Performance and experience with LAPI - a new high-performance communication library for the IBM RS/6000 SP

LAPI is a low-level, high-performance communication interface on the IBM RS/6000 SP system. It provides an active-message-like interface along with remote memory copy and synchronization functionality. It is designed primarily for use by experienced programmers in developing parallel subsystems, libraries and tools, but we also expect power programmers to use it in end-user applications. IBM developed LAPI as a part of a project with Pacific Northwest National Laboratory (PNNL) to optimize the performance of the Global Arrays (GA) toolkit and its applications on the IBM RS/6000 SP. We provide an overview of LAPI characteristics and discuss its differences from other models such as MPI-2. We present some base performance parameters of LAPI including latency and bandwidth and compare it with performance of the MPI/MPL. The Global Arrays library from PNNL was ported to LAPI to exploit the performance benefits of this new interface. Experience using LAPI to implement GA and the performance of the resulting library are presented.