9,776 research outputs found
Code Generation for Efficient Query Processing in Managed Runtimes
In this paper we examine opportunities arising from the conver-gence of two trends in data management: in-memory database sys-tems (IMDBs), which have received renewed attention following the availability of affordable, very large main memory systems; and language-integrated query, which transparently integrates database queries with programming languages (thus addressing the famous ‘impedance mismatch ’ problem). Language-integrated query not only gives application developers a more convenient way to query external data sources like IMDBs, but also to use the same querying language to query an application’s in-memory collections. The lat-ter offers further transparency to developers as the query language and all data is represented in the data model of the host program-ming language. However, compared to IMDBs, this additional free-dom comes at a higher cost for query evaluation. Our vision is to improve in-memory query processing of application objects by introducing database technologies to managed runtimes. We focus on querying and we leverage query compilation to im-prove query processing on application objects. We explore dif-ferent query compilation strategies and study how they improve the performance of query processing over application data. We take C] as the host programming language as it supports language-integrated query through the LINQ framework. Our techniques de-liver significant performance improvements over the default LINQ implementation. Our work makes important first steps towards a future where data processing applications will commonly run on machines that can store their entire datasets in-memory, and will be written in a single programming language employing language-integrated query and IMDB-inspired runtimes to provide transparent and highly efficient querying. 1
Two-lane traffic rules for cellular automata: A systematic approach
Microscopic modeling of multi-lane traffic is usually done by applying
heuristic lane changing rules, and often with unsatisfying results. Recently, a
cellular automaton model for two-lane traffic was able to overcome some of
these problems and to produce a correct density inversion at densities somewhat
below the maximum flow density. In this paper, we summarize different
approaches to lane changing and their results, and propose a general scheme,
according to which realistic lane changing rules can be developed. We test this
scheme by applying it to several different lane changing rules, which, in spite
of their differences, generate similar and realistic results. We thus conclude
that, for producing realistic results, the logical structure of the lane
changing rules, as proposed here, is at least as important as the microscopic
details of the rules
Fundamentals of Traffic Flow
From single vehicle data a number of new empirical results concerning the
density-dependence of the velocity distribution and its moments as well as the
characteristics of their temporal fluctuations have been determined. These are
utilized for the specification of some fundamental relations of traffic flow
and compared with existing traffic theories.Comment: For related work see
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
Presence of Many Stable Nonhomogeneous States in an Inertial Car-Following Model
A new single lane car following model of traffic flow is presented. The model
is inertial and free of collisions. It demonstrates experimentally observed
features of traffic flow such as the existence of three regimes: free,
fluctuative (synchronized) and congested (jammed) flow; bistability of free and
fluctuative states in a certain range of densities, which causes the hysteresis
in transitions between these states; jumps in the density-flux plane in the
fluctuative regime and gradual spatial transition from synchronized to free
flow. Our model suggests that in the fluctuative regime there exist many stable
states with different wavelengths, and that the velocity fluctuations in the
congested flow regime decay approximately according to a power law in time.Comment: 4 pages, 4 figure
Direct current superconducting quantum interferometers with asymmetric shunt resistors
We have investigated asymmetrically shunted Nb/Al-AlO/Nb direct current
(dc) superconducting quantum interference devices (SQUIDs). While keeping the
total resistance identical to a comparable symmetric SQUID with , we shunted only one of the two Josephson junctions with
. Simulations predict that the optimum energy resolution
and thus also the noise performance of such an asymmetric SQUID can
be 3--4 times better than that of its symmetric counterpart. Experiments at a
temperature of 4.2\,K yielded for an asymmetric
SQUID with an inductance of . For a comparable symmetric device
was achieved, confirming our simulation results.Comment: 5 pages, 4 figure
Breakdown and recovery in traffic flow models
Most car-following models show a transition from laminar to ``congested''
flow and vice versa. Deterministic models often have a density range where a
disturbance needs a sufficiently large critical amplitude to move the flow from
the laminar into the congested phase. In stochastic models, it may be assumed
that the size of this amplitude gets translated into a waiting time, i.e.\
until fluctuations sufficiently add up to trigger the transition. A recently
introduced model of traffic flow however does not show this behavior: in the
density regime where the jam solution co-exists with the high-flow state, the
intrinsic stochasticity of the model is not sufficient to cause a transition
into the jammed regime, at least not within relevant time scales. In addition,
models can be differentiated by the stability of the outflow interface. We
demonstrate that this additional criterion is not related to the stability of
the flow. The combination of these criteria makes it possible to characterize
commonalities and differences between many existing models for traffic in a new
way
Discrete stochastic models for traffic flow
We investigate a probabilistic cellular automaton model which has been
introduced recently. This model describes single-lane traffic flow on a ring
and generalizes the asymmetric exclusion process models. We study the
equilibrium properties and calculate the so-called fundamental diagrams (flow
vs.\ density) for parallel dynamics. This is done numerically by computer
simulations of the model and by means of an improved mean-field approximation
which takes into account short-range correlations. For cars with maximum
velocity 1 the simplest non-trivial approximation gives the exact result. For
higher velocities the analytical results, obtained by iterated application of
the approximation scheme, are in excellent agreement with the numerical
simulations.Comment: Revtex, 30 pages, full postscript version (including figures)
available by anonymous ftp from "fileserv1.mi.uni-koeln.de" in the directory
"pub/incoming/" paper accepted for publication in Phys.Rev.
Suppression of spin-state transition in epitaxially strained LaCoO_{3}
Epitaxial thin films of LaCoO_{3} (E-LCO) exhibit ferromagnetic order with a
transition temperature T_c = 85 K, while polycrystalline thin LaCoO_{3} films
(P-LCO) remain paramagnetic. The temperature-dependent spin-state structure for
both E-LCO and P-LCO was studied by x-ray absorption spectroscopy at the Co
L_{2,3} and O K edges. Considerable spectral redistributions over temperature
are observed for P-LCO. The spectra for E-LCO, on the other hand, do not show
any significant changes for temperatures between 30 K and 450 K at both edges,
indicating that the spin state remains constant and that the epitaxial strain
inhibits any population of the low-spin (S = 0) state with decreasing
temperature. This observation identifies an important prerequisite for
ferromagnetism in E-LCO thin films.Comment: 5 pages, 5 figures, submitted to Physical Review
Economics-Based Optimization of Unstable Flows
As an example for the optimization of unstable flows, we present an
economics-based method for deciding the optimal rates at which vehicles are
allowed to enter a highway. It exploits the naturally occuring fluctuations of
traffic flow and is flexible enough to adapt in real time to the transient flow
characteristics of road traffic. Simulations based on realistic parameter
values show that this strategy is feasible for naturally occurring traffic, and
that even far from optimality, injection policies can improve traffic flow.
Moreover, the same method can be applied to the optimization of flows of gases
and granular media.Comment: Revised version of ``Optimizing Traffic Flow'' (cond-mat/9809397).
For related work see http://www.parc.xerox.com/dynamics/ and
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
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