To validate bisectional bandwidth across two racks in a Spectrum-X Ethernet fabric, which NCCL test...

In a large-scale Spectrum-X Ethernet fabric, " East-West " traffic refers to the cross-rack communication between compute nodes. To validate the " Bisectional Bandwidth " (the throughput between two halves of the cluster), administrators use NCCL tests with specific environment variables to control traffic patterns. The NCCL_TESTS_SPLIT variable is used to partition the GPUs into distinct groups for the benchmark. Setting NCCL_TESTS_SPLIT= " DIV 8 " is a standard configuration for multi-node testing on 8-GPU systems. It effectively divides the total number of GPUs by the node count, creating a test environment where each GPU communicates with its corresponding rank on other nodes. By combining this with -g 1 (one GPU per process) across multiple nodes, the engineer can force data to travel across the leaf-and-spine switches rather than staying within the NVLink fabric of a single node. This isolates the physical network performance from the internal GPU-to-GPU bandwidth, providing a true measurement of the fabric ' s ability to handle high-speed AI traffic.