367 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
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
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
Controlled generation of a pn-junction in a waveguide integrated graphene photodetector
With its electrically tunable light absorption and ultrafast photoresponse,
graphene is a promising candidate for high-speed chip-integrated photonics. The
generation mechanisms of photosignals in graphene photodetectors have been
studied extensively in the past years. However, the knowledge about efficient
light conversion at graphene pn-junctions has not yet been translated into
high-performance devices. Here, we present a graphene photodetector integrated
on a silicon slot-waveguide, acting as a dual-gate to create a pn-junction in
the optical absorption region of the device. While at zero bias the
photo-thermoelectric effect is the dominant conversion process, an additional
photoconductive contribution is identified in a biased configuration. Extrinsic
responsivities of 35 mA/W, or 3.5 V/W, at zero bias and 76 mA/W at 300 mV bias
voltage are achieved. The device exhibits a 3 dB-bandwidth of 65 GHz, which is
the highest value reported for a graphene-based photodetector.Comment: 19 pages, 16 figure
Density of states of colloidal glasses
Glasses are structurally liquid-like, but mechanically solid-like. Most
attempts to understand glasses start from liquid state theory. Here we take the
opposite point of view, and use concepts from solid state physics. We determine
the vibrational modes of a colloidal glass experimentally, and find soft
low-frequency modes that are very different in nature from the usual acoustic
vibrations of ordinary solids. These modes extend over surprisingly large
length scales
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