Mini-app driven optimisation of inertial confinement fusion codes

In September 2013, the large laser-based inertial confinement fusion device housed in the National Ignition Facility at Lawrence Livermore National Laboratory, was widely acclaimed to have achieved a milestone in controlled fusion - successfully initiating a reaction that resulted in the release of more energy than the fuel absorbed. Despite this success, we remain some distance from being able to create controlled, self-sustaining fusion reactions. Inertial Confinement Fusion (ICF) represents one leading design for the generation of energy by nuclear fusion. Since the 1950s, ICF has been supported by computing simulations, providing the mathematical foundations for pulse shaping, lasers, and material shells needed to ensure effective and efficient implosion. The research presented here focuses on one such simulation code, EPOCH, a fully relativistic particle-in-cell plasma physics code, developed by a leading network of over 30 UK researchers. A significant challenge in developing large codes like EPOCH is maintaining effective scientific delivery on successive generations of high-performance computing architecture. To support this process, we adopt the use of mini-applications - small code proxies that encapsulate important computational properties of their larger parent counterparts. Through the development of a mini-app for EPOCH (called miniEPOCH), we investigate known time-step scaling issues within EPOCH and explore possible optimisations: (i) Employing loop fission to increase levels of vectorisation, (ii) Enforcing particle ordering to allow the exploitation of domain specific knowledge and, (iii) Changing underlying data storage to improve memory locality. When applied to EPOCH, these improvements represent a 2.02× speed-up in the core algorithm and a 1.55× speed-up to the overall application runtime, when executed on EPCC's Cray XC30 ARCHER platform.