  We present an overview of the Graphics Processing Unit (GPU) based spatial
processing system created for the Canadian Hydrogen Intensity Mapping
Experiment (CHIME). The design employs AMD S9300x2 GPUs and readily-available
commercial hardware in its processing nodes to provide a cost- and
power-efficient processing substrate. These nodes are supported by a
liquid-cooling system which allows continuous operation with modest power
consumption and in all but the most adverse conditions. Capable of continuously
correlating 2048 receiver-polarizations across 400\,MHz of bandwidth, the CHIME
X-engine constitutes the most powerful radio correlator currently in existence.
It receives $6.6$\,Tb/s of channelized data from CHIME's FPGA-based F-engine,
and the primary correlation task requires $8.39\times10^{14}$ complex
multiply-and-accumulate operations per second. The same system also provides
formed-beam data products to commensal FRB and Pulsar experiments; it
constitutes a general spatial-processing system of unprecedented scale and
capability, with correspondingly great challenges in computation, data
transport, heat dissipation, and interference shielding.
