  The CHIME Pathfinder is a new interferometric radio telescope that uses a
hybrid FPGA/GPU FX correlator. The GPU-based X-engine of this correlator
processes over 819 Gb/s of 4+4-bit complex astronomical data from N=256 inputs
across a 400 MHz radio band. A software framework is presented to manage this
real-time data flow, which allows each of 16 processing servers to handle 51.2
Gb/s of astronomical data, plus 8 Gb/s of ancillary data. Each server receives
data in the form of UDP packets from an FPGA F-engine over the eight 10 GbE
links, combines data from these packets into large (32MB-256MB) buffered
frames, and transfers them to multiple GPU co-processors for correlation. The
results from the GPUs are combined and normalized, then transmitted to a
collection server, where they are merged into a single file. Aggressive
optimizations enable each server to handle this high rate of data; allowing the
efficient correlation of 25 MHz of radio bandwidth per server. The solution
scales well to larger values of N by adding additional servers.
