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NCCL NVLS Memory Corruption with Dual-Port NICs

NVLS did not support dual-port NIC transmission, producing duplicate head-rank entries in the proxy loop and heap corruption; fixed in NCCL 2.19.4.

Quick answer

NVLS did not support dual-port NIC transmission, producing duplicate head-rank entries in the proxy loop and heap corruption.

Communication#nccl#nvls#dual-port-nic#memory-corruption#2.19.4

What this failure is

NCCL NVLS Memory Corruption with Dual-Port NICs is a Communication failure seen during ML training runs. NVLS did not support dual-port NIC transmission, producing duplicate head-rank entries in the proxy loop and heap corruption; fixed in NCCL 2.19.4. Common tags: Nccl, Nvls, Dual Port Nic, Memory Corruption.

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Why it happens (the mechanism)

NVLS lacked dual-port NIC transmission support. Duplicate head-rank entries in the proxy loop. Heap corruption from the duplicate entries. 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

  • all_reduce_perf hangs or crashes with dual-port NICs
  • Regression from NCCL 2.16.5 to 2.18.5/2.19.3
  • Only with NVLS enabled

Common symptoms and what they mean

SymptomWhy it happens
free(): invalid next size (fast)NVLS lacked dual-port NIC transmission support
all_reduce_perf hang or crashDuplicate head-rank entries in the proxy loop
NVLS enabled + dual-port NICs (2 ports per HCA)Heap corruption from the duplicate entries

Which systems are affected

  • NCCL 2.18.5 / 2.19.3
  • Dual-port NICs
  • NVLS (NVLink SHARP) enabled

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: free(): invalid next size (fast)
  • ✓Verified signal present: all_reduce_perf hang or crash
  • ✓Verified signal present: NVLS enabled + dual-port NICs (2 ports per HCA)
  • ✓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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NCCL errors in context

NCCL is where a distributed job reports failure, which is not the same as where it failed. The hub lists every common NCCL error next to what it actually indicates, and the environment variables that tell them apart.

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Related failures to investigate next

Nccl Cuda Failure often co-occurs with this failure because both share underlying assumptions about communication; if you fix this entry but still see drift, validate the related primary cause next. nccl cuda failure →
Nccl Bucket Size Mismatch often co-occurs with this failure because both share underlying assumptions about communication; if you fix this entry but still see drift, validate the related neighboring cause next. nccl bucket size mismatch →
Nccl Ras Init Segfault often co-occurs with this failure because both share underlying assumptions about communication; if you fix this entry but still see drift, validate the related neighboring cause next. nccl ras init segfault →

Root cause

  • NVLS lacked dual-port NIC transmission support
  • Duplicate head-rank entries in the proxy loop
  • Heap corruption from the duplicate entries

The fix and how to prevent it

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References

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