Optimising High Performance Computing applications is all about understanding both the application and the target platform. Application developers, or those that port an application to a new supercomputer architecture, worry about memory bandwidth, data placement, cache behaviour and the floating point performance of the target platform in order to deliver the very best performance. Cloud Computing, on the other hand, is all about virtualisation, which hides details of the target architecture from the application. Cloud offers both technical and business model flexibility, which is good – but perhaps not for HPC applications. How does the Uber-Cloud project square this circle? I spoke with Wolfgang Gentzsch to try to answer this – and many more – questions.