Title: Mixed Precision Solver Scalable to 16000 MPI Processes for Lattice Quantum Chromodynamics Simulations on the Oakforest-PACS System,
Abstract: Lattice Quantum Chromodynamics (Lattice QCD) is a quantum field theory on a
finite discretized space-time box so as to numerically compute the dynamics of
quarks and gluons to explore the nature of subatomic world. Solving the
equation of motion of quarks (quark solver) is the most compute-intensive part
of the lattice QCD simulations and is one of the legacy HPC applications. We
have developed a mixed-precision quark solver for a large Intel Xeon Phi (KNL)
system named "Oakforest-PACS", employing the $O(a)$-improved Wilson quarks as
the discretized equation of motion. The nested-BiCGSTab algorithm for the
solver was implemented and optimized using mixed-precision,
communication-computation overlapping with MPI-offloading, SIMD vectorization,
and thread stealing techniques. The solver achieved 2.6 PFLOPS in the
single-precision part on a $400^3\times 800$ lattice using 16000 MPI processes
on 8000 nodes on the system.