872,545 research outputs found
Performance of periodic piezoelectric composite arrays incorporating a passive phase exhibiting anisotropic properties
This paper explores the minimisation of interelement cross talk in 1-D and 2-D periodic composite array structures through the incorporation of a passive phase exhibiting anisotropic elastic properties. Initially the PZFlex finite element code was used to monitor array aperture response as a function of material properties. It is shown that in array structures comprising passive polymer materials possessing low longitudinal loss and high shear loss, inter-element mechanical cross talk is reduced, without a concomitant reduction in element sensitivity. A number of polymer materials with the desired properties were synthesised and their elastic character confirmed through a program of materials characterisation. Finally, a range of experimental devices exhibiting improved directional response, as a result of a significant reduction in interelement cross talk, are presented and the predicted array characteristics are shown to compare favourably in each case
A Program Logic for Verifying Secure Routing Protocols
The Internet, as it stands today, is highly vulnerable to attacks. However,
little has been done to understand and verify the formal security guarantees of
proposed secure inter-domain routing protocols, such as Secure BGP (S-BGP). In
this paper, we develop a sound program logic for SANDLog-a declarative
specification language for secure routing protocols for verifying properties of
these protocols. We prove invariant properties of SANDLog programs that run in
an adversarial environment. As a step towards automated verification, we
implement a verification condition generator (VCGen) to automatically extract
proof obligations. VCGen is integrated into a compiler for SANDLog that can
generate executable protocol implementations; and thus, both verification and
empirical evaluation of secure routing protocols can be carried out in this
unified framework. To validate our framework, we encoded several proposed
secure routing mechanisms in SANDLog, verified variants of path authenticity
properties by manually discharging the generated verification conditions in
Coq, and generated executable code based on SANDLog specification and ran the
code in simulation
Exploiting Inter- and Intra-Memory Asymmetries for Data Mapping in Hybrid Tiered-Memories
Modern computing systems are embracing hybrid memory comprising of DRAM and
non-volatile memory (NVM) to combine the best properties of both memory
technologies, achieving low latency, high reliability, and high density. A
prominent characteristic of DRAM-NVM hybrid memory is that it has NVM access
latency much higher than DRAM access latency. We call this inter-memory
asymmetry. We observe that parasitic components on a long bitline are a major
source of high latency in both DRAM and NVM, and a significant factor
contributing to high-voltage operations in NVM, which impact their reliability.
We propose an architectural change, where each long bitline in DRAM and NVM is
split into two segments by an isolation transistor. One segment can be accessed
with lower latency and operating voltage than the other. By introducing tiers,
we enable non-uniform accesses within each memory type (which we call
intra-memory asymmetry), leading to performance and reliability trade-offs in
DRAM-NVM hybrid memory. We extend existing NVM-DRAM OS in three ways. First, we
exploit both inter- and intra-memory asymmetries to allocate and migrate memory
pages between the tiers in DRAM and NVM. Second, we improve the OS's page
allocation decisions by predicting the access intensity of a newly-referenced
memory page in a program and placing it to a matching tier during its initial
allocation. This minimizes page migrations during program execution, lowering
the performance overhead. Third, we propose a solution to migrate pages between
the tiers of the same memory without transferring data over the memory channel,
minimizing channel occupancy and improving performance. Our overall approach,
which we call MNEME, to enable and exploit asymmetries in DRAM-NVM hybrid
tiered memory improves both performance and reliability for both single-core
and multi-programmed workloads.Comment: 15 pages, 29 figures, accepted at ACM SIGPLAN International Symposium
on Memory Managemen
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The SMFA program for quantum chemistry calculations on large molecules
SMFA is a general program package for performing quantum chemistry calculations on large molecules, using an energy-based fragmentation approach. The program can calculate electronic energies, energy gradients and second derivatives; perform geometry optimization; find first order saddle points (transition states); perform energy optimized scans along a user-defined path; and evaluate various molecular properties. The program can use any of the following quantum chemistry packages: GAMESS(US), GAUSSIAN, NWChem and Q-Chem. In addition, SMFA provides a number of utility programs that, inter alia, calculate vibrational frequencies and infrared spectra with isotopic substitutions, the electrostatic potential on the solvent-accessible-surface, and isodesmic and higher order near-iso-energetic reaction schemes. Calculations of the electronic energy and related properties can be carried out using a scheme that provides a computation time that is linearly dependent on the size of the molecule or, if the user has enough processing units available, in a walltime that is independent of the size of the molecule
