26,444 research outputs found
WMTrace : a lightweight memory allocation tracker and analysis framework
The diverging gap between processor and memory performance has been a well discussed aspect of computer architecture literature for some years. The use of multi-core processor designs has, however, brought new problems to the design of memory architectures - increased core density without matched improvement in memory capacity is reduc- ing the available memory per parallel process. Multiple cores accessing memory simultaneously degrades performance as a result of resource con- tention for memory channels and physical DIMMs. These issues combine to ensure that memory remains an on-going challenge in the design of parallel algorithms which scale. In this paper we present WMTrace, a lightweight tool to trace and analyse memory allocation events in parallel applications. This tool is able to dynamically link to pre-existing application binaries requiring no source code modification or recompilation. A post-execution analysis stage enables in-depth analysis of traces to be performed allowing memory allocations to be analysed by time, size or function. The second half of this paper features a case study in which we apply WMTrace to five parallel scientific applications and benchmarks, demonstrating its effectiveness at recording high-water mark memory consumption as well as memory use per-function over time. An in-depth analysis is provided for an unstructured mesh benchmark which reveals significant memory allocation imbalance across its participating processes
A dynamic scheme for generating number squeezing in Bose-Einstein condensates through nonlinear interactions
We develop a scheme to generate number squeezing in a Bose-Einstein
condensate by utilizing interference between two hyperfine levels and nonlinear
atomic interactions. We describe the scheme using a multimode quantum field
model and find agreement with a simple analytic model in certain regimes. We
demonstrate that the scheme gives strong squeezing for realistic choices of
parameters and atomic species. The number squeezing can result in noise well
below the quantum limit, even if the initial noise on the system is classical
and much greater than that of a poisson distribution.Comment: 4 pages, 3 figure
How an improved implementation of H2 self-shielding influences the formation of massive stars and black holes
High redshift quasars at z>6 have masses up to ~ M. One of the
pathways to their formation includes direct collapse of gas, forming a
supermassive star, precursor of the black hole seed. The conditions for direct
collapse are more easily achievable in metal-free haloes, where atomic hydrogen
cooling operates and molecular hydrogen (H2) formation is inhibited by a strong
external UV flux. Above a certain value of UV flux (J_crit), the gas in a halo
collapses isothermally at ~ K and provides the conditions for
supermassive star formation. However, H2 can self-shield, reducing the effect
of photodissociation. So far, most numerical studies used the local Jeans
length to calculate the column densities for self-shielding. We implement an
improved method for the determination of column densities in 3D simulations and
analyse its effect on the value of J_crit. This new method captures the gas
geometry and velocity field and enables us to properly determine the
direction-dependent self-shielding factor of H2 against photodissociating
radiation. We find a value of J_crit that is a factor of two smaller than with
the Jeans approach (~2000 J_21 vs. ~4000 J_21). The main reason for this
difference is the strong directional dependence of the H2 column density. With
this lower value of J_crit, the number of haloes exposed to a flux >J_crit is
larger by more than an order of magnitude compared to previous studies. This
may translate into a similar enhancement in the predicted number density of
black hole seeds.Comment: 14 pages, 12 figures, published in MNRA
Localization dynamics of fluids in random confinement
The dynamics of two-dimensional fluids confined within a random matrix of
obstacles is investigated using both colloidal model experiments and molecular
dynamics simulations. By varying fluid and matrix area fractions in the
experiment, we find delocalized tracer particle dynamics at small matrix area
fractions and localized motion of the tracers at high matrix area fractions. In
the delocalized region, the dynamics is subdiffusive at intermediate times, and
diffusive at long times, while in the localized regime, trapping in finite
pockets of the matrix is observed. These observations are found to agree with
the simulation of an ideal gas confined in a weakly correlated matrix. Our
results show that Lorentz gas systems with soft interactions are exhibiting a
smoothening of the critical dynamics and consequently a rounded
delocalization-to-localization transition.Comment: 5 pages, 3 figure
Optimal eigenvalues estimate for the Dirac operator on domains with boundary
We give a lower bound for the eigenvalues of the Dirac operator on a compact
domain of a Riemannian spin manifold under the \MIT bag boundary condition.
The limiting case is characterized by the existence of an imaginary Killing
spinor.Comment: 10 page
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