NCCL MNNVL Init Segfault at Large World Size (Stack Overrun)
On GB200 NVL72, NCCL's recursive MNNVL topology search overruns the stack at world_size >= 44, especially with ulimit -s unlimited; fixed in NCCL 2.28.
On GB200 NVL72, NCCL's recursive MNNVL topology search overruns the stack at world_size >= 44, especially with ulimit -s unlimited.
What this failure is
NCCL MNNVL Init Segfault at Large World Size (Stack Overrun) is a Communication failure seen during ML training runs. On GB200 NVL72, NCCL's recursive MNNVL topology search overruns the stack at world_size >= 44, especially with ulimit -s unlimited; fixed in NCCL 2.28. Common tags: Nccl, Mnnvl, Gb200, Stack Overrun.
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Why it happens (the mechanism)
Recursive MNNVL topology search consumes deep stack. With ulimit -s unlimited the recursion overruns into adjacent memory. Stack size insufficient for the recursion depth at scale. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- SIGSEGV during NCCL init at large world size (>=44)
- Smaller world sizes initialize fine
- Crash during init, not during collectives
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| SIGSEGV during NCCL communicator initialization | Recursive MNNVL topology search consumes deep stack |
| Only at world_size >= 44 on GB200 NVL72 | With ulimit -s unlimited the recursion overruns into adjacent memory |
| Crash in recursive topology traversal | Stack size insufficient for the recursion depth at scale |
Which systems are affected
- NCCL 2.27+
- GB200 NVL72 (MNNVL)
- 44+ GPUs, ulimit -s unlimited
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: SIGSEGV during NCCL communicator initialization
- ✓Verified signal present: Only at world_size >= 44 on GB200 NVL72
- ✓Verified signal present: Crash in recursive topology traversal
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Root cause
- Recursive MNNVL topology search consumes deep stack
- With ulimit -s unlimited the recursion overruns into adjacent memory
- Stack size insufficient for the recursion depth at scale
The fix and how to prevent it
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