179 research outputs found
LL/SC and Atomic Copy: Constant Time, Space Efficient Implementations Using Only Pointer-Width CAS
When designing concurrent algorithms, Load-Link/Store-Conditional (LL/SC) is
often the ideal primitive to have because unlike Compare and Swap (CAS), LL/SC
is immune to the ABA problem. However, the full semantics of LL/SC are not
supported by any modern machine, so there has been a significant amount of work
on simulations of LL/SC using Compare and Swap (CAS), a synchronization
primitive that enjoys widespread hardware support. All of the algorithms so far
that are constant time either use unbounded sequence numbers (and thus base
objects of unbounded size), or require space for LL/SC object
(where is the number of processes). We present a constant time
implementation of LL/SC objects using space, where is
the maximum number of overlapping LL/SC operations per process (usually a
constant), and requiring only pointer-sized CAS objects. Our implementation can
also be used to implement -word objects in time (for
both and ) and space. To achieve these bounds, we
begin by implementing a new primitive called Single-Writer Copy which takes a
pointer to a word sized memory location and atomically copies its contents into
another object. The restriction is that only one process is allowed to
write/copy into the destination object at a time. We believe this primitive
will be very useful in designing other concurrent algorithms as well
An Almost Tight RMR Lower Bound for Abortable Test-And-Set
We prove a lower bound of Omega(log n/log log n) for the remote memory reference (RMR) complexity of abortable test-and-set (leader election) in the cache-coherent (CC) and the distributed shared memory (DSM) model. This separates the complexities of abortable and non-abortable test-and-set, as the latter has constant RMR complexity [Wojciech Golab et al., 2010].
Golab, Hendler, Hadzilacos and Woelfel [Wojciech M. Golab et al., 2012] showed that compare-and-swap can be implemented from registers and test-and-set objects with constant RMR complexity. We observe that a small modification to that implementation is abortable, provided that the used test-and-set objects are atomic (or abortable). As a consequence, using existing efficient randomized wait-free implementations of test-and-set [George Giakkoupis and Philipp Woelfel, 2012], we obtain randomized abortable compare-and-swap objects with almost constant (O(log^* n)) RMR complexity
Designing Statistical Language Learners: Experiments on Noun Compounds
The goal of this thesis is to advance the exploration of the statistical
language learning design space. In pursuit of that goal, the thesis makes two
main theoretical contributions: (i) it identifies a new class of designs by
specifying an architecture for natural language analysis in which probabilities
are given to semantic forms rather than to more superficial linguistic
elements; and (ii) it explores the development of a mathematical theory to
predict the expected accuracy of statistical language learning systems in terms
of the volume of data used to train them.
The theoretical work is illustrated by applying statistical language learning
designs to the analysis of noun compounds. Both syntactic and semantic analysis
of noun compounds are attempted using the proposed architecture. Empirical
comparisons demonstrate that the proposed syntactic model is significantly
better than those previously suggested, approaching the performance of human
judges on the same task, and that the proposed semantic model, the first
statistical approach to this problem, exhibits significantly better accuracy
than the baseline strategy. These results suggest that the new class of designs
identified is a promising one. The experiments also serve to highlight the need
for a widely applicable theory of data requirements.Comment: PhD thesis (Macquarie University, Sydney; December 1995), LaTeX
source, xii+214 page
Orchestration : the movement and vocal behavior of free-ranging Norwegian killer whales (Orcinus orca)
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2008Studying the social and cultural transmission of behavior among animals helps to
identify patterns of interaction and information content flowing between individuals. Killer
whales are likely to acquire traits culturally based on their population-specific feeding
behaviors and group-distinctive vocal repertoires. I used digital tags to explore the
contributions of individual Norwegian killer whales to group carousel feeding and the
relationships between vocal and non-vocal activity.
Periods of tail slapping to incapacitate herring during feeding were characterized by
elevated movement variability, heightened vocal activity and call types containing additional
orientation cues. Tail slaps produced by tagged animals were identified using a rapid pitch
change and occurred primarily within 20m of the surface. Two simultaneously tagged
animals maneuvered similarly when tail slapping within 60s of one another, indicating that
the position and composition of the herring ball influenced their behavior.
