5,236 research outputs found

    Parallel Working-Set Search Structures

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    In this paper we present two versions of a parallel working-set map on p processors that supports searches, insertions and deletions. In both versions, the total work of all operations when the map has size at least p is bounded by the working-set bound, i.e., the cost of an item depends on how recently it was accessed (for some linearization): accessing an item in the map with recency r takes O(1+log r) work. In the simpler version each map operation has O((log p)^2+log n) span (where n is the maximum size of the map). In the pipelined version each map operation on an item with recency r has O((log p)^2+log r) span. (Operations in parallel may have overlapping span; span is additive only for operations in sequence.) Both data structures are designed to be used by a dynamic multithreading parallel program that at each step executes a unit-time instruction or makes a data structure call. To achieve the stated bounds, the pipelined data structure requires a weak-priority scheduler, which supports a limited form of 2-level prioritization. At the end we explain how the results translate to practical implementations using work-stealing schedulers. To the best of our knowledge, this is the first parallel implementation of a self-adjusting search structure where the cost of an operation adapts to the access sequence. A corollary of the working-set bound is that it achieves work static optimality: the total work is bounded by the access costs in an optimal static search tree.Comment: Authors' version of a paper accepted to SPAA 201

    A sensitive cloud chamber without radioactive sources

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    We present a sensitive diffusion cloud chamber which does not require any radioactive sources. A major difference from a commonly used chamber is use of a heat sink as its bottom plate. A result of a performance test of the chamber is given.Comment: 8 pages, 8 figures, iopart.cls, figures and references adde

    Oscillation Phenomena in the disk around the massive black hole Sagittarius A*

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    We report the detection of radio QPOs with structure changes using the Very Long Baseline Array (VLBA) at 43 GHz. We found conspicuous patterned changes of the structure with P = 16.8 +- 1.4, 22.2 +- 1.4, 31.2 +- 1.5, 56.4 +- 6 min, very roughly in a 3:4:6:10 ratio. The first two periods show a rotating one-arm structure, while the P = 31.4 min shows a rotating 3-arm structure, as if viewed edge-on. At the central 50 microasec the P = 56.4 min period shows a double amplitude variation of those in its surroundings. Spatial distributions of the oscillation periods suggest that the disk of SgrA* is roughly edge-on, rotating around an axis with PA = -10 degree. Presumably, the observed VLBI images of SgrA* at 43 GHz retain several features of the black hole accretion disk of SgrA* in spite of being obscured and broadened by scattering of surrounding plasma.Comment: 24 pages, 20 figures, revised version submitted to MN main journal (2010, Jan., 12th

    Yearly variations in the low-latitude topside ionosphere

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    International audienceObservations made by the Hinotori satellite have been analysed to determine the yearly variations of the electron density and electron temperature in the low-latitude topside ionosphere. The observations reveal the existence of an equinoctial asymmetry in the topside electron density at low latitudes, i.e. the density is higher at one equinox than at the other. The asymmetry is hemisphere-dependent with the higher electron density occurring at the March equinox in the Northern Hemisphere and at the September equinox in the Southern Hemisphere. The asymmetry becomes stronger with increasing latitude in both hemispheres. The behaviour of the asymmetry has no significant longitudinal and magnetic activity variations. A mechanism for the equinoctial asymmetry has been investigated using CTIP (coupled thermosphere ionosphere plasmasphere model). The model results reproduce the observed equinoctial asymmetry and suggest that the asymmetry is caused by the north-south imbalance of the thermosphere and ionosphere at the equinoxes due to the slow response of the thermosphere arising from the effects of the global thermospheric circulation. The observations also show that the relationship between the electron density and electron temperature is different for daytime and nighttime. During daytime the yearly variation of the electron temperature has negative correlation with the electron density, except at magnetic latitudes lower than 10°. At night, the correlation is positive

    How Does Longitudinal Interaction Promote Second Language Speech Learning? Roles of Learner Experience and Proficiency Levels

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    This study examined how longitudinal interaction impacts the development of second language (L2) oral proficiency in relation to learners’ different experience and proficiency levels. Japanese English-as-a-Foreign-Language learners participated in weekly conversation exchanges with native speakers (NSs) in the US via videoconferencing tools over one academic semester (12 weeks). The participants’ spontaneous speech, elicited from a story telling task before and after the treatment, was analyzed via a set of linguistic measures. In line with the componential view of L2 oral proficiency (De Jong et al., 2012) and development (Bundgaard-Nielsen et al., 2011), our results hinted L2 learners’ experience and proficiency levels as a mediating factor for determining the link between interaction and its impact on different dimensions of L2 speech learning. While the longitudinal interaction equally improved the participants’ grammatical complexity and articulation rate—a fundamental component for defining L2 oral proficiency, the development of less experienced/proficient learners was observed across a wide range of lexicogrammar and fluency features (lexical appropriateness/richness, grammatical accuracy, pause ratio). It was only more experienced/proficient learners that significantly enhanced phonological accuracies (segmentals, word stress) which are thought to gradually develop in the later stages of L2 speech learning. These findings add another piece of evidence for the differential effects of long-term interaction relative to L2 learners’ developmental stages

    Annual and seasonal variations in the low-latitude topside ionosphere

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    Subdiffusive axial transport of granular materials in a long drum mixer

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    Granular mixtures rapidly segregate radially by size when tumbled in a partially filled horizontal drum. The smaller component moves toward the axis of rotation and forms a buried core, which then splits into axial bands. Models have generally assumed that the axial segregation is opposed by diffusion. Using narrow pulses of the smaller component as initial conditions, we have characterized axial transport in the core. We find that the axial advance of the segregated core is well described by a self-similar concentration profile whose width scales as tαt^\alpha, with α0.3<1/2\alpha \sim 0.3 < 1/2. Thus, the process is subdiffusive rather than diffusive as previously assumed. We find that α\alpha is nearly independent of the grain type and drum rotation rate within the smoothly streaming regime. We compare our results to two one-dimensional PDE models which contain self-similarity and subdiffusion; a linear fractional diffusion model and the nonlinear porous medium equation.Comment: 4 pages, 4 figures, 1 table. Submitted to Phys Rev Lett. For more info, see http://www.physics.utoronto.ca/nonlinear