Software bottlenecking in client-server systems and rendezvousnetworks

Software bottlenecks are performance constraints caused by slow execution of a software task, in typical client-server systems a client task must wait in a blocked state for the server task to respond to its requests, so a saturated server will slow down all its clients. A rendezvous network generalizes this relationship to multiple layers of servers with send-and-wait interactions (rendezvous), a two-phase model of task behavior, and to a unified model for hardware and software contention. Software bottlenecks have different symptoms, different behavior when the system is altered, and a different cure from the conventional bottlenecks seen in queueing network models of computer systems, caused by hardware limits. The differences are due to the “push-back” effect of the rendezvous, which spreads the saturation of a server to its clients. The paper describes software bottlenecks by examples, gives a definition, shows how they can be located and alleviated, and gives a method for estimating the performance benefit to be obtained. Ultimately, if all the software bottlenecks can be removed, the performance limit will be due to a conventional hardware bottleneck     

DK UK10-A系列音箱

UK10-A系列适用于各种固定安装的高档演艺吧、慢摇吧及现场表演。该系列采用高功率扬声器单元和轻巧的箱体搭配，通过计算机优化设计，达到平滑的特性、展宽的频响特点和良好的控制覆盖性能。     

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part II. Modeling

In this article, the tensile and fracture properties of a discontinuously reinforced aluminum (DRA) alloy composite are modeled to determine the influence of constituent parameters on material behavior. Comparison of the elastic-modulus calculations to the experimental data suggest that the angular particles are more effective in load transfer than spherical particles, and that a unit cylinder geometry is a good representation of the particles under elastic conditions. This same geometry is used in the finite element-based elastic-plastic model of Bao et al., and reasonably good agreement is obtained between the experimental and predicted yield strengths. A fracture-mechanics model is proposed for predicting the elongation to failure. The model assumes the existence of particle cracks, and criticality is based on the strain required for matrix rupture between cracked particles. The damage criterion of Cockcroft and Latham is utilized, and model predictions are compared to data from different investigations. It is shown that the volume fraction of particles and the work-hardening coefficient of the matrix have a strong influence on the strain to failure. Fracture toughness modeling once again exposes the limitations of existing zero-degree crack-propagation models, such as that of Hahn and Rosenfield, which predict increased toughness with yield strength rather than a decrease, which is observed experimentally. A shear-failure model along a 45-deg direction is proposed for the higher-strength conditions, where concentrated slip bands were observed. The model exhibits the inverse toughness dependence on strength and better correlation to peak-aged (PA) data, but shows poorer agreement with underaged (UA) data. Thus, a transition from zero-degree propagation to 45-deg propagation with increasing strength is suggested. A simplified method for extracting particle stresses is illustrated and is used to estimate a Weibull modulus of 4.9 and a Weibull strength of 2450 MPa for the SiC particles of an average diameter of 10 μm. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Glass and glass-ceramic coatings, versatile materials for industrial and engineering applications

Among various coating systems for industrial and engineering applications, glass and glass-ceramic coatings have advantages of chemical inertness, high temperature stability and superior mechanical properties such as abrasion, impact etc as compared to other coating materials applied by thermal spraying in its different forms viz. PVD, CVD, plasma, etc. Besides imparting required functional properties such as heat, abrasion and corrosion resistance to suit particular end use requirements, the glass and glass-ceramic coatings in general also provide good adherence, defect free surface and refractoriness. Systematic studies covering the basic science of glass and glass-ceramic coatings, the functional properties required for a particular end-use along with the various fields of application have been reviewed in this paper.     

Properties of glass fibre cement — the effect of fibre length and content

The properties of glass fibre reinforced cement composites (grc) containing alkali-resistant fibres of lengths 10 to 40 mm and volume fractions 2 to 8% have been studied. At 28 days the optimum properties of the composite were achieved with 6 vol % fibre addition. These were 4 to 5 times the bending strength, 3 to 4 times the tensile strength and 15 to 20 times the impact strength of the unreinforced cement paste. Further increase in the fibre content increases the porosity of the composite resulting in the lowering of bending and tensile strengths. The stress and strain of the composite at matrix cracking increased with increasing fibre contents. No significant improvements in the modulus of the composite were observed over the range of fibre additions investigated. The trends in the properties of grc as affected by the variations in volume fraction and length of the fibre, and environmental conditions of curing of the composites, are qualitatively related to the degree of cement hydration, changes in porosity of the composites and fibre/matrix interfacial effects. The properties of grc change with time, (strengths tend to decrease) and long term studies are in progress.     

Laboratory-simulation experiment for optical communication through low-visibility atmosphere using a diode laser

A laboratory-simulation experiment has been performed to determine the feasibility of exploiting the scattered (in addition to the unscattered) radiation to improve optical communication through low-visibility atmosphere. A multiple field-of-views (FOV) (0.043-0.945degfull angle) optical receiver was designed which utilizes a narrow-band interference filter (Deltalambda sim 15Å) for background-light suppression. The laser transmitter was a CW GaAlAs laser diode (lambda = 0.8486 mum) capable of emitting ∼ 7 mW power output of 15° divergent beam. Both polydisperse (particle diameters,din the range ofsim0.2-5.17 mum) and monodisperse (d sim 0.2 mum andd = 3.01 mum) latex spheres in water were used for simulating haze, fog, etc. Results include signal-to-noise ratio (SNR) and scattered-to-unscattered signal ratio as a function of field-of-view of reception for various optical thickness of the medium. Also, SNR is plotted as a function of optical thickness for various FOV's. The unique feature of this simulation is that it can simulate worst case solar background where the sun enters the FOV of receiver. Finally a new technique of estimating forward-scattering efficiency and root-mean-square forward scatter angle has also been presented with examples.