  The L-CSC (Lattice Computer for Scientific Computing) is a general purpose
compute cluster built with commodity hardware installed at GSI. Its main
operational purpose is Lattice QCD (LQCD) calculations for physics simulations.
Quantum Chromo Dynamics (QCD) is the physical theory describing the strong
force, one of the four known fundamental interactions in the universe. L-CSC
leverages a multi-GPU design accommodating the huge demand of LQCD for memory
bandwidth. In recent years, heterogeneous clusters with accelerators such as
GPUs have become more and more powerful while supercomputers in general have
shown enormous increases in power consumption making electricity costs and
cooling a significant factor in the total cost of ownership. Using mainly GPUs
for processing, L-CSC is very power-efficient, and its architecture was
optimized to provide the greatest possible power efficiency. This paper
presents the cluster design as well as optimizations to improve the power
efficiency. It examines the power measurements performed for the Green500 list
of the most power-efficient supercomputers in the world which led to the number
1 position as the greenest supercomputer in November 2014.
