5,713 research outputs found
TRIAD: creating synergies between memory, disk and log in log structured key-value stores
We present TRIAD, a new persistent key-value (KV) store based on Log-Structured Merge (LSM) trees. TRIAD improves LSM KV throughput by reducing the write amplification arising in the maintenance of the LSM tree structure. Although occurring in the background, write amplification consumes significant CPU and I/O resources. By reducing write amplification, TRIAD allows these resources to be used instead to improve user-facing throughput. TRIAD uses a holistic combination of three techniques. At the LSM memory component level, TRIAD leverages skew in data popularity to avoid frequent I/O operations on the most popular keys. At the storage level, TRIAD amortizes management costs by deferring and batching multiple I/O operations. At the commit log level, TRIAD avoids duplicate writes to storage. We implement TRIAD as an extension of Facebook's RocksDB and evaluate it with production and synthetic workloads. With these workloads, TRIAD yields up to 193% improvement in throughput. It reduces write amplification by a factor of up to 4x, and decreases the amount of I/O by an order of magnitude
Amplification of Primordial Magnetic Fields by Anisotropic Gravitational Collapse
If a magnetic field is frozen into a plasma that undergoes spherical
compression then the magnetic field B varies with the plasma density \rho
according to B \propto \rho^{2/3}. In the gravitational collapse of
cosmological density perturbations, however, quasi-spherical evolution is very
unlikely. In anisotropic collapses the magnetic field can be a much steeper
function of gas density than in the isotropic case. We investigate the
distribution of amplifications in realistic gravitational collapses from
Gaussian initial fluctuations using the Zel'dovich approximation. Representing
our results using a relation of the form B\propto \rho^{\alpha}, we show that
the median value of \alpha can be much larger than the \alpha=2/3 resulting
from spherical collapse, even if there is no initial correlation between
magnetic field and principal collapse directions. These analytic arguments go
some way towards understanding the results of numerical simulations.Comment: 9 pages, 4 figures. Submitted to MNRA
Dynamic analysis of flexible rotor-bearing systems using a modal approach
The generalized dynamic equations of motion were obtained by the direct stiffness method for multimass flexible rotor-bearing systems. The direct solution of the equations of motion is illustrated on a simple 3-mass system. For complex rotor-bearing systems, the direct solution of the equations becomes very difficult. The transformation of the equations of motion into modal coordinates can greatly simplify the computation for the solution. The use of undamped and damped system mode shapes in the transformation are discussed. A set of undamped critical speed modes is used to transform the equations of motion into a set of coupled modal equations of motion. A rapid procedure for computing stability, steady state unbalance response, and transient response of the rotor-bearing system is presented. Examples of the application of this modal approach are presented. The dynamics of the system is further investigated with frequency spectrum analysis of the transient response
Revisiting the optical -symmetric dimer
Optics has proved a fertile ground for the experimental simulation of quantum
mechanics. Most recently, optical realizations of -symmetric
quantum mechanics have been shown, both theoretically and experimentally,
opening the door to international efforts aiming at the design of practical
optical devices exploiting this symmetry. Here, we focus on the optical
-symmetric dimer, a two-waveguide coupler were the materials show
symmetric effective gain and loss, and provide a review of the linear and
nonlinear optical realizations from a symmetry based point of view. We go
beyond a simple review of the literature and show that the dimer is just the
smallest of a class of planar -waveguide couplers that are the optical
realization of Lorentz group in 2+1 dimensions. Furthermore, we provide a
formulation to describe light propagation through waveguide couplers described
by non-Hermitian mode coupling matrices based on a non-Hermitian generalization
of Ehrenfest theorem.Comment: 25 pages, 12 figure
Low-emittance storage rings
The effects of synchrotron radiation on particle motion in storage rings are
discussed. In the absence of radiation, particle motion is symplectic, and the
beam emittances are conserved. The inclusion of radiation effects in a
classical approximation leads to emittance damping: expressions for the damping
times are derived. Then, it is shown that quantum radiation effects lead to
excitation of the beam emittances. General expressions for the equilibrium
longitudinal and horizontal (natural) emittances are derived. The impact of
lattice design on the natural emittance is discussed, with particular attention
to the special cases of FODO, achromat, and TME style lattices. Finally, the
effects of betatron coupling and vertical dispersion (generated by magnet
alignment and lattice tuning errors) on the vertical emittance are considered.Comment: Presented at the CERN Accelerator School CAS 2013: Advanced
Accelerator Physics Course, Trondheim, Norway, 18-29 August 201
DKVF: A Framework for Rapid Prototyping and Evaluating Distributed Key-value Stores
We present our framework DKVF that enables one to quickly prototype and
evaluate new protocols for key-value stores and compare them with existing
protocols based on selected benchmarks. Due to limitations of CAP theorem, new
protocols must be developed that achieve the desired trade-off between
consistency and availability for the given application at hand. Hence, both
academic and industrial communities focus on developing new protocols that
identify a different (and hopefully better in one or more aspect) point on this
trade-off curve. While these protocols are often based on a simple intuition,
evaluating them to ensure that they indeed provide increased availability,
consistency, or performance is a tedious task. Our framework, DKVF, enables one
to quickly prototype a new protocol as well as identify how it performs
compared to existing protocols for pre-specified benchmarks. Our framework
relies on YCSB (Yahoo! Cloud Servicing Benchmark) for benchmarking. We
demonstrate DKVF by implementing four existing protocols --eventual
consistency, COPS, GentleRain and CausalSpartan-- with it. We compare the
performance of these protocols against different loading conditions. We find
that the performance is similar to our implementation of these protocols from
scratch. And, the comparison of these protocols is consistent with what has
been reported in the literature. Moreover, implementation of these protocols
was much more natural as we only needed to translate the pseudocode into Java
(and add the necessary error handling). Hence, it was possible to achieve this
in just 1-2 days per protocol. Finally, our framework is extensible. It is
possible to replace individual components in the framework (e.g., the storage
component)
The geometry of quantum learning
Concept learning provides a natural framework in which to place the problems
solved by the quantum algorithms of Bernstein-Vazirani and Grover. By combining
the tools used in these algorithms--quantum fast transforms and amplitude
amplification--with a novel (in this context) tool--a solution method for
geometrical optimization problems--we derive a general technique for quantum
concept learning. We name this technique "Amplified Impatient Learning" and
apply it to construct quantum algorithms solving two new problems: BATTLESHIP
and MAJORITY, more efficiently than is possible classically.Comment: 20 pages, plain TeX with amssym.tex, related work at
http://www.math.uga.edu/~hunziker/ and http://math.ucsd.edu/~dmeyer
- …