7,350 research outputs found
Dynamic Physiological Partitioning on a Shared-nothing Database Cluster
Traditional DBMS servers are usually over-provisioned for most of their daily
workloads and, because they do not show good-enough energy proportionality,
waste a lot of energy while underutilized. A cluster of small (wimpy) servers,
where its size can be dynamically adjusted to the current workload, offers
better energy characteristics for these workloads. Yet, data migration,
necessary to balance utilization among the nodes, is a non-trivial and
time-consuming task that may consume the energy saved. For this reason, a
sophisticated and easy to adjust partitioning scheme fostering dynamic
reorganization is needed. In this paper, we adapt a technique originally
created for SMP systems, called physiological partitioning, to distribute data
among nodes, that allows to easily repartition data without interrupting
transactions. We dynamically partition DB tables based on the nodes'
utilization and given energy constraints and compare our approach with physical
partitioning and logical partitioning methods. To quantify possible energy
saving and its conceivable drawback on query runtimes, we evaluate our
implementation on an experimental cluster and compare the results w.r.t.
performance and energy consumption. Depending on the workload, we can
substantially save energy without sacrificing too much performance
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A criterion for condensation in kinetically constrained one-dimensional transport models
We study condensation in one-dimensional transport models with a kinetic
constraint. The kinetic constraint results in clustering of immobile vehicles;
these clusters can grow to macroscopic condensates, indicating the onset of
dynamic phase separation between free flowing and arrested traffic. We
investigate analytically the conditions under which this occurs, and derive a
necessary and sufficient criterion for phase separation. This criterion is
applied to the well-known Nagel-Schreckenberg model of traffic flow to
analytically investigate the existence of dynamic condensates. We find that
true condensates occur only when acceleration out of jammed traffic happens in
a single time step, in the limit of strong overbraking. Our predictions are
further verified with simulation results on the growth of arrested clusters.
These results provide analytic understanding of dynamic arrest and dynamic
phase separation in one-dimensional traffic and transport models
Microscopic origin of nonlinear non-affine deformation and stress overshoot in bulk metallic glasses
The atomic theory of elasticity of amorphous solids, based on the nonaffine
response formalism, is extended into the nonlinear stress-strain regime by
coupling with the underlying irreversible many-body dynamics. The latter is
implemented in compact analytical form using a qualitative method for the
many-body Smoluchowski equation. The resulting nonlinear stress-strain
(constitutive) relation is very simple, with few fitting parameters, yet
contains all the microscopic physics. The theory is successfully tested against
experimental data on metallic glasses, and it is able to reproduce the
ubiquitous feature of stress-strain overshoot upon varying temperature and
shear rate. A clear atomic-level interpretation is provided for the stress
overshoot, in terms of the competition between the elastic instability caused
by nonaffine deformation of the glassy cage and the stress buildup due to
viscous dissipation.Comment: Physical Review B Rapid Comm., in pres
Visualizing the strain evolution during the indentation of colloidal glasses
We use an analogue of nanoindentation on a colloidal glass to elucidate the
incipient plastic deformation of glasses. By tracking the motion of the
individual particles in three dimensions, we visualize the strain field and
glass structure during the emerging deformation. At the onset of flow, we
observe a power-law distribution of strain indicating strongly correlated
deformation, and reflecting a critical state of the glass. At later stages, the
strain acquires a Gaussian distribution, indicating that plastic events become
uncorrelated. Investigation of the glass structure using both static and
dynamic measures shows a weak correlation between the structure and the
emerging strain distribution. These results indicate that the onset of
plasticity is governed by strong power-law correlations of strain, weakly
biased by the heterogeneous glass structure.Comment: 13 pages, 8 figure
Direct Measurement of the Free Energy of Aging Hard-Sphere Colloidal Glasses
The nature of the glass transition is one of the most important unsolved
problems in condensed matter physics. The difference between glasses and
liquids is believed to be caused by very large free energy barriers for
particle rearrangements; however so far it has not been possible to confirm
this experimentally. We provide the first quantitative determination of the
free energy for an aging hard-sphere colloidal glass. The determination of the
free energy allows for a number of new insights in the glass transition,
notably the quantification of the strong spatial and temporal heterogeneity in
the free energy. A study of the local minima of the free energy reveals that
the observed variations are directly related to the rearrangements of the
particles. Our main finding is that the probability of particle rearrangements
shows a power law dependence on the free energy changes associated with the
rearrangements, similarly to the Gutenberg-Richter law in seismology.Comment: 4 pages, 4 figure
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