91 research outputs found
Debye relaxation and 250 K anomaly in glass forming monohydroxy alcohols
A previous dielectric, near-infrared (NIR), and nuclear magnetic resonance
study on the hydrogen-bonded liquid 2-ethyl-1-hexanol [C. Gainaru et al., Phys.
Rev. Lett. 107, 118304 (2011)] revealed anomalous behavior in various static
quantities near 250 K. To check whether corresponding observations can be made
for other monohydroxy alcohols as well, these experimental methods were applied
to such substances with 5, 6, 7, 8, and 10 carbon atoms in their molecular
backbone. All studied liquids exhibit a change of behavior near 250 K which is
tentatively ascribed to effects of hydrogen bond cooperativity. By analyzing
the NIR band intensities, a linear cluster size is derived that agrees with
estimates from dielectric spectroscopy. All studied alcohols, except
4-methyl-3-heptanol, display a dominant Debye-like peak. Furthermore, neat
2-ethyl-1-butanol exhibits a well resolved structural relaxation in its
dielectric loss spectrum which so far has only been observed for diluted
monohydroxy alcohols.Comment: 39 pages including 12 figure
Online Fault Classification in HPC Systems through Machine Learning
As High-Performance Computing (HPC) systems strive towards the exascale goal,
studies suggest that they will experience excessive failure rates. For this
reason, detecting and classifying faults in HPC systems as they occur and
initiating corrective actions before they can transform into failures will be
essential for continued operation. In this paper, we propose a fault
classification method for HPC systems based on machine learning that has been
designed specifically to operate with live streamed data. We cast the problem
and its solution within realistic operating constraints of online use. Our
results show that almost perfect classification accuracy can be reached for
different fault types with low computational overhead and minimal delay. We
have based our study on a local dataset, which we make publicly available, that
was acquired by injecting faults to an in-house experimental HPC system.Comment: Accepted for publication at the Euro-Par 2019 conferenc
FINJ: A Fault Injection Tool for HPC Systems
We present FINJ, a high-level fault injection tool for High-Performance
Computing (HPC) systems, with a focus on the management of complex experiments.
FINJ provides support for custom workloads and allows generation of anomalous
conditions through the use of fault-triggering executable programs. FINJ can
also be integrated seamlessly with most other lower-level fault injection
tools, allowing users to create and monitor a variety of highly-complex and
diverse fault conditions in HPC systems that would be difficult to recreate in
practice. FINJ is suitable for experiments involving many, potentially
interacting nodes, making it a very versatile design and evaluation tool.Comment: To be presented at the 11th Resilience Workshop in the 2018 Euro-Par
conferenc
Nuclear magnetic resonance measurements reveal the origin of the Debye process in monohydroxy alcohols
Monohydroxy alcohols show a structural relaxation and at longer time scales a
Debye-type dielectric peak. From spin-lattice relaxation experiments using
different nuclear probes an intermediate, slower-than-structural dynamics is
identified for n-butanol. Based on these findings and on diffusion
measurements, a model of self-restructuring, transient chains is proposed. The
model is demonstrated to explain consistently the so far puzzling observations
made for this class of hydrogen-bonded glass forming liquids.Comment: 4 pages, 4 figure
Hydrogen-bond equilibria and life times in a supercooled monohydroxy alcohol
Dielectric loss spectra covering 13 decades in frequency were collected for
2-ethyl-1-hexanol, a monohydroxy alcohol that exhibits a prominent Debye-like
relaxation, typical for several classes of hydrogen-bonded liquids. The thermal
variation of the dielectric absorption amplitude agrees well with that of the
hydrogen-bond equilibrium population, experimentally mapped out using near
infrared (NIR) and nuclear magnetic resonance (NMR) measurements. Despite this
agreement, temperature-jump NIR spectroscopy reveals that the hydrogen-bond
switching rate does not define the frequency position of the prominent
absorption peak. This contrasts with widespread notions and models based
thereon, but is consistent with a recent approach.Comment: 4 pages, 4 figure
Improving I/O Performance for Exascale Applications through Online Data Layout Reorganization
