Topological Characterization of Hamming and Dragonfly Networks and its Implications on Routing

Current HPC and datacenter networks rely on large-radix routers. Hamming graphs (Cartesian products of complete graphs) and dragonflies (two-level direct networks with nodes organized in groups) are some direct topologies proposed for such networks. The original definition of the dragonfly topology is very loose, with several degrees of freedom such as the inter- and intra-group topology, the specific global connectivity and the number of parallel links between groups (or trunking level). This work provides a comprehensive analysis of the topological properties of the dragonfly network, providing balancing conditions for network dimensioning, as well as introducing and classifying several alternatives for the global connectivity and trunking level. From a topological study of the network, it is noted that a Hamming graph can be seen as a canonical dragonfly topology with a large level of trunking. Based on this observation and by carefully selecting the global connectivity, the Dimension Order Routing (DOR) mechanism safely used in Hamming graphs is adapted to dragonfly networks with trunking. The resulting routing algorithms approximate the performance of minimal, non-minimal and adaptive routings typically used in dragonflies, but without requiring virtual channels to avoid packet deadlock, thus allowing for lower-cost router implementations. This is obtained by selecting properly the link to route between groups, based on a graph coloring of the network routers. Evaluations show that the proposed mechanisms are competitive to traditional solutions when using the same number of virtual channels, and enable for simpler implementations with lower cost. Finally, multilevel dragonflies are discussed, considering how the proposed mechanisms could be adapted to them

Aerial Searches for Whooping Cranes along the Platte River, Nebraska

The endangered Whooping Cranes (Grus americana) that migrate between Wood Buffalo National Park, Alberta and Northwest Territories, and the Aransas National Wildlife Refuge Area, Texas, roost at many aquatic stopover locations (Austin and Richert 2001) including the central Platte River, Nebraska (Johnson 1982; Lingle et al. 1984, 1986, 1991; Faanes et al. 1992; Richert 1999). Under the U.S. Endangered Species Act, 90 km of the central Platte have been designated as critical habitat for the Whooping Crane, although suitable Platte River habitat for Whooping Crane and Sandhill Crane (Grus canadensis) continues to decline (Sidle et al. 1989, Currier 1997). The Whooping Crane has a long history of using the Platte River, and public agencies and private organizations have endeavored to learn more about Whooping Crane roost sites to enhance conservation of the species through regulatory and other efforts (Sidle et al. 1990a; Faanes 1992; Faanes and Bowman 1992; Ziewitz 1992). On the average, about 7% of the Whooping Cranes use the central Platte River as a stopover during migration (National Research Council 2005). Here we describe our aerial survey technique to locate roosting Whooping Cranes. Knowing the locations of Whooping Cranes roosting on the Platte River is necessary to improve our understanding of crane distribution and habitat characteristics of roost sites on the river. Records of roosting Whooping Cranes have largely relied upon observations reported by the public to government agencies or conservation organizations. There has been a need, however, for a more consistent, objective method of determining roost site locations. One methodical approach to locate Whooping Cranes is to fly in a light aircraft along the Platte River at dawn or dusk. At dawn, the birds are close to leaving the roost to migrate north or south, or to feed in adjacent wet meadows and croplands. At dusk, the birds may be just arriving from meadows and cropland. Whether at dawn or dusk, there is a narrow window to visually detect roosting Whooping Cranes

Platte River ecosystem resources and management, with emphasis on the Big Bend reach in Nebraska. US Fish and Wildlife Service, Grand Island, Nebraska

The Platte River and its tributaries drain over 86,000 square miles in Colorado, Nebraska, and Wyoming. Many habitats for wildlife, including montane pine forest, native grasslands, and eastern deciduous forest, exist in the Platte River ecosystem. The region is steeped in the history of the settlement of the West. The Mormon and Oregon trails, as well as the railroads played important roles in the early settlement of the region. Enactment of several federal laws to facilitate settlement of the region in the 1880s along with the opening of several railroad lines encouraged residents of the eastern United States to move to the region. Along with human settlement came changes in the character of the ecosystem. Wetlands were drained to accommodate intensified agricultural development. A vast acreage f tall grass prairie was converted to monotypic crop fields. Gravity and center-pivot irrigation systems accelerated the transformation of the native grassland communities. Intensified water withdrawal from the river was responsible for changing the character of riverine habitats. Ecological changes benefited some organisms at the expense of others. Enactment of several federal and state laws to conserve fish, wildlife, and ecosystems now plays a role to protect remaining Platte River system biodiversity. In this paper, we present an inventory of some of the ecosystem resources and describe some of the impacts to habitats and species that have occurred in the face of human development. We discuss a series of management alternatives that should be considered to maintain the integrity of the remaining biodiversity

Cache Performance in Vector Supercomputers

Traditional supercomputers use a flat multi-bank SRAM memory organization to supply high bandwidth at low latency. Most other computers use a hierarchical organization with a small SRAM cache and slower, cheaper DRAM for main memory. Such systems rely heavily on data locality for achieving optimum performance. This paper evaluates cache-based memory systems for vector supercomputers. We develop a simulation model for a cache-based version of the Cray Research C90 and use the NAS parallel benchmarks to provide a large scale workload. We show that while caches reduce memory traffic and improve the performance of plain DRAM memory, they still lag behind cacheless SRAM. We identify the performance bottlenecks in DRAM-based memory systems and quantify their contribution to program performance degradation. We find the data fetch strategy to be a significant parameter affecting performance, evaluate the performance of several fetch policies, and show that small fetch sizes improve performance..