47 research outputs found
A case study on latency, bandwidth and energy efficiency of mobile 5G and YouTube Edge service in London. Why the 5G ecosystem and energy efficiency matter?
The advancements in 5G mobile networks and Edge computing offer great
potential for services like augmented reality and Cloud gaming, thanks to their
low latency and high bandwidth capabilities. However, the practical limitations
of achieving optimal latency on real applications remain uncertain. This paper
aims to investigate the actual latency and bandwidth provided by 5G Networks
and YouTube Edge service in London, UK. We analyze how latency and bandwidth
differ between 4G LTE and 5G networks and how the location of YouTube Edge
servers impacts these metrics. Our research reveals over 10 significant
observations and implications, indicating that the primary constraints on 4G
LTE and 5G capabilities are the ecosystem and energy efficiency of mobile
devices down-streaming data. Our study demonstrates that to fully unlock the
potential of 5G and it's applications, it is crucial to prioritize efforts
aimed at improving the ecosystem and enhancing the energy efficiency
Cosmological Fluctuations from Infra-Red Cascading During Inflation
We propose a qualitatively new mechanism for generating cosmological
fluctuations from inflation. The non-equilibrium excitation of interacting
scalar fields often evolves into infra-red (IR) and ultra-violet (UV)
cascading, resulting in an intermediate scaling regime. We observe elements of
this phenomenon in a simple model with inflaton \phi and iso-inflaton \chi
fields interacting during inflation via the coupling g^2 (\phi-\phi_0)^2
\chi^2. Iso-inflaton particles are created during inflation when they become
instantaneously massless at \phi=\phi_0, with occupation numbers not exceeding
unity. We point out that very quickly the produced \chi particles become heavy
and their multiple re-scatterings off the homogeneous condensate \phi(t)
generates bremschtrahlung radiation of light inflaton IR fluctuations with high
occupation numbers. The subsequent evolution of these IR fluctuations is
qualitatively similar to that of the usual inflationary fluctuations, but their
initial amplitude is different. The IR cascading generates a bump-shaped
contribution to the cosmological curvature fluctuations, which can even
dominate over the usual fluctuations for g^2>0.06. The IR cascading curvature
fluctuations are significantly non-gaussian and the strength and location of
the bump are model-dependent, through g^2 and \phi_0. The effect from IR
cascading fluctuations is significantly larger than that from the momentary
slowing-down of \phi(t). With a sequence of such bursts of particle production,
the superposition of the bumps can lead to a new broad band non-gaussian
component of cosmological fluctuations added to the usual fluctuations. Such a
sequence of particle creation events can, but need not, lead to trapped
inflation.Comment: 13 pages, 9 figures. Accepted for publication in Phys. Rev.
Exact identification of the radion and its coupling to the observable sector
Braneworld models in extra dimensions can be tested in laboratory by the
coupling of the radion to the Standard Model fields. The identification of the
radion as a canonically normalized field involves a careful General Relativity
treatment: if a bulk scalar is responsible for the stabilization of the system,
its fluctuations are entangled with the perturbations of the metric and they
also have to be taken into account (similarly to the well-developed theory of
scalar metric perturbations in 4D cosmology with a scalar field). Extracting a
proper dynamical variable in a warped geometry/scalar setting is a nontrivial
task, performed so far only in the limit of negligible backreaction of the
scalar field on the background geometry. We perform the general calculation,
diagonalizing the action up to second order in the perturbations and
identifying the physical eigenmodes of the system for any amplitude of the bulk
scalar. This computation allows us to derive a very simple expression for the
exact coupling of the eigenmodes to the Standard Model fields on the brane,
valid for an arbitrary background configuration. As an application, we discuss
the Goldberger-Wise mechanism for the stabilization of the radion in the
Randall-Sundrum type models. The existing studies, limited to small amplitude
of the bulk scalar field, are characterized by a radion mass which is
significantly below the physical scale at the observable brane. We extend them
beyond the small backreaction regime. For intermediate amplitudes, the radion
mass approaches the electroweak scale, while its coupling to the observable
brane remains nearly constant. At very high amplitudes, the radion mass instead
decreases, while the coupling sharply increases. Severe experimental
constraints are expected in this regime.Comment: 20 pages, 6 figure
Inflationary Theory and Alternative Cosmology
Recently Hollands and Wald argued that inflation does not solve any of the
major cosmological problems. We explain why we disagree with their arguments.
