Is this a hardware or a software issue?

Memory 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 memory 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 CUDA Caching Allocator 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 #Fragmentation. 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 Fragmentation is upgraded. Pin compatible versions and rerun the prevention smoke test on every dependency change.

Is there a fingerprint I can monitor?

Yes. The model crashes with a CUDA OOM error. 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.

CUDA OOM from Caching Allocator Fragmentation

Q: 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 fsdp performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). this usage pattern severely fragments the cuda caching allocator. memory gets trapped in blocks that cannot be merged, meaning pytorch cannot find a contiguous block large enough for a new tensor, even though total free vram is sufficient.. Validating against the symptom list in this entry is the fastest way to confirm the diagnosis.

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 fsdp performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). this usage pattern severely fragments the cuda caching allocator. memory gets trapped in blocks that cannot be merged, meaning pytorch cannot find a contiguous block large enough for a new tensor, even though total free vram is sufficient. 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.

FSDP performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). This usage pattern severely fragments the CUDA caching allocator. Memory gets trapped in blocks that cannot be merged, meaning PyTorch cannot find a contiguous block large enough for a new tensor, even though total free VRAM is sufficient.

Quick answer

FSDP performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding).

Symptom: CUDA out of memory. Tried to allocate...
Root cause: FSDP performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). This usage pattern severely fragments the CUDA caching allocator. Memory gets trapped in blocks that cannot be merged, meaning PyTorch cannot find a contiguous block large enough for a new tensor, even though total free VRAM is sufficient.
Recommended fix: PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True
How Denpex helps: Denpex investigates CUDA OOM from Caching Allocator Fragmentation 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.

Memory#Fragmentation

What this failure is

CUDA OOM from Caching Allocator Fragmentation is a Memory failure seen during ML training runs. FSDP performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). This usage pattern severely fragments the CUDA caching allocator. Memory gets trapped in blocks that cannot be merged, meaning PyTorch cannot find a contiguous block large enough for a new tensor, even though total free VRAM is sufficient. Common tags: Fragmentation.

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Why it happens (the mechanism)

It looks like the model is simply too large for the GPU. Users often waste time reducing batch size or model size, which temporarily hides the issue but doesn't solve the underlying fragmentation.

What you'll observe

CUDA out of memory. Tried to allocate...
allocated and ... reserved in total by PyTorch

Common symptoms and what they mean

Symptom	Why it happens
The model crashes with a CUDA OOM error.	FSDP performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). This usage pattern severely fragments the CUDA caching allocator. Memory gets trapped in blocks that cannot be merged, meaning PyTorch cannot find a contiguous block large enough for a new tensor, even though total free VRAM is sufficient.
The error log states that PyTorch has a large amount of 'reserved' memory but very little 'allocated' memory (e.g., 'Tried to allocate 2.00 GiB. ... 38.00 GiB reserved').	FSDP performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). This usage pattern severely fragments the CUDA caching allocator. Memory gets trapped in blocks that cannot be merged, meaning PyTorch cannot find a contiguous block large enough for a new tensor, even though total free VRAM is sufficient.
Calculations of parameter + gradient + optimizer sizes show they should easily fit in VRAM.	FSDP performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). This usage pattern severely fragments the CUDA caching allocator. Memory gets trapped in blocks that cannot be merged, meaning PyTorch cannot find a contiguous block large enough for a new tensor, even though total free VRAM is sufficient.

Which systems are affected

CUDA Caching Allocator
PyTorch FSDP

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.

✓Read the PyTorch CUDA OOM exception message carefully. Look for a large discrepancy between 'reserved' (memory held by the caching allocator) and 'allocated' (memory actually in use by tensors).
✓Use `torch.cuda.memory_summary()` before the OOM to inspect block sizes.

Root cause

FSDP performs frequent allocations and deallocations of varying sizes (flattening, padding, un-sharding, sharding). This usage pattern severely fragments the CUDA caching allocator. Memory gets trapped in blocks that cannot be merged, meaning PyTorch cannot find a contiguous block large enough for a new tensor, even though total free VRAM is sufficient.

The fix and how to prevent it

Searchable error signature

search key

CUDA out of memory. Tried to allocate...

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

The root cause is on this page and stays free. A free account adds the exact remediation steps, saved history, and the fix on every entry in the encyclopedia.

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Diagnose this failure in VS Code

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CUDA errors in context

CUDA reports errors asynchronously, so the traceback usually points at whatever line synchronised next rather than the one at fault. The hub covers every common CUDA error and how to make it report honestly.

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References

↗PyTorch CUDA Memory Management

Don't just read the fix, diagnose your run

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