SM memcpy performance concerns on GH200

I’m using nvbandwidth to understand the memcpy performance across NVLINK C2C on a GH200 (900GB/s bi-directional)

Command example: ./nvbandwidth --testcase 0 1 2 3 16 17 32 --testSamples 1 --useMean -b 20480

I do not understand the following performance characteristics. What could be the reasons for this?

1. As I understand from https://github.com/NVIDIA/nvbandwidth/issues/23, the SM version should outperform the performance of the Copy Engines due to the available parallel threads. But in this case I am seeing a drastic drop in performance for buffer sizes 20GB and above. Is there copy kernel tuning necessary?
2. Prior work suggests the CE engines should operate in parallel, however for the bi-directional case I see that this is not true and the performance in half of the peak
3. The DtoH and HtoD asymmetry is larger than previously reported estimates.

Start by reproducing the reported command on a GH200 with testcase 0 1 2 3 16 17 32, testSamples 1, useMean, and a 20480 buffer. Investigate the SM versus Copy Engine results, bidirectional throughput, and DtoH/HtoD asymmetry; done would require identifying and documenting the causes or a concrete correction.