Two types of behavioral sequence preceding the tight circling of carousel feeding
were apparent. First, the animals engaged in periods of directional swimming. They were
silent in 2 of 3 instances, suggesting they may have located other foraging groups by
eavesdropping. Second, tagged animals made broad horizontal loops as they dove in a
manner consistent with corralling. All 4 of these occasions were accompanied by vocal
activity, indicating that this and tail slapping may benefit from social communication. No
significant relationship between the call types and the actual movement measurements was found.
Killer whale vocalizations traditionally have been classified into discrete call types.
Using human speech processing techniques, I considered that calls are alternatively
comprised of shared segments that can be recombined to form the stereotyped and variable
repertoire. In a classification experiment, the characterization of calls using the whole call, a
set of unshared segments, or a set of shared segments yielded equivalent performance. The
shared segments required less information to parse the same vocalizations, suggesting a more
parsimonious system of representation.
This closer examination of the movements and vocalizations of Norwegian killer
whales, combined with future work on ontogeny and transmission, will inform our
understanding of whether and how culture plays a role in achieving population-specific
behaviors in this species.Funding sources: The Ocean Life Institute at WHOI and the National Geographic Society, the National Defense Science and Engineering Graduate Fellowship, a National Science Foundation Graduate Fellowship, the Academic Programs Office at WHOI and Dennis McLaughlin at MIT
Recommended from our members
The Shapes of Planet Transits and Planetary Systems
In this Thesis, I explore transiting exoplanets: what we can learn from modeling their light curves, and what we can learn from their arrangement in planetary systems. I begin in Chapter 1 by briefly reviewing the history of transit modeling, from the earliest theoretical models of eclipsing binary stars to the models in current widespread use to model exoplanet transits. In Chapter 2, I model the transits of a sample of Kepler exoplanets with strong prior eccentricity constraints in order to derive correspondingly strong constraints on the density of their host stars, and find that the density constraints I derive are as precise as density constraints from asteroseismology if the transits are observed at high signal-to-noise. In Chapter 3, I apply the same methodology in reverse: using prior knowledge of the stellar density based on Gaia parallax measurements, I model the transits of twelve singly-transiting planets observed by K2 and derive constraints on their periods. In Chapter 4, I consider the general problem of deducing the shape of a transiting object from its light curve alone, which I term ``shadow imaging;'' I explore the mathematical degeneracies of the problem and construct shadow images to explain Dips 5 and 8 of Boyajian's Star.
I next turn to multi-planet systems: in Chapter 5, I investigate the underlying multiplicity distribution of planetary systems orbiting FGK dwarfs observed by Kepler. I find that we can explain the multiplicities of these systems with a single Zipfian multiplicity distribution, without invoking a dichotomous population. In Chapter 6, I consider the arrangement of planets in those systems, and use neural networks inspired by models used for part-of-speech tagging in computational linguistics to model the relationship between exoplanets and their surrounding "context," i.e. their host star and sibling planets. I find that our trained regression model is able to predict the period and radius of an exoplanet to a factor of two better than a naive model which only takes into account basic dynamical stability. I also find that our trained classification model identifies consistent classes of planets in the period-radius plane, and that it is rare for multi-planet systems to contain a neighboring pair of planets from non-contiguous classes.
