DeepSpeed FusedAdam Illegal Memory Access on H100
DeepSpeed FusedAdam optimizer triggers CUDA illegal memory access errors on NVIDIA H100 GPUs. This is a compatibility issue between FusedAdam's CUDA kernels and H100's SM90 architecture. Denpex detects the H100+FusedAdam combination and recommends the fallback optimizer.
DeepSpeed FusedAdam optimizer triggers CUDA illegal memory access errors on NVIDIA H100 GPUs.
What this failure is
DeepSpeed FusedAdam Illegal Memory Access on H100 is a Memory failure seen during ML training runs. DeepSpeed FusedAdam optimizer triggers CUDA illegal memory access errors on NVIDIA H100 GPUs. This is a compatibility issue between FusedAdam's CUDA kernels and H100's SM90 architecture. Denpex detects the H100+FusedAdam combination and recommends the fallback optimizer. Common tags: Deepspeed, Fusedadam, H100, Sm90.
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Why it happens (the mechanism)
FusedAdam's CUDA kernels were compiled for SM80 (A100) and are not compatible with SM90 (H100) architecture. H100's SM90 architecture has different shared memory layout and warp scheduling that causes out-of-bounds access in FusedAdam kernels. The illegal memory access occurs in the Adam update kernel when updating optimizer states (momentum, variance) for large parameter tensors. DeepSpeed's FusedAdam has not been updated to support SM90 architecture natively. 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 on H100 with DeepSpeed FusedAdam crashes with 'CUDA error: illegal memory access'
- The same training works on A100 with FusedAdam
- Switching to the standard PyTorch AdamW optimizer resolves the crash
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| CUDA error: illegal memory access in FusedAdam kernel | FusedAdam's CUDA kernels were compiled for SM80 (A100) and are not compatible with SM90 (H100) architecture |
| Training crashes during the optimizer step (not forward or backward) | H100's SM90 architecture has different shared memory layout and warp scheduling that causes out-of-bounds access in FusedAdam kernels |
| nvidia-smi shows the GPU in a bad state after the crash | The illegal memory access occurs in the Adam update kernel when updating optimizer states (momentum, variance) for large parameter tensors |
| The error is deterministic: it happens at the same step every time | DeepSpeed's FusedAdam has not been updated to support SM90 architecture natively |
Which systems are affected
- DeepSpeed with FusedAdam optimizer on NVIDIA H100 (SM90) GPUs
- DeepSpeed with FusedAdam on H200 GPUs
- DeepSpeed versions using CUDA kernels compiled for SM80 (A100) but running on SM90
- Mixed precision training with FusedAdam and bf16 on H100
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: CUDA error: illegal memory access in FusedAdam kernel
- ✓Verified signal present: Training crashes during the optimizer step (not forward or backward)
- ✓Verified signal present: nvidia-smi shows the GPU in a bad state after the crash
- ✓Verified signal present: The error is deterministic: it happens at the same step every time
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Root cause
- FusedAdam's CUDA kernels were compiled for SM80 (A100) and are not compatible with SM90 (H100) architecture
- H100's SM90 architecture has different shared memory layout and warp scheduling that causes out-of-bounds access in FusedAdam kernels
- The illegal memory access occurs in the Adam update kernel when updating optimizer states (momentum, variance) for large parameter tensors
- DeepSpeed's FusedAdam has not been updated to support SM90 architecture natively
The fix and how to prevent it
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