They also proposed a new speculative mechanism of generation of density
perturbations. We show that in their scenario the inhomogeneities responsible
for the large scale structure observed today were generated at an epoch when
the energy density of the hot universe was 10^{95} times greater than the
Planck density. The only way to avoid this problem is to assume that there was
a stage of inflation in the early universe.Comment: 17 pages, 1 fig, a discussion of a canonical measure of probability
of inflation is adde
Inflation and de Sitter Thermodynamics
We consider the quasi-de Sitter geometry of the inflationary universe. We
calculate the energy flux of the slowly rolling background scalar field through
the quasi-de Sitter apparent horizon and set it equal to the change of the
entropy (1/4 of the area) multiplied by the temperature, dE=TdS. Remarkably,
this thermodynamic law reproduces the Friedmann equation for the rolling scalar
field. The flux of the slowly rolling field through the horizon of the quasi-de
Sitter geometry is similar to the accretion of a rolling scalar field onto a
black hole, which we also analyze. Next we add inflaton fluctuations which
generate scalar metric perturbations. Metric perturbations result in a
variation of the area entropy. Again, the equation dE=TdS with fluctuations
reproduces the linearized Einstein equations. In this picture as long as the
Einstein equations hold, holography does not put limits on the quantum field
theory during inflation. Due to the accumulating metric perturbations, the
horizon area during inflation randomly wiggles with dispersion increasing with
time. We discuss this in connection with the stochastic decsription of
inflation. We also address the issue of the instability of inflaton
fluctuations in the ``hot tin can'' picture of de Sitter horizon.Comment: 19 pages, 5 figure
Power Capping: What Works, What Does Not
Peak power consumption is the first order design constraint of data centers. Though peak power consumption is rarely, if ever, observed, the entire data center facility must prepare for it, leading to inefficient usage of its resources. The most prominent way for addressing this issue is to limit the power consumption of the data center IT facility far below its theoretical peak value. Many approaches have been proposed to achieve that, based on the same small set of enforcement mechanisms, but there has been no corresponding work on systematically examining the advantages and disadvantages of each such mechanism. In the absence of such a study, it is unclear what is the optimal mechanism for a given computing environment, which can lead to unnecessarily poor performance if an inappropriate scheme is used. This paper fills this gap by comparing for the first time five widely used power capping mechanisms under the same hardware/software setting. We also explore possible alternative power capping mechanisms beyond what has been previously proposed and evaluate them under the same setup. We systematically analyze the strengths and weaknesses of each mechanism, in terms of energy efficiency, overhead, and predictable behavior. We show how these mechanisms can be combined in order to implement an optimal power capping mechanism which reduces the slowdown compared to the most widely used mechanism by up to 88%. Our results provide interesting insights regarding the different trade-offs of power capping techniques, which will be useful for designing and implementing highly efficient power capping in the future
ALEA: A Fine-grained Energy Profiling Tool
Energy efficiency is becoming increasingly important, yet few developers understand how source code changes affect the energy and power consumption of their programs. To enable them to achieve energy savings, we must associate energy consumption with software structures, especially at the fine-grained level of functions and loops. Most research in the field relies on direct power/energy measurements taken from on-board sensors or performance counters. However, this coarse granularity does not directly provide the needed fine-grained measurements. This article presents ALEA, a novel fine-grained energy profiling tool based on probabilistic analysis for fine-grained energy accounting. ALEA overcomes the limitations of coarse-grained power-sensing instruments to associate energy information effectively with source code at a fine-grained level. We demonstrate and validate that ALEA can perform accurate energy profiling at various granularity levels on two different architectures: Intel Sandy Bridge and ARM big.LITTLE. ALEA achieves a worst case error of only 2% for coarse-grained code structures and 6% for fine-grained ones, with less than 1% runtime overhead. Our use cases demonstrate that ALEA supports energy optimizations, with energy savings of up to 2.87 times for a latency-critical option pricing workload under a given power budget
On the new string theory inspired mechanism of generation of cosmological perturbations
Recently a non-inflationary mechanism of generation of scale-free
cosmological perturbations of metric was proposed by Brandenberger, Nayeri, and
Vafa in the context of the string gas cosmology. We discuss various problems of
their model and argue that the cosmological perturbations of metric produced in
this model have blue spectrum with a spectral index n = 5, which strongly
disagrees with observations. We conclude that this model in its present form is
not a viable alternative to inflationary cosmology.Comment: 11 pages, 1 figur
Suppressing the lower Multipoles in the CMB Anisotropies
The Cosmic Microwave Background (CMB) anisotropy power on the largest angular
scales observed both by WMAP and COBE DMR appears to be lower than the one
predicted by the standard model of cosmology with almost scale free primordial
perturbations arising from a period of inflation
\cite{cobe,Bennett:2003bz,Spergel,Peiris}. One can either interpret this as a
manifestation of cosmic variance or as a physical effect that requires an
explanation. We discuss various mechanisms that could be responsible for the
suppression of such low multipoles. Features in the late time evolution
of metric fluctuations may do this via the integral Sachs-Wolfe effect. Another
possibility is a suppression of power at large scales in the primordial
spectrum induced by a fast rolling stage in the evolution of the inflaton field
at the beginning of the last 65 e-folds of inflation. We illustrate this effect
in a simple model of inflation and fit the resulting CMB spectrum to the
observed temperature-temperature (TT) power spectrum. We find that the WMAP
observations suggest a cutoff at Mpc at 68% confidence, while only an upper limit of Mpc at 95%. Thus, although it improves the fit of the
data, the presence of a cutoff in power spectrum is only required at a level
close to . This is obtained with a prior which corresponds to equal
distribution wrt . We discuss how other choices (such as an equal
distribution wrt which is natural in the context of inflation) can
affect the statistical interpretation.Comment: 11 pages, 4 figures, replaced with published version, comparison with
recent papers is extende