Strategic allocation of cyclically arriving container vessels to inter-related terminals
We consider a port consisting of a cluster of inter-related terminals, where container vessels arrive cyclically. The problem is to strategically assign a terminal and a time interval of berthing to each of the vessels in the cycle. Restricting properties are terminal quay lengths and quay crane capacity. Conflicting objectives are i) minimizing the number of required quay cranes, ii) minimizing the amount of inter-terminal traffic and iii) minimizing the total weighted deviation from desired berthing intervals. We formulate both a straightforward and an alternative mixed integer linear program to model this system. Results show that the alternative model is much faster solvable and enables to optimize real-life problems within a couple of hours
Probing for Exoplanets Hiding in Dusty Debris Disks: Disk Imaging, Characterization, and Exploration with HST/STIS Multi-Roll Coronagraphy
Spatially resolved scattered-light images of circumstellar (CS) debris in
exoplanetary systems constrain the physical properties and orbits of the dust
particles in these systems. They also inform on co-orbiting (but unseen)
planets, systemic architectures, and forces perturbing starlight-scattering CS
material. Using HST/STIS optical coronagraphy, we have completed the
observational phase of a program to study the spatial distribution of dust in
ten CS debris systems, and one "mature" protoplanetrary disk all with HST
pedigree, using PSF-subtracted multi-roll coronagraphy. These observations
probe stellocentric distances > 5 AU for the nearest stars, and simultaneously
resolve disk substructures well beyond, corresponding to the giant planet and
Kuiper belt regions in our Solar System. They also disclose diffuse very
low-surface brightness dust at larger stellocentric distances. We present new
results inclusive of fainter disks such as HD92945 confirming, and better
revealing, the existence of a narrow inner debris ring within a larger diffuse
dust disk. Other disks with ring-like sub-structures, significant asymmetries
and complex morphologies include: HD181327 with a posited spray of ejecta from
a recent massive collision in an exo-Kuiper belt; HD61005 suggested interacting
with the local ISM; HD15115 & HD32297, discussed also in the context of
environmental interactions. These disks, and HD15745, suggest debris system
evolution cannot be treated in isolation. For AU Mic's edge-on disk,
out-of-plane surface brightness asymmetries at > 5 AU may implicate one or more
planetary perturbers. Time resolved images of the MP Mus proto-planetary disk
provide spatially resolved temporal variability in the disk illumination. These
and other new images from our program enable direct inter-comparison of the
architectures of these exoplanetary debris systems in the context of our own
Solar System.Comment: 109 pages, 43 figures, accepted for publication in the Astronomical
Journa
Software Testing and Metrics for Concurrent Computation
[[abstract]]Verification and validation are two important technologies to assure the reliability and quality of software. Software testing and metrics are two approaches to execute the verification and validation. In sequential computation, a fairly mature process exists, with various methodologies and tools available for use in building and demonstrating the correctness of a program being tested. The emergence of concurrent computation in recent years, however, introduces new testing problems and difficulties that cannot be solved by the traditional sequential program testing techniques. Many concurrent program testing methodologies have been proposed to solve controlled execution and determinism. There have been few discussions of concurrent software testing from the inter-task viewpoint, even though the common characteristics of concurrent programming are the explicit identification of the large-grain parallel computation units (tasks) and the explicit inter-task communication via a rendezvous-style mechanism. In this paper, we focus on testing concurrent programs through task decomposition. We propose four testing criteria to test a concurrent program. A programmer can choose an appropriate testing strategy depending on the properties of the concurrent programs. Associated with the strategies, four equations are provided to measure the complexity of concurrent programs[[conferencetype]]國際[[conferencedate]]19961204~19961204[[conferencelocation]]Seoul, Kore
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