NCCL Tree algorithm deadlocked non-power-of-two rank count NCCL_ALGO Ring

The NCCL tree algorithm stalled on this topology. Tree is chosen automatically for some rank counts and message sizes, and an irregular or asymmetric rank layout can leave it waiting on a peer that never sends. This entry explains how to confirm the cause, apply the fix, and separate it from adjacent nccl-tuning failures.

The literal signature is "NCCL Tree algorithm deadlocked non-power-of-two rank count NCCL_ALGO Ring". 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.

The NCCL tree algorithm stalled on this topology. Tree is chosen automatically for some rank counts and message sizes, and an irregular or asymmetric rank layout can leave it waiting on a peer that never sends. 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 Tree algorithm deadlocked non-power-of-two rank count NCCL_ALGO Ring".
• 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

Apply the following checklist to a small reproduction: each box below is a positive signal that you are looking at this exact failure rather than a sibling in the same taxonomy.

• ✓Find the first occurrence of "NCCL Tree algorithm deadlocked non-power-of-two rank count NCCL_ALGO Ring" and preserve at least 100 lines before it.
• ✓Identify which rank, node, device, or process emitted the first related warning.
• ✓NCCL_DEBUG=INFO NCCL_DEBUG_SUBSYS=GRAPH shows the chosen algorithm and the constructed topology. Irregular rank counts and mismatched per-node GPU counts are the common trigger; confirm every node contributes the same number of ranks.
• ✓Repeat the same input after the targeted change and require the signature to disappear.
• ✓Resume from latest checkpoint only after the control passes.

NCCL Tree algorithm deadlocked non-power-of-two rank count NCCL_ALGO Ring

Timestamps and exact values vary across runs, but the pattern. An info-level start, an early WARN, an ERROR carrying the symptom. Is the actual fingerprint you should alert on. The Denpex platform flags this combination automatically.

force the ring algorithm to unblock the run, export NCCL_ALGO=Ring. This is a valid production setting, not just a diagnostic. 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 Tree algorithm deadlocked non-power-of-two rank count NCCL_ALGO Ring" <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.

• The NCCL tree algorithm stalled on this topology. Tree is chosen automatically for some rank counts and message sizes, and an irregular or asymmetric rank layout can leave it waiting on a peer that never sends.
• The decisive evidence is the first log line that precedes "NCCL Tree algorithm deadlocked non-power-of-two rank count NCCL_ALGO Ring" and differs from a healthy run.
• A nearby failure remains a competing hypothesis until a control separates configuration, capacity, transport, and hardware causes.