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[Bug fix] Create gloo subgroups among their members only - #369
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With the default process group bound to a device, creating a subgroup on a rank outside it runs an ncclCommSplit that every world rank must join. For a gloo subgroup the members never call it, so a deployment with a rank outside every parallel group (Qwen3-Omni's Thinker on TP2 with the Talker on its own GPU) hung at startup: the non-member blocked in new_group while the members waited at the startup barrier. use_local_synchronization makes the gloo groups members-only and names them by their ranks, so the shared group counter stays in step. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
NSagan271
marked this pull request as ready for review
October 8, 2026 19:15
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What does this PR do?
Fixes a bug introduced in #258 that prevented Qwen3-Omni with Thinker TP2 from starting.
With the default process group bound to a device, creating a subgroup on a rank outside it runs an ncclCommSplit that every world rank must join. For a gloo subgroup the members never call it, so a deployment with a rank outside every parallel group (Qwen3-Omni's Thinker on TP2 with the Talker on its own GPU) hung at startup: the non-member blocked in new_group while the members waited at the startup barrier. use_local_synchronization makes the gloo groups members-only and names them by their ranks, so the shared group counter stays in step.
How was it tested?
Verified Qwen3-Omni with Thinker TP 2 starts and runs as normal.
Checklist
ruff check .passes