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Deveci M.,Ohio State University | Kaya K.,SabanciUniversity | Kaya K.,Ohio State University
Journal of Parallel and Distributed Computing | Year: 2015

We investigate hypergraph partitioning-based methods for efficient parallelization of communicating tasks. A good partitioning method should divide the load among the processors as evenly as possible and minimize the inter-processor communication overhead. The total communication volume is the most popular communication overhead metric which is reduced by the existing state-of-the-art hypergraph partitioners. However, other metrics such as the total number of messages, the maximum amount of data transferred by a processor, or a combination of them are equally, if not more, important. Existing hypergraph-based solutions use a two phase approach to minimize such metrics where in each phase, they minimize a different metric, sometimes at the expense of others. We propose a one-phase approach where all the communication cost metrics can be effectively minimized in a multi-objective setting and reductions can be achieved for all metrics together. For an accurate modeling of the maximum volume and the number of messages sent and received by a processor, we propose the use of directed hypergraphs. The directions on hyperedges necessitate revisiting the standard partitioning heuristics. We do so and propose a multi-objective, multi-level hypergraph partitioner called UMPa. The partitioner takes various prioritized communication metrics into account, and optimizes all of them together in the same phase. Compared to the state-of-the-art methods which only minimize the total communication volume, we show on a large number of problem instances that UMPa produces better partitions in terms of several communication metrics. © 2014 Elsevier Inc. All rights reserved. Source

Gruber D.,Max Planck Institute for Extraterrestrial Physics | Goldstein A.,University of Alabama in Huntsville | Von Ahlefeld V.W.,Max Planck Institute for Extraterrestrial Physics | Von Ahlefeld V.W.,University of Edinburgh | And 31 more authors.
Astrophysical Journal, Supplement Series | Year: 2014

In this catalog we present the updated set of spectral analyses of gamma-ray bursts (GRBs) detected by the Fermi Gamma-Ray Burst Monitor during its first four years of operation. It contains two types of spectra, time-integrated spectral fits and spectral fits at the brightest time bin, from 943 triggered GRBs. Four different spectral models were fitted to the data, resulting in a compendium of more than 7500 spectra. The analysis was performed similarly but not identically to Goldstein et al. All 487 GRBs from the first two years have been re-fitted using the same methodology as that of the 456 GRBs in years three and four. We describe, in detail, our procedure and criteria for the analysis and present the results in the form of parameter distributions both for the observer-frame and rest-frame quantities. The data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center. © 2014. The American Astronomical Society. All rights reserved.. Source

Dufosse F.,French Institute for Research in Computer Science and Automation | Kaya K.,SabanciUniversity | Ucar B.,University of Lyon
Journal of Parallel and Distributed Computing | Year: 2015

We propose two heuristics for the bipartite matching problem that are amenable to shared-memory parallelization. The first heuristic is very intriguing from a parallelization perspective. It has no significant algorithmic synchronization overhead and no conflict resolution is needed across threads. We show that this heuristic has an approximation ratio of around 0.632 under some common conditions. The second heuristic is designed to obtain a larger matching by employing the well-known Karp-Sipser heuristic on a judiciously chosen subgraph of the original graph. We show that the Karp-Sipser heuristic always finds a maximum cardinality matching in the chosen subgraph. Although the Karp-Sipser heuristic is hard to parallelize for general graphs, we exploit the structure of the selected subgraphs to propose a specialized implementation which demonstrates very good scalability. We prove that this second heuristic has an approximation guarantee of around 0.866 under the same conditions as in the first algorithm. We discuss parallel implementations of the proposed heuristics on a multicore architecture. Experimental results, for demonstrating speed-ups and verifying the theoretical results in practice, are provided. © 2015 Elsevier Inc. Source

Huppenkothen D.,University of Amsterdam | D'Angelo C.,University of Amsterdam | Watts A.L.,University of Amsterdam | Heil L.,University of Amsterdam | And 11 more authors.
Astrophysical Journal | Year: 2014

The discovery of quasi-periodic oscillations (QPOs) in magnetar giant flares has opened up prospects for neutron star asteroseismology. The scarcity of giant flares makes a search for QPOs in the shorter, far more numerous bursts from soft gamma repeaters (SGRs) desirable. In Huppenkothen et al., we developed a Bayesian method for searching for QPOs in short magnetar bursts, taking into account the effects of the complicated burst structure, and have shown its feasibility on a small sample of bursts. Here we apply the same method to a much larger sample from a burst storm of 286 bursts from SGR J1550-5418. We report a candidate signal at 260 Hz in a search of the individual bursts, which is fairly broad. We also find two QPOs at ∼93 Hz, and one at 127 Hz, when averaging periodograms from a number of bursts in individual triggers, at frequencies close to QPOs previously observed in magnetar giant flares. Finally, for the first time, we explore the overall burst variability in the sample and report a weak anti-correlation between the power-law index of the broadband model characterizing aperiodic burst variability and the burst duration: shorter bursts have steeper power-law indices than longer bursts. This indicates that longer bursts vary over a broader range of timescales and are not simply longer versions of the short bursts. © 2014. The American Astronomical Society. All rights reserved. Source

Kaneko Y.,SabanciUniversity | Gou E.,SabanciUniversity | Kouveliotou C.,NASA | Granot J.,University of Hertfordshire | And 11 more authors.
Astrophysical Journal | Year: 2010

SGRJ1550-5418 (previously known as AXP1E1547.0-5408 or PSRJ1550-5418) went into three active bursting episodes in 2008 October and in 2009 January and March, emitting hundreds of typical soft gamma repeater bursts in soft gamma rays. The second episode was especially intense, and our untriggered burst search on Fermi/Gamma-ray Burst Monitor (GBM) data (8-1000 keV) revealed 450 bursts emitted over 24 hr during the peak of this activity. Using the GBM data, we identified a 150 s long enhanced persistent emission during 2009 January 22 that exhibited intriguing timing and spectral properties: (1) clear pulsations up to 110 keV at the spin period of the neutron star (P 2.07 s, the fastest of all magnetars); (2) an additional (to a power-law) blackbody component required for the enhanced emission spectra with kT 17 keV; and (3) pulsed fraction that is strongly energy dependent and highest in the 50-74 keV energy band. A total isotropic-equivalent energy emitted during this enhanced emission is estimated to be 2.9 × 1040(D/5 kpc)2 erg. The estimated area of the blackbody emitting region of 0.046(D/5 kpc)2 km2 (roughly a few ×10-5 of the neutron star area) is the smallest "hot spot" ever measured for a magnetar and most likely corresponds to the size of magnetically confined plasma near the neutron star surface. © 2010. The American Astronomical Society. Source

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