2,724 research outputs found
Horizon Tunneling Revisited: The Case of Higher Dimensional Black Holes
We study the tunneling of massless scalars across black hole horizons in any
number of spacetime dimensions greater than three. Our analysis finds that
corrections due to backreaction and the inverse dimensional expansion are
naturally concomitant, and furnishes a simple proof of the classic relation
between entropy and area in all spacetime dimensions, finite or infinite. We
conclude with a discussion of the limit in which the the number of spacetime
dimensions is taken to infinity, where we find that thermodynamic quantities
are related to the "thickness" of the membrane on which all the curvature is
localized.Comment: 22 page
Polynomial mechanics and optimal control
We describe a new algorithm for trajectory optimization of mechanical
systems. Our method combines pseudo-spectral methods for function approximation
with variational discretization schemes that exactly preserve conserved
mechanical quantities such as momentum. We thus obtain a global discretization
of the Lagrange-d'Alembert variational principle using pseudo-spectral methods.
Our proposed scheme inherits the numerical convergence characteristics of
spectral methods, yet preserves momentum-conservation and symplecticity after
discretization. We compare this algorithm against two other established methods
for two examples of underactuated mechanical systems; minimum-effort swing-up
of a two-link and a three-link acrobot.Comment: Final version to EC
Optimal strategies for throwing accurately
Accuracy of throwing in games and sports is governed by how errors at
projectile release are propagated by flight dynamics. To address the question
of what governs the choice of throwing strategy, we use a simple model of
throwing with an arm modelled as a hinged bar of fixed length that can release
a projectile at any angle and angular velocity. We show that the amplification
of deviations in launch parameters from a one parameter family of solution
curves is quantified by the largest singular value of an appropriate Jacobian.
This allows us to predict a preferred throwing style in terms of this singular
value, which itself depends on target location and the target shape. Our
analysis also allows us to characterize the trade-off between speed and
accuracy despite not including any effects of signal-dependent noise. Using
nonlinear calculations for propagating finite input-noise, we find that an
underarm throw to a target leads to an undershoot, but an overarm throw does
not. Finally, we consider the limit of the arm-length vanishing, i.e. shooting
a projectile, and find that the most accurate shooting angle bifurcates as the
ratio of the relative noisiness of the initial conditions crosses a threshold.Comment: 18 pages, 8 figure
A Temperature and Abundance Retrieval Method for Exoplanet Atmospheres
We present a new method to retrieve molecular abundances and temperature
profiles from exoplanet atmosphere photometry and spectroscopy. We run millions
of 1D atmosphere models in order to cover the large range of allowed parameter
space, and present error contours in the atmospheric properties, given the
data. In order to run such a large number of models, we have developed a
parametric pressure-temperature (P-T) profile coupled with line-by-line
radiative transfer, hydrostatic equilibrium, and energy balance, along with
prescriptions for non-equilibrium molecular composition and energy
redistribution. We apply our temperature and abundance retrieval method to the
atmospheres of two transiting exoplanets, HD 189733b and HD 209458b, which have
the best available Spitzer and HST observations. For HD 189733b, we find
efficient day-night redistribution of energy in the atmosphere, and molecular
abundance constraints confirming the presence of H2O, CO, CH4, and CO2. For HD
209458b, we confirm and constrain the day-side thermal inversion in an average
1D temperature profile. We also report independent detections of HO, CO,
CH and CO on the dayside of HD 209458b, based on six-channel Spitzer
photometry. We report constraints for HD 189733b due to individual data sets
separately; a few key observations are variable in different data sets at
similar wavelengths. Moreover, a noticeably strong carbon dioxide absorption in
one data set is significantly weaker in another. We must, therefore,
acknowledge the strong possibility that the atmosphere is variable, both in its
energy redistribution state and in the chemical abundances.Comment: 20 pages in emulateapj format, 11 figures. Final version, after proof
correction
Tromino: Demand and DRF Aware Multi-Tenant Queue Manager for Apache Mesos Cluster
Apache Mesos, a two-level resource scheduler, provides resource sharing
across multiple users in a multi-tenant cluster environment. Computational
resources (i.e., CPU, memory, disk, etc. ) are distributed according to the
Dominant Resource Fairness (DRF) policy. Mesos frameworks (users) receive
resources based on their current usage and are responsible for scheduling their
tasks within the allocation. We have observed that multiple frameworks can
cause fairness imbalance in a multiuser environment. For example, a greedy
framework consuming more than its fair share of resources can deny resource
fairness to others. The user with the least Dominant Share is considered first
by the DRF module to get its resource allocation. However, the default DRF
implementation, in Apache Mesos' Master allocation module, does not consider
the overall resource demands of the tasks in the queue for each user/framework.
This lack of awareness can result in users without any pending task receiving
more resource offers while users with a queue of pending tasks starve due to
their high dominant shares. We have developed a policy-driven queue manager,
Tromino, for an Apache Mesos cluster where tasks for individual frameworks can
be scheduled based on each framework's overall resource demands and current
resource consumption. Dominant Share and demand awareness of Tromino and
scheduling based on these attributes can reduce (1) the impact of unfairness
due to a framework specific configuration, and (2) unfair waiting time due to
higher resource demand in a pending task queue. In the best case, Tromino can
significantly reduce the average waiting time of a framework by using the
proposed Demand-DRF aware policy
Exploring the Fairness and Resource Distribution in an Apache Mesos Environment
Apache Mesos, a cluster-wide resource manager, is widely deployed in massive
scale at several Clouds and Data Centers. Mesos aims to provide high cluster
utilization via fine grained resource co-scheduling and resource fairness among
multiple users through Dominant Resource Fairness (DRF) based allocation. DRF
takes into account different resource types (CPU, Memory, Disk I/O) requested
by each application and determines the share of each cluster resource that
could be allocated to the applications. Mesos has adopted a two-level
scheduling policy: (1) DRF to allocate resources to competing frameworks and
(2) task level scheduling by each framework for the resources allocated during
the previous step. We have conducted experiments in a local Mesos cluster when
used with frameworks such as Apache Aurora, Marathon, and our own framework
Scylla, to study resource fairness and cluster utilization. Experimental
results show how informed decision regarding second level scheduling policy of
frameworks and attributes like offer holding period, offer refusal cycle and
task arrival rate can reduce unfair resource distribution. Bin-Packing
scheduling policy on Scylla with Marathon can reduce unfair allocation from
38\% to 3\%. By reducing unused free resources in offers we bring down the
unfairness from to 90\% to 28\%. We also show the effect of task arrival rate
to reduce the unfairness from 23\% to 7\%
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