  Parallel algorithms on CPU and GPU are implemented for the Unified
Gas-Kinetic Scheme and their performances are investigated and compared by a
two dimensional channel flow case. The parallel CPU algorithm has a one
dimensional block partition that parallelizes only the spatial space. Due to
the intrinsic feature of the UGKS, a compromised two-level parallelization is
adopted for GPU algorithm. A series of meshes with different sizes are tested
to reveal the performance evolution of the algorithms with respect to problem
size. Then special attentions are paid to UGKS applications where the molecular
velocity space range is large. The comparison confirms that GPU has relative
elevated accelerations with the latest device having a speedup of 118.38x.
Parallel CPU algorithm, on the contrary, might provide better performances when
the grid point number in velocity space is large.
