The use of virtualization in HPC clusters can provide rich software environments, application isolation and efficient workload management mechanisms, but system-level virtualization introduces a software layer on the computing nodes that reduces performance and inhibits the direct use of hardware devices. We present an unobtrusive user-level platform to execute virtual machines inside batch jobs that does not handicap the computing cluster's ability to execute the most demanding applications. A per-user platform uses a static mode in which the VMs run entirely within the resources of a single batch job and a dynamic mode in which the VMs navigate at runtime between the continuously allocated jobs node time-slots. In the dynamic mode fault-tolerant system agents are integrated using group communication to control the system, to execute user commands and to implement user-defined scheduling policies. In our tests compute intensive applications suffered negligible performance overhead compared to the native configuration, but the user-mode network overlay introduced a significant penalty on the more taxing networked applications.
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