HPSim was developed to provide fast and realistic beam simulations for the 800-MeV proton linear accelerator at the Los Alamos Neutron Science Center. The code was designed with the idea that it should be fast enough for a control room setting and easy to use with the same control interface as the actual machine. This led us to create the computing kernels with CUDA which takes full advantage of the NVIDIA GPU performance and a user interface with Python that enables ease-of-use and automation. Once connected to the accelerator control sys tem, the simulator can rapidly respond to control set point changes and predict beam properties in near real time. HPSim has been used to troubleshoot and guide accelerator operations and served as a test bed for novel control scheme development. This presentation will cover the motivation, code design, features, GPU performance, and applications of HPSim.