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The past few years have seen a rise in popularity of massively parallel architectures that use fat-trees as their interconnection networks. In this paper we study the communication performance of a parametric family of fat-trees, the k-ary n-trees, built with constant arity switches interconnected in a regular topology. Through simulation on a 4-ary 4-tree with 256 nodes, we analyze some variants of an adaptive algorithm that utilize wormhole routing with one, two and four virtual channels. The experimental results show that the uniform, bit reversal and transpose trafic pattems are very sensitive to the jlow control strategy. In all these cases, the saturation points are between 35 -40% of the network capacity with one virtual channel, 55-60% with two virtual channels and around 75% with four virtual channels. The complement trafic, a representative of the class of the congestion-free communication patterns, reaches un optimal performance, with a saturation point at 97% of the capacity for allJlow control strategies.

This paper presents a parallel programming methodology that ensures easy programming, e ciency, and portability of programs to di erent m a c hines belonging to the class of the generalpurpose, distributed memory, MIMD architectures. The methodology is based on the de nition of a new, high-level, explicitly parallel language, called P 3 L, and of a set of static tools that automatically adapt the program features for each target architecture. P 3 L does not require programmers to specify process activations, the actual parallelism degree, scheduling, or interprocess communications, i.e. all those features that need to be adjusted to harness each speci c target machine. Parallelism is, on the other hand, expressed in a structured and qualitative w ay, b y hierarchical composition of a restricted set of language constructs, corresponding to those forms of parallelism that are frequently encountered in parallel applications, and that can e ciently be implemented.The e cient portability o f P 3 L applications is guaranteed by the compiler along with the novel structure of the support. The compiler automatically adapts the program features for each speci c architecture, accessing the costs (in terms of performance) of the low-level mechanisms exported by t h e a r c hitecture itself. In our methodology, these costs, along with other features of the architecture, are viewed through an abstract machine, whose mechanism interface is used by the compiler to produce the nal object code.1

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ASSIST (A Software development System based upon Integrated Skeleton Technology) is a programming environment oriented to the development of parallel and distributed high-performance applications according to a unified approach. The language and implementation features of ASSIST are a result of our long-term research in parallel programming models and tools. ASSIST is evolving towards programming environments for high-performance complex enabling platforms, especially Grids. In this paper, we show how ASSIST can act as a valid research vehicle to study, experiment and realize Grid-aware programming environments for high-performance applications. Special emphasis is put on the innovative methodologies, strategies and tools for dynamically adaptive applications, that represent the necessary step for the success of Grid platforms.First we discuss the conceptual framework for Grid-aware programming environments, based upon structured parallel programming and components technology, anticipating how ASSIST possesses the essential features required by such framework. Then we summarize the ASSIST programming model, showing its evolution, along the line of structured parallel programming, to solve critical problems of expressive power, flexibility, interoperability and efficiency; some examples, both of kernels and of complex applications, are used to point out the ASSIST features. The modular compiler model and the current implementation for heterogeneous platforms and Globus-based Grids are illustrated. We show the features that allow ASSIST programs to be used in CORBA infrastructures, that represents our basic starting point towards interoperability in Grid applications. Finally, the presentation of all the previous issues is used to derive an ASSIST-based model for supporting dynamically adaptive applications.--