What does this failure look like in production?

In production this failure typically appears during the first tens-to-hundreds of training steps: loss curves become non-monotonic, validation metrics plateau above the baseline, and downstream checkpoints either diverge or fail to improve. Engineers frequently misread it as data drift when the underlying mechanism is zero-3 parameter gathering traverses the model hierarchy recursively, entering moe router logic during the gather phase. Validating against the symptom list in this entry is the fastest way to confirm the diagnosis.

Is this a hardware or a software issue?

Distributed Training failures can look like either layer depending on the symptom. The discriminator is the resolution of the symptom to the recommended fix: if applying the steps under "How to fix it" eliminates the symptom, this is a software/configuration class issue regardless of the hardware it surfaced on. Repeat the fix on a fresh node to confirm.

How long does the fix typically take?

Most resolutions from this entry take 10 to 30 minutes of engineer time once the diagnosis is confirmed. Long fixes come from skipping the diagnosis step and chasing apparent symptoms (validation drift, memory growth, slow throughput) instead of the actual root cause listed above. Apply the fix in a small experiment first, then roll it forward.

Does this affect inference or only training?

Primarily this is a distributed training failure observed during training, but the same misconfiguration can leak into downstream evaluation and serving. Treat the prevention checklist as a guardrail: any pipeline that touches a trained checkpoint should re-validate the failure mode before promoting the model.

What's the cheapest way to confirm this is the problem?

Cheapest signal: reproduce the symptom on a 1-GPU toy run with the same configuration. If the toy run exhibits the same behavior, the cause is the configuration rather than the cluster. If only the multi-GPU run is affected, the failure is a distributed interaction, so DeepSpeed ZeRO-3 with Mixture-of-Experts models (Mixtral, Switch Transformer, etc.) is the most likely layer to inspect.

Can the Denpex platform detect this automatically?

Yes. Denpex correlates the symptoms in this entry against your training logs in real time and surfaces the failure taxonomy, root cause, and exact remediation command on the console. Three full diagnoses are included free during signup, and the encyclopedia entry you are reading is generated from the same signal library that powers the platform.

What related failures should I check next?

Closely related failures cluster around #deepspeed, #moe, #zero3. After applying this fix, validate against the related entries below to confirm no overlapping cause is masking as this one. Engineers routinely discover that the dominant failure mode is actually a second, sibling failure compounded on top.

Will this recur after the fix?

Applied once with the prevention checklist, this failure does not recur under normal operation. The recurring variant most engineers encounter is the re-emergence of the same symptom on a different model or stack after deepspeed is upgraded. Pin compatible versions and rerun the prevention smoke test on every dependency change.

Is there a fingerprint I can monitor?

Yes. Training hangs with NCCL timeout during the first forward pass through MoE layers is the highest-signal fingerprint. Wire a metric or alert on that signal (or its absence after the fix) so you can detect the recurrence automatically. The Denpex platform encodes this exact fingerprint and surfaces it directly in the dashboard.

What is the official documentation that covers this?

Reference documentation is listed in the References section at the bottom of this entry. The most authoritative URLs are the upstream framework docs and the issue trackers cited; the Denpex entry synthesizes both into the executive, engineer, and on-call perspectives so each audience can act without reading every linked source.

Does this entry include working code?

Yes. The "Code Examples" section contains copy-paste-ready snippets that target the most common stacks (PyTorch, Hugging Face Trainer, DeepSpeed, Megatron-LM, vLLM/Transformers). Each snippet is annotated with why it works so you can adapt the pattern to your own framework without losing fidelity.

DeepSpeed MoE + ZeRO-3 Hang. Missing Leaf Module Marking

Q: How does this scale from 1 GPU to 1000 GPUs?

At single-GPU scale the symptom usually looks like slow convergence or a single anomalous loss value. At 64+ GPU scale, distributed interactions amplify zero-3 parameter gathering traverses the model hierarchy recursively, entering moe router logic during the gather phase through collectives, and the failure can manifest as a full job abort. Always reproduce the fix at the smallest cluster configuration that still exhibits the symptom before promoting to production scale.

DeepSpeed ZeRO-3 with Mixture-of-Experts (MoE) models hangs during training because MoE blocks need to be marked as leaf modules for ZeRO-3 parameter gathering. Without this marking, ZeRO-3 attempts to gather parameters recursively through the MoE router, causing deadlocks. Denpex detects the MoE+ZeRO-3 hang pattern and recommends the leaf module fix.

Quick answer

DeepSpeed ZeRO-3 with Mixture-of-Experts (MoE) models hangs during training because MoE blocks need to be marked as leaf modules for ZeRO-3 parameter gathering.

