583 research outputs found
Linux kernel compaction through cold code swapping
There is a growing trend to use general-purpose operating systems like Linux in embedded systems. Previous research focused on using compaction and specialization techniques to adapt a general-purpose OS to the memory-constrained environment, presented by most, embedded systems. However, there is still room for improvement: it has been shown that even after application of the aforementioned techniques more than 50% of the kernel code remains unexecuted under normal system operation. We introduce a new technique that reduces the Linux kernel code memory footprint, through on-demand code loading of infrequently executed code, for systems that support virtual memory. In this paper, we describe our general approach, and we study code placement algorithms to minimize the performance impact of the code loading. A code, size reduction of 68% is achieved, with a 2.2% execution speedup of the system-mode execution time, for a case study based on the MediaBench II benchmark suite
Ballistic electron transport in stubbed quantum waveguides: experiment and theory
We present results of experimental and theoretical investigations of electron
transport through stub-shaped waveguides or electron stub tuners (ESTs) in the
ballistic regime. Measurements of the conductance G as a function of voltages,
applied to different gates V_i (i=bottom, top, and side) of the device, show
oscillations in the region of the first quantized plateau which we attribute to
reflection resonances. The oscillations are rather regular and almost periodic
when the height h of the EST cavity is small compared to its width. When h is
increased, the oscillations become less regular and broad depressions in G
appear. A theoretical analysis, which accounts for the electrostatic potential
formed by the gates in the cavity region, and a numerical computation of the
transmission probabilities successfully explains the experimental observations.
An important finding for real devices, defined by surface Schottky gates, is
that the resonance nima result from size quantization along the transport
direction of the EST.Comment: Text 20 pages in Latex/Revtex format, 11 Postscript figures. Phys.
Rev. B,in pres
Electron transport in Coulomb- and tunnel-coupled one-dimensional systems
We develop a linear theory of electron transport for a system of two
identical quantum wires in a wide range of the wire length L, unifying both the
ballistic and diffusive transport regimes. The microscopic model, involving the
interaction of electrons with each other and with bulk acoustical phonons
allows a reduction of the quantum kinetic equation to a set of coupled
equations for the local chemical potentials for forward- and backward-moving
electrons in the wires. As an application of the general solution of these
equations, we consider different kinds of electrical contacts to the
double-wire system and calculate the direct resistance, the transresistance, in
the presence of tunneling and Coulomb drag, and the tunneling resistance. If L
is smaller than the backscattering length l_P, both the tunneling and the drag
lead to a negative transresistance, while in the diffusive regime (L >>l_P) the
tunneling opposes the drag and leads to a positive transresistance. If L is
smaller than the phase-breaking length, the tunneling leads to interference
oscillations of the resistances that are damped exponentially with L.Comment: Text 14 pages in Latex/Revtex format, 4 Postscript figure
Inferring Energy Bounds via Static Program Analysis and Evolutionary Modeling of Basic Blocks
The ever increasing number and complexity of energy-bound devices (such as
the ones used in Internet of Things applications, smart phones, and mission
critical systems) pose an important challenge on techniques to optimize their
energy consumption and to verify that they will perform their function within
the available energy budget. In this work we address this challenge from the
software point of view and propose a novel parametric approach to estimating
tight bounds on the energy consumed by program executions that are practical
for their application to energy verification and optimization. Our approach
divides a program into basic (branchless) blocks and estimates the maximal and
minimal energy consumption for each block using an evolutionary algorithm. Then
it combines the obtained values according to the program control flow, using
static analysis, to infer functions that give both upper and lower bounds on
the energy consumption of the whole program and its procedures as functions on
input data sizes. We have tested our approach on (C-like) embedded programs
running on the XMOS hardware platform. However, our method is general enough to
be applied to other microprocessor architectures and programming languages. The
bounds obtained by our prototype implementation can be tight while remaining on
the safe side of budgets in practice, as shown by our experimental evaluation.Comment: Pre-proceedings paper presented at the 27th International Symposium
on Logic-Based Program Synthesis and Transformation (LOPSTR 2017), Namur,
Belgium, 10-12 October 2017 (arXiv:1708.07854). Improved version of the one
presented at the HIP3ES 2016 workshop (v1): more experimental results (added
benchmark to Table 1, added figure for new benchmark, added Table 3),
improved Fig. 1, added Fig.
