Wilkes is the name, Cambridge is the host, and 3631 MFLOP/Watt is the figure you're looking for. That was back in November 2013.

At ISC2017, the latest Green 500 includes Cambridge University's Wilkes-2 system -- further details to follow

In Nov2013, the partnership of Cambridge University High Performance Computing Service, Dell and Mellanox (with part sponsorship by STFC, Rolls Royce and Mitsubishi) delivered Wilkes, which also has the peak performance (240 TFLOPS) of all UK universities' high end compute. The cluster comprises 128 Dell T620 servers, 256 NVIDIA GPUs interconnected by 256 Mellanox Connect IB cards.

But, perhaps surprisingly, these are K20 GPUs not the recently announced K40s.

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