543 research outputs found
Linearization Errors in Discrete Goal-Oriented Error Estimation
Goal-oriented error estimation provides the ability to approximate the
discretization error in a chosen functional quantity of interest. Adaptive mesh
methods provide the ability to control this discretization error to obtain
accurate quantity of interest approximations while still remaining
computationally feasible. Traditional discrete goal-oriented error estimates
incur linearization errors in their derivation. In this paper, we investigate
the role of linearization errors in adaptive goal-oriented error simulations.
In particular, we develop a novel two-level goal-oriented error estimate that
is free of linearization errors. Additionally, we highlight how linearization
errors can facilitate the verification of the adjoint solution used in
goal-oriented error estimation. We then verify the newly proposed error
estimate by applying it to a model nonlinear problem for several quantities of
interest and further highlight its asymptotic effectiveness as mesh sizes are
reduced. In an adaptive mesh context, we then compare the newly proposed
estimate to a more traditional two-level goal-oriented error estimate. We
highlight that accounting for linearization errors in the error estimate can
improve its effectiveness in certain situations and demonstrate that localizing
linearization errors can lead to more optimal adapted meshes
Distilling the Real Cost of Production Garbage Collectors
Abridged abstract: despite the long history of garbage collection (GC) and
its prevalence in modern programming languages, there is surprisingly little
clarity about its true cost. Without understanding their cost, crucial
tradeoffs made by garbage collectors (GCs) go unnoticed. This can lead to
misguided design constraints and evaluation criteria used by GC researchers and
users, hindering the development of high-performance, low-cost GCs. In this
paper, we develop a methodology that allows us to empirically estimate the cost
of GC for any given set of metrics. By distilling out the explicitly
identifiable GC cost, we estimate the intrinsic application execution cost
using different GCs. The minimum distilled cost forms a baseline. Subtracting
this baseline from the total execution costs, we can then place an empirical
lower bound on the absolute costs of different GCs. Using this methodology, we
study five production GCs in OpenJDK 17, a high-performance Java runtime. We
measure the cost of these collectors, and expose their respective key
performance tradeoffs. We find that with a modestly sized heap, production GCs
incur substantial overheads across a diverse suite of modern benchmarks,
spending at least 7-82% more wall-clock time and 6-92% more CPU cycles relative
to the baseline cost. We show that these costs can be masked by concurrency and
generous provisioning of memory/compute. In addition, we find that newer
low-pause GCs are significantly more expensive than older GCs, and,
surprisingly, sometimes deliver worse application latency than stop-the-world
GCs. Our findings reaffirm that GC is by no means a solved problem and that a
low-cost, low-latency GC remains elusive. We recommend adopting the
distillation methodology together with a wider range of cost metrics for future
GC evaluations.Comment: Camera-ready versio
Adjusting the melting point of a model system via Gibbs-Duhem integration: application to a model of Aluminum
Model interaction potentials for real materials are generally optimized with
respect to only those experimental properties that are easily evaluated as
mechanical averages (e.g., elastic constants (at T=0 K), static lattice
energies and liquid structure). For such potentials, agreement with experiment
for the non-mechanical properties, such as the melting point, is not guaranteed
and such values can deviate significantly from experiment. We present a method
for re-parameterizing any model interaction potential of a real material to
adjust its melting temperature to a value that is closer to its experimental
melting temperature. This is done without significantly affecting the
mechanical properties for which the potential was modeled. This method is an
application of Gibbs-Duhem integration [D. Kofke, Mol. Phys.78, 1331 (1993)].
As a test we apply the method to an embedded atom model of aluminum [J. Mei and
J.W. Davenport, Phys. Rev. B 46, 21 (1992)] for which the melting temperature
for the thermodynamic limit is 826.4 +/- 1.3K - somewhat below the experimental
value of 933K. After re-parameterization, the melting temperature of the
modified potential is found to be 931.5K +/- 1.5K.Comment: 9 pages, 5 figures, 4 table
The Canada-UK Deep Submillimetre Survey: First Submillimetre Images, the Source Counts, and Resolution of the Background
We present the first results of a deep unbiased submillimetre survey carried
out at 450 and 850 microns. We detected 12 sources at 850 microns, giving a
surface density of sources with 850-micron flux densities > 2.8mJy of of
0.49+-0.16 per square arcmin. The sources constitute 20-30% of the background
radiation at 850 microns and thus a significant fraction of the entire
background radiation produced by stars. This implies, through the connection
between metallicity and background radiation, that a significant fraction of
all the stars that have ever been formed were formed in objects like those
detected here. The combination of their large contribution to the background
radiation and their extreme bolometric luminosities make these objects
excellent candidates for being proto-ellipticals. Optical astronomers have
recently shown that the UV-luminosity density of the universe increases by a
factor of about 10 between z=0 and z=1 and then decreases again at higher
redshifts. Using the results of a parallel submillimetre survey of the local
universe, we show that both the submillimetre source density and background can
be explained if the submillimetre luminosity density evolves in a similar way
to the UV-luminosity density. Thus, if these sources are ellipticals in the
process of formation, they may be forming at relatively modest redshifts.Comment: 8 pages (LATEX), 6 postscript figures, submitted to ApJ Letter
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