Part of a cluster can be reserved during week days (roughly less than 20% of the cluster and at most 2 hours or so) which would be convenient to test the benchmarks work as expected during a short period.
An entire cluster can be reserved during a week-end (from friday night to monday morning, i.e. roughly 60 hours) to run the actual benchmark.
When getting the machines, a tool should be used to install the desired operating system (Debian GNU/Linux for instance) and an ssh root access is provided in return. The machines can then be configured with no restrictions.