1,103 research outputs found
Litter fall and chemical cycling in an aspen (Populus tremuloides) woodland ecosystem in the Canadian Rockies
A Relativistic Mean Field Model for Entrainment in General Relativistic Superfluid Neutron Stars
General relativistic superfluid neutron stars have a significantly more
intricate dynamics than their ordinary fluid counterparts. Superfluidity allows
different superfluid (and superconducting) species of particles to have
independent fluid flows, a consequence of which is that the fluid equations of
motion contain as many fluid element velocities as superfluid species. Whenever
the particles of one superfluid interact with those of another, the momentum of
each superfluid will be a linear combination of both superfluid velocities.
This leads to the so-called entrainment effect whereby the motion of one
superfluid will induce a momentum in the other superfluid. We have constructed
a fully relativistic model for entrainment between superfluid neutrons and
superconducting protons using a relativistic mean field model
for the nucleons and their interactions. In this context there are two notions
of ``relativistic'': relativistic motion of the individual nucleons with
respect to a local region of the star (i.e. a fluid element containing, say, an
Avogadro's number of particles), and the motion of fluid elements with respect
to the rest of the star. While it is the case that the fluid elements will
typically maintain average speeds at a fraction of that of light, the
supranuclear densities in the core of a neutron star can make the nucleons
themselves have quite high average speeds within each fluid element. The
formalism is applied to the problem of slowly-rotating superfluid neutron star
configurations, a distinguishing characteristic being that the neutrons can
rotate at a rate different from that of the protons.Comment: 16 pages, 5 figures, submitted to PR
Implementation and Performance of Munin
Munin is a distributed shared memory (DSM) system that allows shared memory parallel programs to be executed efficiently on distributed memory multiprocessors. Munin is unique among existing DSM systems in its use of multiple consistency protocols and in its use of release consistency. In Munin, shared program variables are annotated with their expected access pattern, and these annotations are then used by the runtime system to choose a consistency protocol best suited to that access pattern. Release consistency allows Munin to mask network latency and reduce the number of messages required to keep memory consistent. Munin's multiprotocol release consistency is implemented in software using a delayed update queue that buffers and merges pending outgoing writes. A sixteenprocessor prototype of Munin is currently operational. We evaluate its imple mentation and describe the execution of two Munin programs that achieve performance within ten percent of message passing implementations of the same programs. Munin achieves this level of performance with only minor annotations to the shared memory programs
Techniques for Reducing Consistency-Related Communication in Distributed Shared Memory System
Distributed shared memory 8DSM) is an abstraction of shared memory on a distributed memory machine. Hardware DSM systems support this abstraction at the architecture level; software DSM systems support the abstraction within the runtime system. One of the key problems in building an efficient software DSM system is to reduce the amount of communication needed to keep the distributed memories consistent. In this paper we present four techniques for doing so: 1) software release consistency; 2) multiple consistency protocols; (3) write-shared protocols; and (4) an update-with-timeout mechanism. These techniques have been implemented in the Munin DSM system. We compare the performance of seven Munin application programs, first to their performance when implemented using message passing, and then to their performance when running on a conventional software DSM system that does not embody the above techniques. On a 16-processor cluster of workstations, Munin’s performance is within 5% of message passing for four out of the seven applications. For the other three, performance is within 29% to 33%. Detailed analysis of two of these three applications indicates that the addition of a function shipping capability would bring their performance to within 7% of the message passing performance. Compared to a conventional DSM system, Munin achieves performance improvements ranging from a few to several hundred percent, depending on the application
Munin: Distributed Shared Memory Based on Type-Specific Memory Coherence
We are developing Munin y , a system that allows pro grams written for shared memory multiprocessors to be executed efficiently on distributed memory machines. Thus, Munin overcomes the architectural limitations of shared memory machines, while maintaining their advantages in terms of ease of programming. A unique characteristic of Munin is the mechanism by which the shared memory programming model is translated to the distributed memory hardware. This translation is performed by runtime software, with the aid of semantic hints provided by the user. Each shared data object is supported by a memory coherence mechanism appropriate to the manner in which the object is accessed. This paper focuses on Munin's memory coherence mechanisms, and compares our approach to previous work in this area. This research was supported in part by the National Science Foundationunder Grants CCR8716914 and DCA8619893 and by a National Science Foundation Fellowship. y In Norse mythology, the ravens Munin (Memory) and Hugin (Thought) perched on Odin's shoulder, and each evening they flew across the world to bring Odin knowledge of man's memories and thoughts. Thus, the raven Munin can be considered to have been the first distributed shared memory mechanism
