[thomas-huber]

Motivation

NCCL 2.32.3 made ncclTopoCompareGraphs keep an even-channel reference over an odd-channel graph when the reference has at least the same aggregate bandwidth, but only while the graph has fewer channels than the system has NICs (NVIDIA/nccl@24264d1499). RCCL picked it up with the 2.32.3 sync in #12321. CompareGraphs_OddCountTiesGoToTheEvenReference (#13605) covers the preference; nothing covered the NIC condition, so dropping or loosening it would go unnoticed.

Technical Details

• Two new SearchMicrotest cases in rccl-UnitTestsMicroTopo tie an odd-channel graph that has fewer hops against an even-channel reference with the same aggregate bandwidth, where the NIC condition does not hold, and expect the hop count to decide, as it did before the sync:
  • CompareGraphs_WithoutNicsOddCountTiesGoToFewerHops: two GPUs and no NICs; 1 channel at 40 GB/s against 2 at 20 GB/s.
  • CompareGraphs_OddCountAtTheNicCountTiesGoToFewerHops: three GPUs with a NIC each; 3 channels at 40 GB/s against 4 at 30 GB/s.
• Registration: "Search*" in test_categories_micro_topo.yaml already covers them.
• Test-only; no source changes. Upstream: NVIDIA/nccl@24264d1499.

Issue Tracking

Test Plan

• The Search* cases of rccl-UnitTestsMicroTopo from the standalone build (test/host/run_host_tests.sh build) and the in-tree build, on develop (98f6a72dcc49): the five CompareGraphs_* cases three runs each, the other 74 once.
• The same with ncclTopoCompareGraphs changed three ways, one at a time: the even-channel check disabled, the NIC condition removed, and its < relaxed to <=.
• MI355X (gfx950), ROCm 7.0.2, release build (install.sh -l -t).

Test Result

• On develop: all 79 Search* cases pass in both builds, the CompareGraphs_* cases 3/3.
• Even-channel check disabled: CompareGraphs_OddCountTiesGoToTheEvenReference fails 3/3; the two new cases pass.
• NIC condition removed: both new cases fail 3/3 (the even reference wins the tie); CompareGraphs_OddCountTiesGoToTheEvenReference still passes.
• <= instead of <: only CompareGraphs_OddCountAtTheNicCountTiesGoToFewerHops fails 3/3.
• The other 74 Search* cases pass with every change, so none of the full searches on the test topologies depends on the preference.

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/rocm-systems/blob/develop/CONTRIBUTING.md.

[therock-pr-bot]

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🧪 Unit Test	⚠️ Warning	Error: Source/code files changed without an accompanying unit test. Expected: add at least one test file named like test_<name>.py / test_<name>.cpp (or <name>_test.*). Current: code file(s) changed: projects/rccl/test/host/search-test.cc; no test file found
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