Massively parallel quantum chromodynamics

Quantum chromodynamics (QCD), the theory of the strong nuclear force, can be numerically simulated on massively parallel supercomputers using the method of lattice gauge theory. We describe the special programming requirements of lattice QCD (LQCD) as well as the optimal supercomputer hardware architectures for which LQCD suggests a need. We demonstrate these methods on the IBM Blue Gene/L™ (BG/L) massively parallel supercomputer and argue that the BG/L architecture is very well suited for LQCD studies. This suitability arises from the fact that LQCD is a regular lattice discretization of space into lattice sites, while the BG/L supercomputer is a discretization of space into compute nodes. Both LQCD and the BG/L architecture are constrained by the requirement of short-distance exchanges. This simple relation is technologically important and theoretically intriguing. We demonstrate a computational speedup of LQCD using up to 131,072 CPUs on the largest BG/L supercomputer available in 2007. As the number of CPUs is increased, the speedup increases linearly with sustained performance of about 20% of the maximum possible hardware speed. This corresponds to a maximum of 70.5 sustained teraflops. At these speeds, LQCD and the BG/L supercomputer are able to produce theoretical results for the next generation of strong-interaction physics.