1,348 research outputs found
Lingering issues in distributed scheduling
Recent advances have resulted in queue-based algorithms for medium access control which operate in a distributed fashion, and yet achieve the optimal throughput performance of centralized scheduling algorithms. However, fundamental performance bounds reveal that the "cautious" activation rules involved in establishing throughput optimality tend to produce extremely large delays, typically growing exponentially in 1/(1-r), with r the load of the system, in contrast to the usual linear growth. Motivated by that issue, we explore to what extent more "aggressive" schemes can improve the delay performance. Our main finding is that aggressive activation rules induce a lingering effect, where individual nodes retain possession of a shared resource for excessive lengths of time even while a majority of other nodes idle. Using central limit theorem type arguments, we prove that the idleness induced by the lingering effect may cause the delays to grow with 1/(1-r) at a quadratic rate. To the best of our knowledge, these are the first mathematical results illuminating the lingering effect and quantifying the performance impact. In addition extensive simulation experiments are conducted to illustrate and validate the various analytical results
High Resolution Linear Polarimetric Imaging for the Event Horizon Telescope
Images of the linear polarization of synchrotron radiation around Active
Galactic Nuclei (AGN) identify their projected magnetic field lines and provide
key data for understanding the physics of accretion and outflow from
supermassive black holes. The highest resolution polarimetric images of AGN are
produced with Very Long Baseline Interferometry (VLBI). Because VLBI
incompletely samples the Fourier transform of the source image, any image
reconstruction that fills in unmeasured spatial frequencies will not be unique
and reconstruction algorithms are required. In this paper, we explore
extensions of the Maximum Entropy Method (MEM) to linear polarimetric VLBI
imaging. In contrast to previous work, our polarimetric MEM algorithm combines
a Stokes I imager that uses only bispectrum measurements that are immune to
atmospheric phase corruption with a joint Stokes Q and U imager that operates
on robust polarimetric ratios. We demonstrate the effectiveness of our
technique on 7- and 3-mm wavelength quasar observations from the VLBA and
simulated 1.3-mm Event Horizon Telescope observations of Sgr A* and M87.
Consistent with past studies, we find that polarimetric MEM can produce
superior resolution compared to the standard CLEAN algorithm when imaging
smooth and compact source distributions. As an imaging framework, MEM is highly
adaptable, allowing a range of constraints on polarization structure.
Polarimetric MEM is thus an attractive choice for image reconstruction with the
EHT.Comment: 19 pages, 9 figures. Accepted for publication in ApJ. Imaging code
available at https://github.com/achael/eht-imaging
Different Observational Methods and the Detection of Seasonal and Atlantic Influence Upon Phytoplankton Communities in the Western Barents Sea
Phytoplankton community composition, and its dependency on environmental variation, are key to understanding marine primary production, processes of trophic transfer and the role of marine phytoplankton in global biogeochemical cycles. Understanding changes in phytoplankton community composition on Arctic shelves is important, because these productive environments are experiencing rapid change. Many different methods have been employed by researchers to quantify phytoplankton community composition. Previous studies have demonstrated that the way in which community composition is quantified can influence the interpretation of environmental dependencies. Researchers must consider both the suitability of the data they collect for monitoring marine ecosystems, as well as the research effort required to collect representative datasets. We therefore seek to understand how the representation of phytoplankton community structure in the western Barents Sea, a rapidly changing Arctic shelf sea, influences the interpretation of environmental dependencies. We compare datasets of cell counts, phytoplankton pigments and bio-optics (absorption spectra), relating them to a suite of environmental conditions with multivariate exploratory analyses. We show that, while cell counts reveal the greatest insight into environmental dependencies, pigment and absorption spectral datasets still provide useful information about seasonal succession and the influence of Atlantic water masses– two key subjects of great research interest in this region. As pigments and optical properties influence remotely-sensed ocean-colour, these findings hold implications for remote detection of phytoplankton community composition
Observing---and Imaging---Active Galactic Nuclei with the Event Horizon Telescope
Originally developed to image the shadow region of the central black hole in
Sagittarius A* and in the nearby galaxy M87, the Event Horizon Telescope (EHT)
provides deep, very high angular resolution data on other AGN sources too. The
challenges of working with EHT data have spurred the development of new image
reconstruction algorithms. This work briefly reviews the status of the EHT and
its utility for observing AGN sources, with emphasis on novel imaging
techniques that offer the promise of better reconstructions at 1.3 mm and other
wavelengths.Comment: 10 pages, proceedings contribution for Blazars through Sharp
Multi-Wavelength Eyes, submitted to Galaxie
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