The reverse-acceleration model for programming petascale hybrid systems

Current technology trends favor hybrid architectures, typically with each node in a cluster containing both general-purpose and specialized accelerator processors. The typical model for programming such systems is host-centric: The general-purpose processor orchestrates the computation, offloading performance-critical work to the accelerator, and data are communicated only among general-purpose processors. In this paper, we propose a radically different hybrid-programming approach, which we call the reverse-acceleration model. In this model, the accelerators orchestrate the computation, offloading work that cannot be accelerated to the general-purpose processors. Data is communicated among accelerators, not among general-purpose processors. Our thesis is that the reverse-acceleration model simplifies porting codes to hybrid systems and facilitates performance optimization. We present a case study of a legacy neutron-transport code that we modified to use reverse acceleration and ran across the full 122,400 cores (general-purpose plus accelerator) of the Los Alamos National Laboratory Roadrunner supercomputer. Results indicate a substantial performance improvement over the unaccelerated version of the code.