An Eye-Opening Approach to Developing and Communicating Integrated Environmental Assessments
Communication among managers, the public, and scientists is the key to successful ecosystem management; however, the varied perspectives and interests of these groups can make such communication difficult. One way to achieve effective communication is to develop a common knowledge base by combining syntheses of key scientific results with information-rich visual elements. Within a management landscape, integrated environmental assessments provide a useful framework for evaluating resources and directing management efforts. The integrated assessment process involves (1) initial investigation, (2) development of a conceptual framework, (3) data navigation, (4) environmental report cards, and (5) science communication. Each step requires the synthesis and visualization of information on the status and trends connected with multiple natural resources. We provide a case study, using examples from selected National Park Service sites in the mid-Atlantic region of the United States. Visual elements (conceptual diagrams, maps, graphs, tables, and photographs) were used to facilitate comparative assessments and to provide a more visual, or eye-opening , approach to effective environmental decision making
Slowly Rotating General Relativistic Superfluid Neutron Stars with Relativistic Entrainment
Neutron stars that are cold enough should have two or more
superfluids/supercondutors in their inner crusts and cores. The implication of
superfluidity/superconductivity for equilibrium and dynamical neutron star
states is that each individual particle species that forms a condensate must
have its own, independent number density current and equation of motion that
determines that current. An important consequence of the quasiparticle nature
of each condensate is the so-called entrainment effect, i.e. the momentum of a
condensate is a linear combination of its own current and those of the other
condensates. We present here the first fully relativistic modelling of slowly
rotating superfluid neutron stars with entrainment that is accurate to the
second-order in the rotation rates. The stars consist of superfluid neutrons,
superconducting protons, and a highly degenerate, relativistic gas of
electrons. We use a relativistic - mean field model for the
equation of state of the matter and the entrainment. We determine the effect of
a relative rotation between the neutrons and protons on a star's total mass,
shape, and Kepler, mass-shedding limit.Comment: 30 pages, 10 figures, uses ReVTeX
Effects of acceleration on the collision of particles in the rotating black hole spacetime
We study the collision of two geodesic particles in the accelerating and
rotating black hole spacetime and probe the effects of the acceleration of
black hole on the center-of-mass energy of the colliding particles and on the
high-velocity collision belts. We find that the dependence of the
center-of-mass energy on the acceleration in the near event-horizon collision
is different from that in the near acceleration-horizon case. Moreover, the
presence of the acceleration changes the shape and position of the
high-velocity collision belts. Our results show that the acceleration of black
holes brings richer physics for the collision of particles.Comment: 7 pages, 2 figures, The corrected version accepted for publication in
EPJ
6D Dyonic String With Active Hyperscalars
We derive the necessary and sufficient conditions for the existence of a
Killing spinor in N=(1,0) gauge supergravity in six dimensions coupled to a
single tensor multiplet, vector multiplets and hypermultiplets. These are shown
to imply most of the field equations and the remaining ones are determined. In
this framework, we find a novel 1/8 supersymmetric dyonic string solution with
nonvanishing hypermultiplet scalars. The activated scalars parametrize a 4
dimensional submanifold of a quaternionic hyperbolic ball. We employ an
identity map between this submanifold and the internal space transverse to the
string worldsheet. The internal space forms a 4 dimensional analog of the
Gell-Mann-Zwiebach tear-drop which is noncompact with finite volume. While the
electric charge carried by the dyonic string is arbitrary, the magnetic charge
is fixed in Planckian units, and hence necessarily non-vanishing. The source
term needed to balance a delta function type singularity at the origin is
determined. The solution is also shown to have 1/4 supersymmetric AdS_3 x S^3
near horizon limit where the radii are proportional to the electric charge.Comment: 28 pages, latex, minor corrections mad
Stealth Branes
We discuss the brane world model of Dvali, Gabadadze and Porrati in which
branes evolve in an infinite bulk and the brane curvature term is added to the
action. If Z_2 symmetry between the two sides of the brane is not imposed, we
show that the model admits the existence of "stealth branes" which follow the
standard 4D internal evolution and have no gravitational effect on the bulk
space. Stealth branes can nucleate spontaneosly in a Minkowski bulk. This
process is described by the standard 4D quantum cosmology formalism with
tunneling boundary conditions for the brane world wave function. The notorious
ambiguity in the choice of boundary conditions is fixed in this case due to the
presence of the embedding spacetime. We also point to some problematic aspects
of models admitting stealth brane solutions.Comment: 24 pages; Final version, to appear in Phys. Rev. D. The discussion of
"embeddability obstruction" is removed (thanks to Takahiro Tanaka who
convinced us that there is no such obstruction
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