Proximity as a Scalability Measure

In distributed systems decisions should be based on currently available local information whenever possible. When weak-consistency suffices, partial system state information is used to arrive at scalable and adaptive decisions. Adaptive partitioning which is part of a load sharing algorithm is proposed in [6] resulting in improved system performance. Adaptive partitioning is applicable to applications requiring scalability and possessing some criteria (such as load state) for node mutual interest. Also, distributed systems such as the WWW cover vast geographical areas. In such systems communication may suffer lengthy network delays. This paper proposes to further improve performance by handling proximity in addition to system size. The benefit of the extended adaptive partitioning algorithm is demonstrated with simulation results produced by the NEST [4] simulator for the extended flexible load sharing algorithm (FLS). Introduction Scale is recognized as a primary factor influencing t..

Rigorous Analysis of (Distributed) Simulation Results

Formal static analysis of the correctness and complexity of scalable and adaptive algorithms for distributed systems is difficult and often not appropriate. Rather, tool support is required to facilitate the 'trial and error' approach which is often adopted. Simulation supports this experimental approach well. In this paper we discuss the need for a rigorous approach to simulation results analysis and model validation. These aspects are often neglected in simulation studies, particularly in distributed simulation. Our aim is to provide the practitioner with a set of guidelines which can be used as a `recipe' in different simulation environments, making sound techniques (simulation and statistics) accessible to users. We demonstrate use of the suggested analysis method with two different distributed simulators (CNCSIM [8]) and (NEST[3]) thus illustrating its generality. The same guidelines may be used with other simulation tools to ensure meaningful results while obviating the need to a..

Co-evolution of eukaryotes and ocean oxygenation in the Neoproterozoic era

The Neoproterozoic era (about 1,000 to 542 million years ago) was a time of turbulent environmental change. Large fluctuations in the carbon cycle were associated with at least two severe-possible Snowball Earth-glaciations. There were also massive changes in the redox state of the oceans, culminating in the oxygenation of much of the deep oceans. Amid this environmental change, increasingly complex life forms evolved. The traditional view is that a rise in atmospheric oxygen concentrations led to the oxygenation of the ocean, thus triggering the evolution of animals. We argue instead that the evolution of increasingly complex eukaryotes, including the first animals, could have oxygenated the ocean without requiring an increase in atmospheric oxygen. We propose that large eukaryotic particles sank quickly through the water column and reduced the consumption of oxygen in the surface waters. Combined with the advent of benthic filter feeding, this shifted oxygen demand away from the surface to greater depths and into sediments, allowing oxygen to reach deeper waters. The decline in bottom-water anoxia would hinder the release of phosphorus from sediments, potentially triggering a potent positive feedback: phosphorus removal from the ocean reduced global productivity and ocean-wide oxygen demand, resulting in oxygenation of the deep ocean. That, in turn, would have further reinforced eukaryote evolution, phosphorus removal and ocean oxygenation

Underwater microscopy for in situ studies of benthic ecosystems

Microscopic-scale processes significantly influence benthic marine ecosystems such as coral reefs and kelp forests. Due to the ocean's complex and dynamic nature, it is most informative to study these processes in the natural environment yet it is inherently difficult. Here we present a system capable of non-invasively imaging seafloor environments and organisms in situ at nearly micrometre resolution. We overcome the challenges of underwater microscopy through the use of a long working distance microscopic objective, an electrically tunable lens and focused reflectance illumination. The diver-deployed instrument permits studies of both spatial and temporal processes such as the algal colonization and overgrowth of bleaching corals, as well as coral polyp behaviour and interspecific competition. By enabling in situ observations at previously unattainable scales, this instrument can provide important new insights into micro-scale processes in benthic ecosystems that shape observed patterns at much larger scales