This book constitutes the refereed post-proceedings of the 9th European Performance Engineering Workshop, EPEW 2012, held in Munich, Germany, and the 28th UK Performance Engineering Workshop, UKPEW 2012, held in Edinburgh, UK, in July 2012. The 15 regular papers and one poster presentation paper presented together with 2 invited talks were carefully reviewed and selected from numerous submissions. The papers cover a wide range of topics from classical performance modeling areas such as wireless network protocols and parallel execution of scientific codes to hot topics such as energy-aware computing to unexpected ventures into ranking professional tennis players. In addition to new case studies, the papers also present new techniques for dealing with the modeling challenges brought about by the increasing complexity and scale of systems today.