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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.

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