Random redundant storage in disk arrays: Complexity of retrieval problems

Random redundant data storage strategies have proven to be a good choice for efficient data storage in multimedia servers. These strategies lead to a retrieval problem in which it is decided for each requested data block which disk to use for its retrieval. In this paper, we give a complexity classification of retrieval problems for random redundant storage

Reliable and randomized data distribution strategies for large scale storage systems

The ever-growing amount of data requires highly scalable storage solutions. The most flexible approach is to use storage pools that can be expanded and scaled down by adding or removing storage devices. To make this approach usable, it is necessary to provide a solution to locate data items in such a dynamic environment. This paper presents and evaluates the Random Slicing strategy, which incorporates lessons learned from table-based, rule-based, and pseudo-randomized hashing strategies and is able to provide a simple and efficient strategy that scales up to handle exascale data. Random Slicing keeps a small table with information about previous storage system insert and remove operations, drastically reducing the required amount of randomness while delivering a perfect load distribution.Peer ReviewedPostprint (author’s final draft

The resolution bias: low resolution feedback simulations are better at destroying galaxies

Feedback from super-massive black holes (SMBHs) is thought to play a key role in regulating the growth of host galaxies. Cosmological and galaxy formation simulations using smoothed particle hydrodynamics (SPH), which usually use a fixed mass for SPH particles, often employ the same sub-grid Active galactic nuclei (AGN) feedback prescription across a range of resolutions. It is thus important to ask how the impact of the simulated AGN feedback on a galaxy changes when only the numerical resolution (the SPH particle mass) changes. We present a suite of simulations modelling the interaction of an AGN outflow with the ambient turbulent and clumpy interstellar medium (ISM) in the inner part of the host galaxy at a range of mass resolutions. We find that, with other things being equal, degrading the resolution leads to feedback becoming more efficient at clearing out all gas in its path. For the simulations presented here, the difference in the mass of the gas ejected by AGN feedback varies by more than a factor of ten between our highest and lowest resolution simulations. This happens because feedback-resistant high density clumps are washed out at low effective resolutions. We also find that changes in numerical resolution lead to undesirable artifacts in how the AGN feedback affects the AGN immediate environment.Comment: 15 pages, 12 figures, accepted for publication in MNRA

A Comparison of Numerical Modelling Techniques for Tidal Stream Turbine Analysis

To fully understand the performance of tidal stream turbines for the development of ocean renewable energy, a range of computational models is required. We review and compare results from several models of horizontal axis turbines at different spatial scales. Models under review include blade element momentum theory (BEMT), blade element actuator disk, Reynolds averaged Navier Stokes (RANS) CFD (BEM-CFD), blade-resolved moving reference frame and coastal models based on the shallow water equations. To evaluate the BEMT, a comparison is made to experiments with three different rotors. We demonstrate that, apart from the near-field wake, there are similarities in the results between the BEM-CFD approach and a coastal area model using a simplified turbine fence at a headland case

Dust attenuation, dust content and geometry of star-forming galaxies

We analyse the joint distribution of dust attenuation and projected axis ratios, together with galaxy size and surface brightness profile information, to infer lessons on the dust content and star/dust geometry within star-forming galaxies at 0 < z <2.5. To do so, we make use of large observational datasets from KiDS+VIKING+HSC-SSP and extend the analysis out to redshift z = 2.5 using the HST surveys CANDELS and 3D-DASH. We construct suites of SKIRT radiative transfer models for idealized galaxies observed under random viewing angles with the aim of reproducing the aforementioned distributions, including the level and inclination dependence of dust attenuation. We find that attenuation-based dust mass estimates are at odds with constraints from far-infrared observations, especially at higher redshifts, when assuming smooth star and dust geometries of equal extent. We demonstrate that UV-to-near-IR and far-infrared constraints can be reconciled by invoking clumpier dust geometries for galaxies at higher redshifts and/or very compact dust cores. We discuss implications for the significant wavelength- and redshift-dependent differences between half-light and half-mass radii that result from spatially varying dust columns within -- especially massive -- star-forming galaxies.Comment: Accepted for publication in MNRA

Spatial agent-based modelling

Archaeologists were among some of the earliest users of agent-based modelling, but recent years have undoubtedly seen a surge of interest in the use of this technique to infer past behaviour or help develop new theories and methods. Although ABM software is much easier to use than it was even 20 years ago and sufficiently powerful computers are more readily available, the success of a modelling project is still largely determined by decisions made about the purpose and design of the model, and the subsequent experimental regime. This chapter guides the reader through those key issues. It covers epistemological topics such as the role of the model in a wider project, the trade-off between realism and generality, the idea of generative modelling and the importance of adequate experimentation. It also discusses technical issues such as options for the integration of ABM and GIS, and even the dangers inherent in poor design decisions about the scheduling of agent behaviour

Mind out of matter: topics in the physical foundations of consciousness and cognition

This dissertation begins with an exploration of a brand of dual aspect monism and some problems deriving from the distinction between a first person and third person point of view. I continue with an outline of one way in which the conscious experience of the subject might arise from organisational properties of a material substrate. With this picture to hand, I first examine theoretical features at the level of brain organisation which may be required to support conscious experience and then discuss what bearing some actual attributes of biological brains might have on such experience. I conclude the first half of the dissertation with comments on information processing and with artificial neural networks meant to display simple varieties of the organisational features initially described abstractly.While the first half begins with a view of conscious experience and infers downwards in the organisational hierarchy to explore neural features suggested by the view, attention in the second half shifts towards analysing low level dynamical features of material substrates and inferring upwards to possible effects on experience. There is particular emphasis on clarifying the role of chaotic dynamics, and I discuss relationships between levels of description of a cognitive system and comment on issues of complexity, computability, and predictability before returning to the topic of representation which earlier played a central part in isolating features of brain organisation which may underlie conscious experience.Some themes run throughout the dissertation, including an emphasis on understanding experience from both the first person and the third person points of view and on analysing the latter at different levels of description. Other themes include a sustained effort to integrate the picture offered here with existing empirical data and to situate current problems in the philosophy of mind within the new framework, as well as an appeal to tools from mathematics, computer science, and cognitive science to complement the more standard philosophical repertoire

The Missing Baryon Problem via Cosmological Zoom-in Simulations

This thesis explores the missing baryon problem in a computational context. An overview of the problem is given, along with a discussion regarding the relevance of the Circumgalactic Medium (CMG) and cosmological Zoom-in simulations. The mechanisms underlying the N-body code ChaNGa (H. Menon, et al., Computational Astrophysics and Cosmology 2, 1 (2015), arXiv:1409.1929), as well as the data visualization and analysis tools yt (M. J. Turk, et al., 192, 9 (2011), arXiv:1011.3514) and trident (Hummels, et al., 847, 59 (2017), arXiv:1612.03935) are presented at a conceptual level. Finally, a series of synthetic quasar absorption spectra produced by using trident on a ChaNGa dataset from (S. Roca-Fabrega, et al., 917, 64 (2021), arXiv:2106.09738) at redshift of $z\sim4$ are shown. The low relative flux exhibited by these spectra render absorption features indistinguishable from background noise, and possible explanations for this phenomena such as high redshift are discussed. Though the resulting spectra exhibit serious obstacles for both qualitative and quantitative interpretation, they provide a "proof-of-concept" for future work, demonstrating trident's compatibility with ChaNGa's data format. Future prospects for using trident to analyze the CGM as simulated by ChaNGa are discussed, as well as possible extensions of this project.Comment: Written as part of the undergraduate program at Reed Colleg