NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled

Traffic spread across multiple NICs arrived out of order. RoCE is highly sensitive to reordering, go-back-N retransmission means a reordered flow collapses throughput rather than degrading gracefully, and can stall the pipeline entirely. This entry explains how to confirm the cause, apply the fix, and separate it from adjacent nccl-tuning failures.

Symptom

NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled

Root cause

Traffic spread across multiple NICs arrived out of order. RoCE is highly sensitive to reordering, go-back-N retransmission means a reordered flow collapses throughput rather than degrading gracefully, and can stall the pipeline entirely. The decisive evidence is the first log line that precedes "NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled" and differs from a healthy run.

Recommended fix

NCCL_CROSS_NIC=0

How Denpex helps

Denpex investigates NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled using the evidence you provide or your connected workload collects. Earlier rank, host or application evidence is needed to distinguish an initiating failure from a downstream report.

The literal signature is "NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled". It is a communication failure associated with NCCL algorithm, buffer, and multi-NIC tuning. The line identifies the failing operation or subsystem, while the surrounding evidence decides whether it is the initiating fault or a downstream symptom.

Traffic spread across multiple NICs arrived out of order. RoCE is highly sensitive to reordering, go-back-N retransmission means a reordered flow collapses throughput rather than degrading gracefully, and can stall the pipeline entirely. The failure becomes visible at this call site because the operation first requires the missing resource, valid state, healthy peer, or correct result. Earlier log lines and a known-good control carry more causal value than the final wrapper exception.

• The workload stops or loses forward progress after emitting "NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled".
• A retry on the same configuration reproduces the failure because the causal state has not changed.
• The outer framework exception can hide the rank, node, allocation, or dependency that failed first.
• Increasing timeouts or reducing workload size can suppress the symptom without correcting the cause.

• NCCL algorithm, buffer, and multi-NIC tuning
• production-shaped multi-accelerator workloads
• containerized and bare-metal deployments of the same stack

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.

• ✓Find the first occurrence of "NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled" and preserve at least 100 lines before it.
• ✓Identify which rank, node, device, or process emitted the first related warning.
• ✓reordering across NICs usually means ECMP hashing is spreading one QP's packets over multiple paths. Confirm the fabric hashes on the full 5-tuple and that both NICs are in the same layer-2 domain with matching MTU. Check port counters for out-of-sequence on the receiving HCA.
• ✓Repeat the same input after the targeted change and require the signature to disappear.
• ✓Resume from latest checkpoint only after the control passes.

• Traffic spread across multiple NICs arrived out of order. RoCE is highly sensitive to reordering, go-back-N retransmission means a reordered flow collapses throughput rather than degrading gracefully, and can stall the pipeline entirely.
• The decisive evidence is the first log line that precedes "NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled" and differs from a healthy run.
• A nearby failure remains a competing hypothesis until a control separates configuration, capacity, transport, and hardware causes.

NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled

Use this text as a lookup key in logs and upstream issue trackers. It is not presented as a captured customer log. Confirm the cause from your own preceding events, versions, configuration and the cited references.

disable multi-NIC striping for a single flow, export NCCL_CROSS_NIC=0, and re-measure. If throughput recovers, reordering was the cause. This changes the condition in the causal diagnosis instead of hiding the outer exception. The repeated control proves ownership before recovery from latest checkpoint.

# Preserve evidence before restarting
NCCL_DEBUG=INFO NCCL_DEBUG_SUBSYS=INIT,NET,COLL torchrun train.py
ibstat
ethtool -S <interface>

# Find the exact signature in the complete log
rg -n -F -- "NCCL multi-NIC packet reordering detected RoCE packets arrived out of sequence pipeline stalled" <log-file>

Adapt the snippet to your framework. The same pattern holds for PyTorch Lightning, Hugging Face Trainer, DeepSpeed, Megatron-LM, and vLLM training wrappers. Where the wrapper exposes a config flag (for examplelr_scheduler_type in Trainer), prefer the flag over the imperative API to keep the schedule declarative and reproducible.

Select the traceback or open the failed terminal, then run Denpex locally to see the initiating rank, collateral failures, exact fix, and verification command without uploading the log.

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.