main-sequence phase. It stays within its Roche lobe while it
gradually becomes a helium star. It is the less massive star, in which the
effects of rotational mixing are less pronounced, which fills its Roche lobe
first. We propose that this evolution path provides an alternative channel for
the formation of tight Wolf-Rayet binaries with a main-sequence companion and
might explain massive black hole binaries such as the intriguing system M33
X-7.Comment: Accepted for publication in A&A, in pres
Resource prediction and quality control for parallel execution of heterogeneous medical imaging tasks
We have established a novel control system for combining the parallel execution of deterministic and non-deterministic medical imaging applications on a single platform, sharing the same constrained resources. The control system aims at avoiding resource overload and ensuring throughput and latency of critical applications, by means of accurate resource-usage prediction. Our approach is based on modeling the required computation tasks, by employing a combination of weighted moving-average filtering and scenario-based Markov chains to predict the execution. Experimental validation on medical image processing shows an accuracy of 97%. As a result, the latency variation within non-deterministic analysis applications is reduced by 70% by adaptively splitting/merging of tasks. Furthermore, the parallel execution of a deterministic live-viewing application features constant throughput and latency by dynamically switching between quality modes. Interestingly, our solution can successfully be reused for alternative applications with several parallel streams, like in surveillance
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