3,009 research outputs found
Prioritized Garbage Collection: Explicit GC Support for Software Caches
Programmers routinely trade space for time to increase performance, often in
the form of caching or memoization. In managed languages like Java or
JavaScript, however, this space-time tradeoff is complex. Using more space
translates into higher garbage collection costs, especially at the limit of
available memory. Existing runtime systems provide limited support for
space-sensitive algorithms, forcing programmers into difficult and often
brittle choices about provisioning.
This paper presents prioritized garbage collection, a cooperative programming
language and runtime solution to this problem. Prioritized GC provides an
interface similar to soft references, called priority references, which
identify objects that the collector can reclaim eagerly if necessary. The key
difference is an API for defining the policy that governs when priority
references are cleared and in what order. Application code specifies a priority
value for each reference and a target memory bound. The collector reclaims
references, lowest priority first, until the total memory footprint of the
cache fits within the bound. We use this API to implement a space-aware
least-recently-used (LRU) cache, called a Sache, that is a drop-in replacement
for existing caches, such as Google's Guava library. The garbage collector
automatically grows and shrinks the Sache in response to available memory and
workload with minimal provisioning information from the programmer. Using a
Sache, it is almost impossible for an application to experience a memory leak,
memory pressure, or an out-of-memory crash caused by software caching.Comment: to appear in OOPSLA 201
Improvements on ORCA for Fast Computation of Graphlet Degree Vectors of any Graphlet Order
It is increasingly common to find real-life structures or behaviors represented as graphs in many areas of the computing sciences. Comparing these graphs is a hard task, especially when we are interested in assigning a non-binary similarity score between two large graphs based on some domain-specific context. In bioinformatics, social network analysis and other areas is frequently necessary to compute graph similarities based on the local topological information of each vertex of the given graphs. This is why graphlet degree vectors have become more and more popular in these areas. They provide a simple yet detailed representation of a vertex\u27s topology by counting the number of times such vertex touches a list of small predefined sub-structures called graphlets. In this thesis, we study the state-of-the-art algorithm to compute graphlet degree vectors, the Orbit Counting Algorithm (ORCA). ORCA generates a triangular system of linear equations that can be quickly solved to obtain the graphlet degree vector of a vertex. We make theoretical and practical improvements to this algorithm and measure the difference in speed after these improvements. The theoretical improvement consists of finding automorphisms of graphlets given a fixed vertex that is required to map to itself in such automorphisms. We observe that one piece of the algorithm runs much faster than before with this improvement, especially for larger graphlet orders. This helps the algorithm take less time in generating the linear system that we use to find the desired graphlet degree vector. The practical improvement consists of making a flexible implementation of the algorithm, which can take any graphlet size as input, any number of input graphs, and compute the graphlet degree vector for every vertex in each one of those graphs
Friction force on slow charges moving over supported graphene
We provide a theoretical model that describes the dielectric coupling of a 2D
layer of graphene, represented by a polarization function in the Random Phase
Approximation, and a semi-infinite 3D substrate, represented by a surface
response function in a non-local formulation. We concentrate on the role of the
dynamic response of the substrate for low-frequency excitations of the combined
graphene-substrate system, which give rise to the stopping force on slowly
moving charges above graphene. A comparison of the dielectric loss function
with experimental HREELS data for graphene on a SiC substrate is used to
estimate the damping rate in graphene and to reveal the importance of phonon
excitations in an insulating substrate. A signature of the hybridization
between graphene's pi plasmon and the substrate's phonon is found in the
stopping force. A friction coefficient that is calculated for slow charges
moving above graphene on a metallic substrate shows an interplay between the
low-energy single-particle excitations in both systems.Comment: 13 pages, 5 figures, submitted to Nanotechnology for a special issue
related to the NGC 2009 conference (http://asdn.net/ngc2009/index.shtml
Energy controlled non-intrusive code coupling for dynamic fluid structure failure
This paper presents a non intrusive approach to couple a SPH fluid code with a FEM or SPH solid code to treat large strains and failure of solids in case of strongly non-linear fluid-structure interaction
Superconformal Gauge Theories and Non-Critical Superstrings
We consider effective actions for six-dimensional non-critical superstrings.
We show that the addition of units of R-R flux and of space-time
filling D5-branes produces solutions with curvature
comparable to the string scale. These solutions have the right structure to be
dual to supersymmetric SU(N) gauge theories with flavors. We
further suggest bounds on the mass-squared of tachyonic fields in this
background that should restrict the theory to the conformal window.Comment: 16 pages, 1 figure v4: Minor change
Spin transfer in an antiferromagnet
An electrical current can transfer spin angular momentum to a ferromagnet.
This novel physical phenomenon, called spin transfer, offers unprecedented
spatial and temporal control over the magnetic state of a ferromagnet and has
tremendous potential in a broad range of technologies, including magnetic
memory and recording. Recently, it has been predicted that spin transfer is not
limited to ferromagnets, but can also occur in antiferromagnetic materials and
even be stronger under some conditions. In this paper we demonstrate transfer
of spin angular momentum across an interface between ferromagnetic and
antiferromagnetic metals. The spin transfer is mediated by an electrical
current of high density (~10^12 A/m^2) and revealed by variation in the
exchange bias at the ferromagnet/antiferromagnet interface. We find that,
depending on the polarity of the electrical current flowing across the
interface, the strength of the exchange bias can either increase or decrease.
This finding is explained by the theoretical prediction that a spin polarized
current generates a torque on magnetic moments in the antiferromagnet.
Current-mediated variation of exchange bias can be used to control the magnetic
state of spin-valve devices, e.g., in magnetic memory applications.Comment: 5 pages, 3 figure
Tomonaga-Luttinger liquids and Coulomb blockade in multiwall carbon nanotubes under pressure
We report that the conductance of macroscopic multiwall nanotube (MWNT)
bundles under pressure shows power laws in temperature and voltage, as
corresponding to a network of bulk-bulk connected Tomonaga-Luttinger Liquids
(LL). Contrary to individual MWNT, where the observed power laws are attributed
to Coulomb blockade, the measured ratio for the end and bulk obtained
exponents, ~2.4, can only be accounted for by LL theory. At temperatures
characteristic of interband separation, it increases due to thermal population
of the conducting sheets unoccupied bands.Comment: 16 pages, 3 Figures, .pdf. Accepted in Phys. Rev. Let
Band-structure trend in hole-doped cuprates and correlation with Tcmax
By calculation and analysis of the bare conduction bands in a large number of
hole-doped high-temperature superconductors, we have identified the energy of
the so-called axial-orbital as the essential, material-dependent parameter. It
is uniquely related to the range of the intra-layer hopping. It controls the Cu
4s-character, influences the perpendicular hopping, and correlates with the
observed Tc at optimal doping. We explain its dependence on chemical
composition and structure, and present a generic tight-binding model.Comment: 5 pages, Latex, 5 eps figure
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