NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable

NCCL could not bring up a transport (IB/EFA/NET) between two ranks. "Resource temporarily unavailable" here usually means the RDMA device ran out of queue pairs or memory registrations, or the container lacks the device/ulimit needed to allocate them. This entry explains how to confirm the cause, apply the fix, and separate it from adjacent nccl failures.

Quick answer

NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable means NCCL could not bring up a transport (IB/EFA/NET) between two ranks. "Resource temporarily unavailable" here usually means the RDMA device ran out of queue pairs or memory registrations, or the container lacks the device/ulimit needed to allocate them. Preserve the first preceding error, then run the targeted control below.

Symptom: NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable
Root cause: NCCL could not bring up a transport (IB/EFA/NET) between two ranks. "Resource temporarily unavailable" here usually means the RDMA device ran out of queue pairs or memory registrations, or the container lacks the device/ulimit needed to allocate them. The decisive evidence is the first log line that precedes "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable" and differs from a healthy run.
Recommended fix: NCCL_DEBUG=INFO
How Denpex helps: Denpex investigates NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable 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.

Communication#nccl#init#transport#net

What this failure is

The literal signature is "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable". It is a communication failure associated with NCCL and ProcessGroupNCCL collectives. The line identifies the failing operation or subsystem, while the surrounding evidence decides whether it is the initiating fault or a downstream symptom.

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Why it happens (the mechanism)

NCCL could not bring up a transport (IB/EFA/NET) between two ranks. "Resource temporarily unavailable" here usually means the RDMA device ran out of queue pairs or memory registrations, or the container lacks the device/ulimit needed to allocate them. 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.

What you'll observe

The workload stops or loses forward progress after emitting "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable".
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.

Common symptoms and what they mean

Symptom	Why it happens
NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable	NCCL could not bring up a transport (IB/EFA/NET) between two ranks. "Resource temporarily unavailable" here usually means the RDMA device ran out of queue pairs or memory registrations, or the container lacks the device/ulimit needed to allocate them.
The same operation fails at a consistent stage of NCCL and ProcessGroupNCCL collectives.	The decisive evidence is the first log line that precedes "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable" and differs from a healthy run.
The first related warning appears before the final exception and names the causal subsystem.	A nearby failure remains a competing hypothesis until a control separates configuration, capacity, transport, and hardware causes.
A known-good control changes one variable and either reproduces or clears the failure.	NCCL could not bring up a transport (IB/EFA/NET) between two ranks. "Resource temporarily unavailable" here usually means the RDMA device ran out of queue pairs or memory registrations, or the container lacks the device/ulimit needed to allocate them.

Which systems are affected

NCCL and ProcessGroupNCCL collectives
production-shaped multi-accelerator workloads
containerized and bare-metal deployments of the same stack

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.

✓Find the first occurrence of "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable" 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=INIT,NET and read which transport failed for which rank pair. Confirm the HCA is visible in the container (ibv_devinfo) and the ports are ACTIVE. Check /dev/infiniband is mounted.
✓Repeat the same input after the targeted change and require the signature to disappear.
✓Resume from job start only after the control passes.

Root cause

NCCL could not bring up a transport (IB/EFA/NET) between two ranks. "Resource temporarily unavailable" here usually means the RDMA device ran out of queue pairs or memory registrations, or the container lacks the device/ulimit needed to allocate them.
The decisive evidence is the first log line that precedes "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable" and differs from a healthy run.
A nearby failure remains a competing hypothesis until a control separates configuration, capacity, transport, and hardware causes.

The fix and how to prevent it

Searchable error signature

search key

NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable

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.

The fix and the prevention pattern

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Why the recommended fix works

check locked-memory limits inside the container, ulimit -l must be unlimited for RDMA. Under Docker add --ulimit memlock=-1:-1; under Kubernetes set the equivalent in the pod spec. This changes the condition in the causal diagnosis instead of hiding the outer exception. The repeated control proves ownership before recovery from job start.

Code examples

snippet

# 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 WARN Failed to initialize transport NET IB Resource temporarily unavailable" <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.

Best practices by model family

Model / Stack	Recommendation	Notes
First response	Preserve the first failure	Keep the context before "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable" so aggregation does not erase causality.
Confirmation	Change one variable	Use a known-good node, rank, input, or configuration as the control.
Recovery	Resume from job start	Resume only after the literal signature no longer appears in the same control.

With the fix vs without the fix

Dimension	With the fix	Without the fix
Evidence	First preceding error and one controlled comparison	Only the final aggregated exception
Fix	check locked-memory limits inside the container, ulimit -l must be unlimited for RDMA. Under Docker add --ulimit memlock=-1:-1; under Kubernetes set the equivalent in the pod spec.	Retrying the unchanged workload
Exit criterion	"NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable" is absent in the repeated control	The job happened to run once

Diagnostic note

“Treat "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable" as a search key and an investigation checkpoint, not as proof of every cause associated with the phrase. The high-value evidence is what changed immediately before it and whether the failure follows the workload, node, or configuration.”

Visual fingerprint

Decision path for NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable

literal error captured
        |
        v
find first preceding failure
        |
        v
run one known-good control
        |
        +-- follows workload --> inspect input or configuration
        +-- follows node ------> inspect hardware or platform
        +-- disappears --------> validate the targeted fix

The control separates workload, configuration, and node ownership before recovery from job start.

Diagnose this failure in VS Code

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.

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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.

Compare every nccl error side by side

Related failures to investigate next

Frequently asked questions

Questions engineers and on-call staff commonly ask about this failure.

What does "NCCL WARN Failed to initialize transport NET IB Resource temporarily unavailable" mean?

Is this line always the root cause?

No. It can be the direct failure or the point where an earlier failure becomes visible. The first preceding error and a controlled comparison decide which.

What should I collect before restarting?

Collect complete log context, the emitting rank or node, component versions, resolved configuration, and the diagnostic output shown above.

What is the fastest confirmation?

NCCL_DEBUG=INFO NCCL_DEBUG_SUBSYS=INIT,NET and read which transport failed for which rank pair. Confirm the HCA is visible in the container (ibv_devinfo) and the ports are ACTIVE. Check /dev/infiniband is mounted.

How do I prevent it from recurring?

put memlock=unlimited and the RDMA device mounts in the job template; validate with a two-node ib_write_bw before a large run.

References

Don't just read the fix, diagnose your run

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