An engineer needs to validate NVLink Switch functionality on a DGX H100 system with 8...

The NVIDIA Collective Communications Library (NCCL) " Tests " are used to verify the maximum achievable bandwidth of the interconnects. On a DGX H100, the GPUs are connected via a dedicated high-bandwidth NVLink Switch fabric (NVLink 4), which provides significantly higher throughput than PCIe. To validate the intra-node (within a single server) performance, the all_reduce_perf test is used. The command in Option D is specifically designed to stress all 8 GPUs (-g 8) across a wide range of message sizes (8 bytes to 16G). The use of the environment variable NCCL_TESTS_SPLIT with the bitwise " OR " or " AND " masks allows the engineer to isolate specific traffic patterns or groups of GPUs to ensure the NVLink switches are distributing the load evenly. For a standard 8-GPU H100 tray, achieving a " Bus Bandwidth " of ~450 GB/s to 900 GB/s (depending on the precision and message size) confirms that the NVLink fabric is operating at its theoretical peak. Using only 4 GPUs (Option B) or 1 GPU (Option C) would not provide a complete picture of the NVLink switch bisection bandwidth.