  Computational biology has increasingly turned to agent-based modeling to
explore complex biological systems. Biological diffusion (diffusion, decay,
secretion, and uptake) is a key driver of biological tissues. GPU computing can
vastly accelerate the diffusion and decay operators in the partial differential
equations used to represent biological transport in an agent-based biological
modeling system. In this paper, we utilize OpenACC to accelerate the diffusion
portion of PhysiCell, a cross-platform agent-based biosimulation framework. We
demonstrate an almost 40x speedup on the state-of-the-art NVIDIA A100 GPU
compared to a serial run on AMD's EPYC 7742. We also demonstrate 9x speedup on
the 64 core AMD EPYC 7742 multicore platform. By using OpenACC for both the
CPUs and the GPUs, we maintain a single source code base, thus creating a
portable yet performant solution. With the simulator's most significant
computational bottleneck significantly reduced, we can continue cancer
simulations over much longer times.