In Chapter 7, I summarize these results and briefly discuss avenues for future work, including the application of our methods to planets and planetary systems discovered by TESS
Behavioural analysis of marine predator movements in relation to heterogeneous environments
An understanding of the spatio-temporal dynamics of marine predator populations is essential for the sustainable management of marine resources. Tagging studies are providing ever more information about the movements and migrations of marine predators and much has been learned about where these predators spend their time. However little is known about their underlying motivations, making it difficult to make predictions about how apex predators will respond to changing environments. While much progress has been made in behavioural ecology through the use of optimality models, in the marine environment the necessary costs and benefits are difficult to quantify making this approach less successful than with terrestrial studies. One aspect of foraging behaviour that has proved tractable however is the optimisation of random searches. Work by statistical physicists has shown that a specialised movement, known as Lévy flight, can optimise the rate of new prey patch encounters when new prey patches are beyond sensory range. The resulting Lévy flight foraging (LFF) hypothesis makes testable predictions about marine predator search behaviour that can be addressed with the theoretical and empirical studies that form the basis of this thesis. Results presented here resolve the controversy surrounding the hypothesis, demonstrating the optimality of Lévy searches under a broader set of conditions than previously considered, including whether observed Lévy patterns are innate or emergent. Empirical studies provide robust evidence for the prevalence of Lévy search patterns in the movements of diverse marine pelagic predators such as sharks, tunas and billfish as well as in the foraging patterns of albatrosses, overturning a previous study. Predictions from the LFF hypothesis concerning fast moving prey are confirmed leading to simulation studies of ambush predator’s activity patterns. Movement analysis is then applied to the assessment of by-catch mitigation efforts involving VMS data from long-liners and simulated sharks
Supporting multimedia user interface design using mental models and representational expressiveness
This thesis addresses the problem of output media allocation in the design of
multimedia user interfaces. The literature survey identifies a formal definition of the
representational capabilities of different media.as important in this task. Equally
important, though less prominent in the literature, is that the correct mental model of
a domain is paramount for the successful completion of tasks.
The thesis proposes an original linguistic and cognitive based descriptive framework,
in two parts. The first part defines expressiveness, the amount of representational
abstraction a medium provides over any domain. The second part describes how this
expressiveness is linked to the mental models that media induce, and how this in turn
affects task performance. It is postulated that the mental models induced by different
media, will reflect the abstractive representation those media offer over the task
domain. This must then be matched to the abstraction required by tasks to allow them
to be effectively accomplished.
A 34 subject experiment compares five media, of two levels of expressiveness, over a
range of tasks, in a complex and dynamic domain. The results indicate that
expressiveness may allow media to be matched more closely to tasks, if the mental
models they are known to induce are considered.
Finally, the thesis proposes a tentative framework for media allocation, and two
example interfaces are designed using this framework. This framework is based on
the matching of expressiveness to the abstraction of a domain required by tasks. The
need for the methodology to take account of the user's cognitive capabilities is
stressed, and the experimental results are seen as the beginning of this procedure
A Taxonomy for and Analysis of Anonymous Communications Networks
Any entity operating in cyberspace is susceptible to debilitating attacks. With cyber attacks intended to gather intelligence and disrupt communications rapidly replacing the threat of conventional and nuclear attacks, a new age of warfare is at hand. In 2003, the United States acknowledged that the speed and anonymity of cyber attacks makes distinguishing among the actions of terrorists, criminals, and nation states difficult. Even President Obama’s Cybersecurity Chief-elect recognizes the challenge of increasingly sophisticated cyber attacks. Now through April 2009, the White House is reviewing federal cyber initiatives to protect US citizen privacy rights. Indeed, the rising quantity and ubiquity of new surveillance technologies in cyberspace enables instant, undetectable, and unsolicited information collection about entities. Hence, anonymity and privacy are becoming increasingly important issues. Anonymization enables entities to protect their data and systems from a diverse set of cyber attacks and preserves privacy. This research provides a systematic analysis of anonymity degradation, preservation and elimination in cyberspace to enhance the security of information assets. This includes discovery/obfuscation of identities and actions of/from potential adversaries. First, novel taxonomies are developed for classifying and comparing well-established anonymous networking protocols. These expand the classical definition of anonymity and capture the peer-to-peer and mobile ad hoc anonymous protocol family relationships. Second, a unique synthesis of state-of-the-art anonymity metrics is provided. This significantly aids an entity’s ability to reliably measure changing anonymity levels; thereby, increasing their ability to defend against cyber attacks. Finally, a novel epistemic-based mathematical model is created to characterize how an adversary reasons with knowledge to degrade anonymity. This offers multiple anonymity property representations and well-defined logical proofs to ensure the accuracy and correctness of current and future anonymous network protocol design
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