16,812 research outputs found
Improving Performance of Iterative Methods by Lossy Checkponting
Iterative methods are commonly used approaches to solve large, sparse linear
systems, which are fundamental operations for many modern scientific
simulations. When the large-scale iterative methods are running with a large
number of ranks in parallel, they have to checkpoint the dynamic variables
periodically in case of unavoidable fail-stop errors, requiring fast I/O
systems and large storage space. To this end, significantly reducing the
checkpointing overhead is critical to improving the overall performance of
iterative methods. Our contribution is fourfold. (1) We propose a novel lossy
checkpointing scheme that can significantly improve the checkpointing
performance of iterative methods by leveraging lossy compressors. (2) We
formulate a lossy checkpointing performance model and derive theoretically an
upper bound for the extra number of iterations caused by the distortion of data
in lossy checkpoints, in order to guarantee the performance improvement under
the lossy checkpointing scheme. (3) We analyze the impact of lossy
checkpointing (i.e., extra number of iterations caused by lossy checkpointing
files) for multiple types of iterative methods. (4)We evaluate the lossy
checkpointing scheme with optimal checkpointing intervals on a high-performance
computing environment with 2,048 cores, using a well-known scientific
computation package PETSc and a state-of-the-art checkpoint/restart toolkit.
Experiments show that our optimized lossy checkpointing scheme can
significantly reduce the fault tolerance overhead for iterative methods by
23%~70% compared with traditional checkpointing and 20%~58% compared with
lossless-compressed checkpointing, in the presence of system failures.Comment: 14 pages, 10 figures, HPDC'1
Fabrication of bismuth nanowires with a silver nanocrystal shadowmask
We fabricated bismuth (Bi) nanowires with low energy electron beam lithography using silver (Ag) nanocrystal shadowmasks and a subsequent chlorine reactive ion etching. Submicron-size metal contacts on the single Bi nanowire were successfully prepared by in situ focused ion beam metal deposition for transport measurements. The temperature dependent resistance measurements on the 50 nm wide Bi nanowires showed that the resistance increased with decreasing temperature, which is characteristic of semiconductors and insulators
Redshift Evolution of the Nonlinear Two-Point Correlation Function
This paper presents a detailed theoretical study of the two-point correlation
function for both dark matter halos and the matter density field in five
cosmological models with varying matter density and neutrino
fraction . The objectives of this systematic study are to evaluate
the nonlinear gravitational effects on , to contrast the behavior of
for halos vs. matter, and to quantify the redshift evolution of and its
dependence on cosmological parameters. Overall, for halos exhibits
markedly slower evolution than for matter, and its redshift dependence is
much more intricate than the single power-law parameterization used in the
literature. Of particular interest is that the redshift evolution of the
halo-halo correlation length depends strongly on and
, being slower in models with lower or higher
. Measurements of to higher redshifts can therefore be a
potential discriminator of cosmological parameters. The evolution rate of
for halos within a given model increases with time, passing the phase of fixed
comoving clustering at to 3 toward the regime of stable clustering at
. The shape of the halo-halo , on the other hand, is well
approximated by a power law with slope -1.8 in all models and is not a
sensitive model discriminator.Comment: 22 pages, 8 postscript figures, AAS LaTeX v4.0. Accepted for
publication in The Astrophysical Journal, Vol. 510 (January 1 1999
Different Ways of Reading, or Just Making the Right Noises?
What does reading look like? Can learning to read be reduced to the acquisition of a set of isolable skills, or proficiency in reading be equated with the independence of the solitary, silent reader of prose fiction? These conceptions of reading and reading development, which figure strongly in educational policy, may appear to be simple common sense. But both ethnographic data and evidence from literary texts suggest that such paradigms offer, at most, a partial and ahistorical picture of reading. An important dimension, neglected in the dominant paradigms, is the irreducibly social quality of reading practices
Physicality and Cooperative Design
CSCW researchers have increasingly come to realize that material work setting and its population of artefacts play a crucial part in coordination of distributed or co-located work. This paper uses the notion of physicality as a basis to understand cooperative work. Using examples from an ongoing fieldwork on cooperative design practices, it provides a conceptual understanding of physicality and shows that material settings and co-workerâs working practices play an important role in understanding physicality of cooperative design
Kondo regime in triangular arrangements of quantum dots: Molecular orbitals, interference and contact effects
Transport properties of an interacting triple quantum dot system coupled to
three leads in a triangular geometry has been studied in the Kondo regime.
Applying mean-field finite-U slave boson and embedded cluster approximations to
the calculation of transport properties unveils a set of rich features
associated to the high symmetry of this system. Results using both calculation
techniques yield excellent overall agreement and provide additional insights
into the physical behavior of this interesting geometry. In the case when just
two current leads are connected to the three-dot system, interference effects
between degenerate molecular orbitals are found to strongly affect the overall
conductance. An S=1 Kondo effect is also shown to appear for the perfect
equilateral triangle symmetry. The introduction of a third current lead results
in an `amplitude leakage' phenomenon, akin to that appearing in beam splitters,
which alters the interference effects and the overall conductance through the
system.Comment: 14 pages, 9 figures, submitted to PR
Optimal combinations of imperfect objects
We address the question of how to make best use of imperfect objects, such as
defective analog and digital components. We show that perfect, or near-perfect,
devices can be constructed by taking combinations of such defects. Any
remaining objects can be recycled efficiently. In addition to its practical
applications, our `defect combination problem' provides a novel generalization
of classical optimization problems.Comment: 4 pages, 3 figures, minor change
Sediment budget in a deep-sea core from the central equatorial Pacific
Stratigraphic, mineralogic, chemical, and geochronologic measurements on a core from 8°20\u27N, 153°W show that sediment has been accumulating at a rate of 160 g/cm2/106 years. Of this, 125 g is fresh Quaternary sediment while the remainder is lower and middle Tertiary material eroded from nearby outcrops...
The Taurus Tunable Filter Field Galaxy Survey: Sample Selection and Narrowband Number-Counts
Recent evidence suggests a falling volume-averaged star-formation rate (SFR)
over z ~ 1. It is not clear, however, the extent to which the selection of such
samples influences the measurement of this quantity. Using the Taurus Tunable
Filter (TTF) we have obtained an emission-line sample of faint star-forming
galaxies over comparable lookback times: the TTF Field Galaxy Survey. By
selecting through emission-lines, we are screening galaxies through a quantity
that scales directly with star-formation activity for a given choice of initial
mass function. The scanning narrowband technique furnishes a galaxy sample that
differs from traditional broadband-selected surveys in both its volume-limited
nature and selection of galaxies through emission-line flux. Three discrete
wavelength intervals are covered, centered at H-alpha redshifts z = 0.08, 0.24
and 0.39.
Galaxy characteristics are presented and comparisons made with existing
surveys of both broadband and emission-line selection. When the number-counts
of emission-line objects are compared with those expected on the basis of
existing H-alpha surveys, we find an excess of ~ 3 times at the faintest
limits. While these detections are yet to be independently confirmed,
inspection of the stronger subsample of galaxies detected in both the line and
continuum (line-on-continuum subsample; 13 %) is sufficient to support an
excess population. This increase in the emission-line field population implies
higher star-formation densities over z ~ 0.4. However, further study in the
form of multi-object spectroscopic follow-up is necessary to quantify this and
confirm the faintest detections in the sample.Comment: 48 pages, 12 figures. To appear in the Astrophysical Journal. An
abridged version of the Abstract is shown her
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