To calculate the N-Body gravity equations for 128 particles, each nbody() kernel calculates the N-Body gravity equations for 32 particles. However, in order to calculate acceleration and the new velocities, an nbody() kernel needs to know the data in the other kernels. For example, if particle 0 is mapped to nbody_kernel[0] and particle 32 is mapped to nbody_kernel[1], then nbody_kernel[0] needs to know the data in nbody_kernel[1] to accurately calculate the summation equation for acceleration and then calculate the new velocity of particle 0.
This is where the input_j stream plays a vital role in data sharing. Even though the input_j data stream has a window size for 32 particles worth of data, the LOOP_COUNT_J value can be set to allow the nbody() kernels to take in any number of 32 particles worth of data at a time. For a single instance of the nbody_subsystem graph, the LOOP_COUNT_J should be set to 4 to stream in data for all four kernels. For the final AI Engine graph, which contains 100 instances of the nbody_subsystem graph, the LOOP_COUNT_J value is set to 400 to stream in data for all 400 kernels to each nbody() kernel.
For example, to calculate the new velocity of particle 0 mapped in nbody_kernel[0], the nbody_kernel[0] can retrieve the data value of particle 32 from the input_j stream. This way, all nbody() kernels will have the data values for all other particles mapped in the other nbody() kernels through data streaming from input_j.