The applications being developed within the U.S. Exascale Computing Project (ECP) to run on imminent Exascale computers will generate scientific results with unprecedented fidelity and record turn-around time. Many of these codes are based on particle-mesh methods and use advanced algorithms, especially dynamic load-balancing and mesh-refinement, to achieve high performance on Exascale machines. Yet, as such algorithms improve parallel application efficiency, they raise new challenges for I/O logic due to their irregular and dynamic data distributions. Thus, while the enormous data rates of Exascale simulations already challenge existing file system write strategies, the need for efficient read and processing of generated data introduces additional constraints on the data layout strategies that can be used when writing data to secondary storage. We review these I/O challenges and introduce two online data layout reorganization approaches for achieving good tradeoffs between read and write performance. We demonstrate the benefits of using these two approaches for the ECP particle-in-cell simulation WarpX, which serves as a motif for a large class of important Exascale applications. We show that by understanding application I/O patterns and carefully designing data layouts we can increase read performance by more than 80 percent
NMR and dielectric studies of hydrated collagen and elastin: Evidence for a delocalized secondary relaxation
Using a combination of dielectric spectroscopy and solid-state deuteron NMR,
the hydration water dynamics of connective tissue proteins is studied at
sub-ambient temperatures. In this range, the water dynamics follows an
Arrhenius law. A scaling analysis of dielectric losses, 'two-phase' NMR
spectra, and spin-lattice relaxation times consistently yield evidence for a
Gaussian distribution of energy barriers. With the dielectric data as input,
random-walk simulations of a large-angle, quasi-isotropic water reorientation
provide an approximate description of stimulated-echo data on hydrated elastin.
This secondary process takes place in an essentially rigid energy landscape,
but in contrast to typical {\beta}-relaxations it is quasi-isotropic and
delocalized. The delocalization is inferred from previous NMR diffusometry
experiments. To emphasize the distinction from conventional {\beta}-processes,
for aqueous systems such a matrix-decoupled relaxation was termed a
{\nu}-process. It is emphasized that the phenomenology of this time-honored,
'new' process is shared by many non-aqueous binary glasses in which the
constituent components exhibit a sufficient dynamical contrast
Shear-Modulus Investigations of Monohydroxy Alcohols: Evidence for a Short-Chain-Polymer Rheological Response
In addition to the ubiquitous structural relaxation of viscous supercooled
liquids, monohydroxy alcohols and several other hydrogen-bonded systems display
a strong single-exponential electrical low-frequency absorption. So far, this
so-called Debye process could be observed only using dielectric techniques.
Exploiting a combination of broad-band and high-resolution rheology experiments
for three isomeric octanols, unambiguous mechanical evidence for the Debye
process is found. Its spectral signature is similar to the viscoelastic
fingerprint of small-chain polymers, enabling us to estimate the effective
molecular weight for the supramolecular structure formed by the studied
monohydroxy alcohols. This finding opens the venue for the application of
further non-dielectric techniques directed at unraveling the microscopic nature
of the Debye process and for an understanding of this phenomenon in terms of
polymer concepts.Comment: 12 pages, 4 figure
Corresponding States of Structural Glass Formers
The variation with respect to temperature T of transport properties of 58
fragile structural glass forming liquids (68 data sets in total) are analyzed
and shown to exhibit a remarkable degree of universality. In particular,
super-Arrhenius behaviors of all super-cooled liquids appear to collapse to one
parabola for which there is no singular behavior at any finite temperature.
This behavior is bounded by an onset temperature To above which liquid
transport has a much weaker temperature dependence. A similar collapse is also
demonstrated, over the smaller available range, for existing numerical
simulation data.Comment: 6 pages, 2 figures. Updated References, Table Values, Submitted for
Publicatio
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