A network model of barrier synchronization algorithms

We consider two algorithms for the barrier synchronization ofN processes: the Dissemination algorithm⁽²⁾ and Brooks algorithm.^(1,2) Both algorithms comprise a number of binary communications amongst the processes, organized into a sequence of stages. It is shown that Brooks' algorithm⁽¹⁾ requires between LogN(log₂ N) and 2 LogN stages, the lower bound being guaranteed only in the case thatN is a power of 2 (cubic) and the upper bound seemingly needed for most otherN. On the other hand, it is shown⁽²⁾ that the Dissemination algorithm requires only LogN stages regardless ofN, making it apparently superior to the Brooks algorithm. We introduce a network model of local barrier algorithms. Using it we obtain a rigorous correctness proof for local barrier algorithms, and show that the number of stages in the Brooks algorithm is bounded above by LogN+1. The Brooks algorithms is therefore essentially equivalent in time complexity to the Dissemination algorithm. We then address the question of which values ofN admit exactly LogN Brooks stages. We find a sufficient condition, and conjecture that it is also necessary.     

The Doomsday distributed termination detection protocol

Distributed termination detection (DTD) algorithms are important since they detect globally stable states in distributed computations. Here we introduce a new DTD mechanism, the Doomsday protocol together with its proof of correctness. Doomsday is generic since it forms the basis for a number of new and existing DTD algorithms for which the correctness proof may be reused. The paper describes the Doomsday protocol, provides its formal proof, derives one new DTD algorithm and shows how other hitherto unrelated algorithms, Dijkstra–Scholten, Task Balancing and Credit Recovery, can be derived from the protocol. The paper concludes by examining various properties of the protocol in the context of existing DTD algorithms. This work was supported in part by Visiting Fellowship grant EPSRC GR/R84481/01 “The Doomsday Protocol” and by Australian Research Council ARC Linkage International Grant LX0349049 “Extending a Family of Garbage Collectors”.     

Turbulence and Secondary Flow over Sediment Stripes in Weakly Bimodal Bed Material

Longitudinal stripes are a common bed form in heterogeneous alluvial sediments and consist of periodic, spanwise variations in bed texture and elevation that are aligned parallel to the mean flow direction. This paper quantifies mean and turbulent flow structures over self-formed sediment stripes in a weakly bimodal sand and gravel mixture. Turbulence anisotropy generates two secondary circulation cells across the channel half-width, which produce a cross-stream perturbation in boundary shear stress. The interaction between this flow structure and the selective transport of bed material generates spanwise sediment sorting that is symmetrical about the centerline. Finer sediments are entrained from regions of high shear stress, transported laterally by the secondary flow, and deposited in regions of lower shear stress. Lateral changes in bed texture further enhance the near-bed secondary flow, which provides a positive feedback mechanism for stripe growth. In bimodal sediments, at shear stresses just above the entrainment threshold, stripes may replace lower-stage plane beds. At higher shear stresses the coarser sediment becomes more mobile and the stripes are replaced by flow transverse bed forms.     

Transient response of a cold atomic beam in the presence of a far-off resonance light guide

Abstract

A low velocity intense source (LVIS) of cold rubidium atoms is guided along a co-propagating far-off resonance light guide. An 8% enhancement of the guided atomic flux is observed in the steady state. Data are presented for the transient response of the LVIS to the guide beam observing a non-adiabatic kick and a transient 25% enhancement of the guided flux. A characteristic decay time of 0.45 s is recorded for the return to the steady state flux.     

Identifying changes in gene expression in the nervous system: mRNA differential display

The majority of cellular and developmental processes are characterized by changes in gene expression. Recently, several polymerase chain reaction-based methods have been introduced for detecting genes whose expressions differ between cells or tissues. It is now possible to investigate whether novel gene expression occurs within many systems as a response to extracellular signals, and to identify systematically those genes. The characterization of cell-, tissue- or stage-specific gene expression will broaden our understanding of many complex biological processes.     

Functional differentiation within the neostriatum of the rat using electrical (blocking) stimulation during discrimination learning.

16 male albino rats, 8 with bipolar electrodes implanted bilaterally in the anterodorsal head of the caudate-putamen and 8 with similar electrodes in the posteroventral caudate-putamen, learned a spatial and a form discrimination task and their reversals while receiving "continuous" stimulation. Ss receiving stimulation to the anterodorsal caudate-putamen were impaired on spatial reversal learning compared with the posteroventral group. On form discrimination reversal the posteroventral group was impaired compared with the anterodorsal group. This dissociation appears to be related to the particular cortical neostriatal projection system for the region stimulated and to demonstrate a behavioral differentiation in rat neostriatum comparable with that observed in the monkey. (30 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Disruption of passive avoidance learning in the rat by electrical stimulation of the hippocampus.

Five experiments, with 184 male albino rats, examined the effect of electrical stimulation to discrete regions of the dorsal hippocampus on passive avoidance. Ss stepped from a brightly lit, white compartment to a dark compartment. Following footshock in the dark compartment, step-through latencies were recorded 1 min, 24 hrs, and 48 hrs after footshock. When given 20 step-through trials prior to footshock, dentate-stimulated Ss exhibited lack of passive avoidance with particularly short latencies at 1 min, and CA1-stimulated Ss showed reduced latencies compared with latencies of cortical and nonoperated controls. In Exp II, Ss given stimulation in the same regions performed the avoidance task without prior step-through experience. All groups showed less passive avoidance than the control group in Exp I, and there were no significant differences between groups. With a single step-through trial before the footshock trial, longer avoidance latencies were recorded, but again groups did not differ significantly. Dentate-implanted Ss, given 20 prior exposure trials but with stimulation at different stages of the task sequence, demonstrated a passive avoidance deficit at 1 min after footshock. Results are discussed in terms of the generation of expectancy that the black compartment was a safe place and the effects of brain stimulation on the expectancy, with particular reference to O. S. Vinogradova's (1975) theory of hippocampal function. (32 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)