1,771 research outputs found
Beltway: Getting Around Garbage Collection Gridlock
We present the design and implementation of a new garbage collection framework that significantly generalizes existing copying collectors. The Beltway framework exploits and separates object age and incrementality. It groups objects in one or more increments on queues called belts, collects belts independently, and collects increments on a belt in first-in-first-out order. We show that Beltway configurations, selected by command line options, act and perform the same as semi-space, generational, and older-first collectors, and encompass all previous copying collectors of which we are aware. The increasing reliance on garbage collected languages such as Java requires that the collector perform well. We show that the generality of Beltway enables us to design and implement new collectors that are robust to variations in heap size and improve total execution time over the best generational copying collectors of which we are aware by up to 40%, and on average by 5 to 10%, for small to moderate heap sizes. New garbage collection algorithms are rare, and yet we define not just one, but a new family of collectors that subsumes previous work. This generality enables us to explore a larger design space and build better collectors
Reorganization Plan No. 3 of 1979: Revamping the U.S. Trade Machinery
The United States is no longer the successful competitor it once was in the international marketplace.I During the last decade, concern has grown in both the national and international communities about the way in which United States trade policy is formulated and imple- mented. This concern has resulted in a reorganized and strengthened U.S. trade machinery. Briefly, the Carter Administration\u27s new Reor- ganization Plan No. 3 of 19792 has placed the responsibility for the negotiation of foreign trade matters and for the formulation of trade policy in the Office of the United States Trade Representative (USTR), and has called upon the Department of Commerce to implement these trade policies.\u2
Reorganization Plan No. 3 of 1979: Revamping the U.S. Trade Machinery
The United States is no longer the successful competitor it once was in the international marketplace.I During the last decade, concern has grown in both the national and international communities about the way in which United States trade policy is formulated and imple- mented. This concern has resulted in a reorganized and strengthened U.S. trade machinery. Briefly, the Carter Administration\u27s new Reor- ganization Plan No. 3 of 19792 has placed the responsibility for the negotiation of foreign trade matters and for the formulation of trade policy in the Office of the United States Trade Representative (USTR), and has called upon the Department of Commerce to implement these trade policies.\u2
On the Energetics of Advection-Dominated Accretion Flows
Using mean field MHD, we discuss the energetics of optically thin, two
temperature, advection-dominated accretion flows (ADAFs). If the magnetic field
is tangled and roughly isotropic, flux freezing is insufficient to maintain the
field in equipartition with the gas. In this case, we expect a fraction of the
energy generated by shear in the flow to be used to build up the magnetic field
strength as the gas flows in; the remaining energy heats the particles. We
argue that strictly equipartition magnetic fields are incompatible with a
priori reasonable levels of particle heating; instead, the plasma in
ADAFs (defined to be the gas pressure divided by magnetic/turbulent pressure)
is likely to be \gsim 5; correspondingly, the viscosity parameter is
likely to be \lsim 0.2Comment: 24 pages, ApJ submitte
Model Complexity of Program Phases
In resource limited computing systems, sequence prediction models must
operate under tight constraints. Various models are available that cater to
prediction under these conditions that in some way focus on reducing the cost
of implementation. These resource constrained sequence prediction models, in
practice, exhibit a fundamental tradeoff between the cost of implementation and
the quality of its predictions. This fundamental tradeoff seems to be largely
unexplored for models for different tasks. Here we formulate the necessary
theory and an associated empirical procedure to explore this tradeoff space for
a particular family of machine learning models such as deep neural networks. We
anticipate that the knowledge of the behavior of this tradeoff may be
beneficial in understanding the theoretical and practical limits of creation
and deployment of models for resource constrained tasks
Radiation Pressure Supported Starburst Disks and AGN Fueling
We consider the structure of marginally Toomre-stable starburst disks under
the assumption that radiation pressure on dust grains provides the dominant
vertical support against gravity. This is particularly appropriate when the
disk is optically thick to its own IR radiation, as in the central regions of
ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for
star formation changes in the optically-thick limit, with the star formation
rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface
density and kappa is the mean opacity. We show that optically thick starburst
disks have a characteristic flux and dust effective temperature of F ~ 10^{13}
L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with
observations and find good agreement. We extend our model from many-hundred
parsec scales to sub-parsec scales and address the problem of fueling AGN. We
assume that angular momentum transport proceeds via global torques rather than
a local viscosity. We account for the radial depletion of gas due to star
formation and find a strong bifurcation between two classes of disk models: (1)
solutions with a starburst on large scales that consumes all of the gas with
little fueling of a central AGN and (2) models with an outer large-scale
starburst accompanied by a more compact starburst on 1-10 pc scales and a
bright central AGN. The luminosity of the latter models is in many cases
dominated by the AGN. We show that the vertical thickness of the starburst disk
on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration
in some Type 2 AGN. We also argue that the disk of young stars in the Galactic
Center may be the remnant of such a compact nuclear starburst.Comment: 26 pages, 9 figures, emulateapj, accepted to ApJ, minor changes,
discussion tightened, references adde
Extragalactic Zeeman Detections in OH Megamasers
We have measured the Zeeman splitting of OH megamaser emission at 1667 MHz
from five (ultra)luminous infrared galaxies ([U]LIRGs) using the 305 m Arecibo
telescope and the 100 m Green Bank Telescope. Five of eight targeted galaxies
show significant Zeeman-splitting detections, with 14 individual masing
components detected and line-of-sight magnetic field strengths ranging from
~0.5-18 mG. The detected field strengths are similar to those measured in
Galactic OH masers, suggesting that the local process of massive star formation
occurs under similar conditions in (U)LIRGs and the Galaxy, in spite of the
vastly different large-scale environments. Our measured field strengths are
also similar to magnetic field strengths in (U)LIRGs inferred from synchrotron
observations, implying that milligauss magnetic fields likely pervade most
phases of the interstellar medium in (U)LIRGs. These results provide a
promising new tool for probing the astrophysics of distant galaxies.Comment: 32 pages, 14 figures, 8 tables. Accepted for publication in The
Astrophysical Journal v680n2, June 20, 2008; corrected 2 typo
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