Running large MPI-applications with resource demands exceeding the local site's cluster capacity could be distributed across a number of clusters in a Grid instead, to satisfy the demand. However, there are a number of drawbacks limiting the applicability of this approach: communication paths between compute nodes of different clusters usually provide lower bandwidth and higher latency than the cluster internal ones, MPI libraries use dedicated I/Onodes for inter-cluster communication which become a bottleneck, missing tools for co-ordinating the availability of the different clusters across different administrative domains is another issue. To make the Grid approach efficient several prerequisites must be in place: an implementation of MPI providing high-performance communication mechanisms across the borders of clusters, a network connection with high bandwidth and low latency dedicated to the application, compute nodes made available to the application exclusively, and finally a Grid middleware glueing together everything. In this paper we present work recently completed in the VIOLA project: MetaMPICH, user controlled QoS of clusters and interconnecting network, a MetaScheduling Service and the UNICORE integration.

The UNICORE (UNiform Interface to COmputing REsources) software provides a Grid infrastructure together with a computing portal for engineers and scientists to access supercomputer centres from anywhere on the Internet. While UNICORE is primarily designed for the submission and control of batch jobs, it is also feasible to establish an on-line connection between an application and the UNICORE user-client. This opens up the possibility of performing on-line visualization and computational steering of applications under UNICORE control while maintaining the security provided by this system. This contribution describes the design of a steering extension to UNICORE based on the steering toolkit VISIT (VISualization Interface Toolkit). VISIT is a lightweight library that supports bidirectional data exchange between visualizations and parallel applications. As an example application, a parallel simulation of a laser-plasma interaction that can be steered by an AVS/Express application is presented.

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