Automatic parallelization of irregular and pointer-based computations: perspectives from logic and constraint programming
Irregular computations pose some of the most interesting and
challenging problems in automatic parallelization. Irregularity appears in certain kinds of numerical problems and is pervasive in symbolic applications. Such computations often use dynamic data structures which make heavy use of pointers. This complicates all the steps of a parallelizing compiler, from independence detection to task partitioning and placement. In the past decade there has been significant progress in the development of parallelizing compilers for logic programming and, more recently, constraint programming. The typical applications of these paradigms frequently involve irregular computations, which arguably makes the techniques used in these compilers potentially interesting. In this paper we
introduce in a tutorial way some of the problems faced by parallelizing compilers for logic and constraint programs. These include the need for inter-procedural pointer aliasing analysis for independence detection and having to manage speculative and irregular computations through task
granularity control and dynamic task allocation. We also provide pointers to some of the progress made in these áreas. In the associated talk we demónstrate representatives of several generations of these parallelizing compilers
Universal conductance fluctuations in three dimensional metallic single crystals of Si
In this paper we report the measurement of conductance fluctuations in single
crystals of Si made metallic by heavy doping (n \approx 2-2.5n_c, n_c being
critical composition at Metal-Insulator transition). Since all dimensions (L)
of the samples are much larger than the electron phase coherent length L_\phi
(L/L_\phi \sim 10^3), our system is truly three dimensional. Temperature and
magnetic field dependence of noise strongly indicate the universal conductance
fluctuations (UCF) as predominant source of the observed magnitude of noise.
Conductance fluctuations within a single phase coherent region of L_\phi^3 was
found to be saturated at \approx (e^2/h)^2. An accurate
knowledge of the level of disorder, enables us to calculate the change in
conductance \delta G_1 due to movement of a single scatterer as \delta G_1 \sim
e^2/h, which is \sim 2 orders of magnitude higher than its theoretically
expected value in 3D systems.Comment: Text revised version. 4 eps figs unchange
Determinacy analysis for logic programs using mode and type information
We propose an analysis for detecting procedures and goals
that are deterministic (i.e. that produce at most one solution), or predicates whose clause tests are mutually exclusive (which implies that at most one of their clauses will succeed) even if they are not deterministic (because they cali other predicates that can produce more than one solution). Applications of such determinacy information include detecting programming errors, performing certain high-level program transformations for improving search efñciency, optimizing low level code generation and parallel execution, and estimating tighter upper bounds on the computational costs of goals and data sizes, which can be used for program debugging, resource consumption and granularity control, etc. We have implemented the analysis and integrated it in the CiaoPP system, which also infers automatically the mode and type information that our analysis takes as input. Experiments performed on this implementation show that the analysis is fairly accurate and efncient
Mesoscopic fluctuations of Coulomb drag between quasi-ballistic 1D-wires
Quasiballistic 1D quantum wires are known to have a conductance of the order
of 2e^2/h, with small sample-to-sample fluctuations. We present a study of the
transconductance G_12 of two Coulomb-coupled quasiballistic wires, i.e., we
consider the Coulomb drag geometry. We show that the fluctuations in G_12
differ dramatically from those of the diagonal conductance G_ii: the
fluctuations are large, and can even exceed the mean value, thus implying a
possible reversal of the induced drag current. We report extensive numerical
simulations elucidating the fluctuations, both for correlated and uncorrelated
disorder. We also present analytic arguments, which fully account for the
trends observed numerically.Comment: 10 pages including 7 figures. Minor changes according to referee
report. Accepted for PR
Automatic Inference of Determinacy and Mutual Exclusion for Logic Programs Using Mode and Type Analyses
We propose an analysis for detecting procedures and goals
that are deterministic (i.e., that produce at most one solution at most once),or predicates whose clause tests are mutually exclusive (which implies that at most one of their clauses will succeed) even if they are not deterministic.
The analysis takes advantage of the pruning operator in order to improve the detection of mutual exclusion and determinacy. It also supports arithmetic equations and disequations, as well as equations and disequations on terms,for which we give a complete satisfiability testing algorithm, w.r.t. available type information. Information about determinacy can be used for program debugging and optimization, resource consumption and granularity control,
abstraction carrying code, etc. We have implemented the analysis and integrated it in the CiaoPP system, which also infers automatically the mode and type information that our analysis takes as input. Experiments performed on this implementation show that the analysis is fairly accurate and efficient
Coulomb Drag in Coherent Mesoscopic Systems
We present a theory for Coulomb drag between two mesoscopic systems. Our
formalism expresses the drag in terms of scattering matrices and wave
functions, and its range of validity covers both ballistic and disordered
systems. The consequences can be worked out either by analytic means, such as
the random matrix theory, or by numerical simulations. We show that Coulomb
drag is sensitive to localized states, which usual transport measurements do
not probe. For chaotic 2D-systems we find a vanishing average drag, with a
nonzero variance. Disordered 1D-wires show a finite drag, with a large
variance, giving rise to a possible sign change of the induced current.Comment: 4 pages including 2 figures. Minor changes. Accepted for publication
in Phys. Rev. Let
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