Symptom: nvidia-smi shows 0% GPU utilization on all ranks during the hang
Root cause: ZeRO-3 parameter gathering traverses the model hierarchy recursively, entering MoE router logic during the gather phase. MoE routers have conditional parameter access (only active experts per token), which creates deadlocks when ZeRO-3 tries to gather all expert parameters simultaneously. Without leaf module marking, ZeRO-3 doesn't know to treat MoE blocks as atomic units that should not be recursively decomposed.
Recommended fix: Mark MoE blocks as leaf modules: deepspeed.zero.set_z3_leaf_modules(model, [MoEBlockClass])
How Denpex helps: Denpex investigates DeepSpeed MoE + ZeRO-3 Hang. Missing Leaf Module Marking 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.

Distributed Training#deepspeed#moe#zero3#leaf-module#hang#mixture-of-experts

What this failure is

DeepSpeed MoE + ZeRO-3 Hang. Missing Leaf Module Marking is a Distributed Training failure seen during ML training runs. DeepSpeed ZeRO-3 with Mixture-of-Experts (MoE) models hangs during training because MoE blocks need to be marked as leaf modules for ZeRO-3 parameter gathering. Without this marking, ZeRO-3 attempts to gather parameters recursively through the MoE router, causing deadlocks. Denpex detects the MoE+ZeRO-3 hang pattern and recommends the leaf module fix. Common tags: Deepspeed, Moe, Zero3, Leaf Module.

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Why it happens (the mechanism)

ZeRO-3 parameter gathering traverses the model hierarchy recursively, entering MoE router logic during the gather phase. MoE routers have conditional parameter access (only active experts per token), which creates deadlocks when ZeRO-3 tries to gather all expert parameters simultaneously. Without leaf module marking, ZeRO-3 doesn't know to treat MoE blocks as atomic units that should not be recursively decomposed. The hang occurs because rank 0 is waiting for rank 1's expert parameters while rank 1 is stuck in the MoE router. 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

Training with DeepSpeed ZeRO-3 and MoE models hangs at the first forward pass
NCCL timeout occurs during parameter gathering for MoE layers
The hang is specific to MoE models. Non-MoE models with the same ZeRO-3 config work fine

Common symptoms and what they mean

Symptom	Why it happens
Training hangs with NCCL timeout during the first forward pass through MoE layers	ZeRO-3 parameter gathering traverses the model hierarchy recursively, entering MoE router logic during the gather phase
nvidia-smi shows 0% GPU utilization on all ranks during the hang	MoE routers have conditional parameter access (only active experts per token), which creates deadlocks when ZeRO-3 tries to gather all expert parameters simultaneously
DeepSpeed logs show parameter gathering initiated but never completing for MoE parameters	Without leaf module marking, ZeRO-3 doesn't know to treat MoE blocks as atomic units that should not be recursively decomposed
No error output. The job just hangs indefinitely	The hang occurs because rank 0 is waiting for rank 1's expert parameters while rank 1 is stuck in the MoE router

Which systems are affected

DeepSpeed ZeRO-3 with Mixture-of-Experts models (Mixtral, Switch Transformer, etc.)
DeepSpeed with expert-parallel training
Models using Megablocks or ScatterMoE with DeepSpeed ZeRO-3
DeepSpeed versions before v0.14.0

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: Training hangs with NCCL timeout during the first forward pass through MoE layers
✓Verified signal present: nvidia-smi shows 0% GPU utilization on all ranks during the hang
✓Verified signal present: DeepSpeed logs show parameter gathering initiated but never completing for MoE parameters
✓Verified signal present: No error output. The job just hangs indefinitely
✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.

Root cause

ZeRO-3 parameter gathering traverses the model hierarchy recursively, entering MoE router logic during the gather phase
MoE routers have conditional parameter access (only active experts per token), which creates deadlocks when ZeRO-3 tries to gather all expert parameters simultaneously
Without leaf module marking, ZeRO-3 doesn't know to treat MoE blocks as atomic units that should not be recursively decomposed
The hang occurs because rank 0 is waiting for rank 1's expert parameters while rank 1 is stuck in the MoE router

The fix and how to prevent it

Searchable error signature

search key

nvidia-smi shows 0% GPU utilization on all ranks during the hang

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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Diagnose this failure in VS Code

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DeepSpeed errors in context

DeepSpeed changes when parameters, gradients and optimizer state are created, partitioned, gathered and offloaded. The hub separates ZeRO, memory, checkpoint and pipeline failures by lifecycle phase.

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