Megatron-LM Hangs at Fused Kernel Compilation
Megatron-LM training hangs at startup while compiling fused kernels (right after 'using torch.float16 for parameters ...') and never proceeds. A stale or contended megatron/fused_kernels/build directory blocks the ninja build; deleting it (or pre-compiling once) unblocks the run.
Megatron-LM training hangs at startup while compiling fused kernels (right after 'using torch.
What this failure is
Megatron-LM Hangs at Fused Kernel Compilation is a Training Stability failure seen during ML training runs. Megatron-LM training hangs at startup while compiling fused kernels (right after 'using torch.float16 for parameters ...') and never proceeds. A stale or contended megatron/fused_kernels/build directory blocks the ninja build; deleting it (or pre-compiling once) unblocks the run. Common tags: Megatron Lm, Fused Kernels, Jit, Ninja.
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Why it happens (the mechanism)
A stale or partially written megatron/fused_kernels/build directory (from an interrupted prior build) leaves a ninja lock that never clears. Multiple ranks compiling into the same build dir simultaneously can deadlock the first-time build. 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 freezes during fused kernel compilation at launch
- No progress and no error message
- Multi-rank launches hang before the first step
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Process stuck at 'using torch.float16 for parameters ...' | A stale or partially written megatron/fused_kernels/build directory (from an interrupted prior build) leaves a ninja lock that never clears |
| Hang while compiling fused_kernels | Multiple ranks compiling into the same build dir simultaneously can deadlock the first-time build |
| Deleting megatron/fused_kernels/build fixes it | A stale or partially written megatron/fused_kernels/build directory (from an interrupted prior build) leaves a ninja lock that never clears |
Which systems are affected
- Megatron-LM with JIT-compiled fused kernels
- Multi-rank launches that race the same build directory
- Images without pre-built kernels
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: Process stuck at 'using torch.float16 for parameters ...'
- ✓Verified signal present: Hang while compiling fused_kernels
- ✓Verified signal present: Deleting megatron/fused_kernels/build fixes it
- ✓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
- A stale or partially written megatron/fused_kernels/build directory (from an interrupted prior build) leaves a ninja lock that never clears
- Multiple ranks compiling into the same build dir simultaneously can deadlock the first-time build
The fix and how to prevent it
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