Gradient Explosion Caused by Corrupted All-Reduce (Not Learning Rate)
Gradient norms explode suddenly while loss remains normal because the cross-rank reduction itself is corrupting values. In-network reduction (SHARP) faults, NCCL data corruption over a marginal link, or a mixed-precision comm dtype bug. Per-rank local gradients are healthy; only the aggregated result is wrong.
Gradient norms explode suddenly while loss remains normal because the cross-rank reduction itself is corrupting values. In-network reduction (SHARP) faults, NCCL data corruption over a marginal link, or a mixed-precision comm dtype bug.
What this failure is
Gradient Explosion Caused by Corrupted All-Reduce (Not Learning Rate) is a Training Stability failure seen during ML training runs. Gradient norms explode suddenly while loss remains normal because the cross-rank reduction itself is corrupting values. In-network reduction (SHARP) faults, NCCL data corruption over a marginal link, or a mixed-precision comm dtype bug. Per-rank local gradients are healthy; only the aggregated result is wrong. Common tags: Gradient Explosion, Nccl, Sharp, Infiniband.
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Why it happens (the mechanism)
Switch ALU corruption when NCCL_SHARP is enabled on faulty firmware. Bit corruption over a link failing CRC-silently. Comm-hook dtype truncation overflowing fp16 during reduce. 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
- grad_norm jumps 3-6 orders of magnitude in one step with no LR change
- loss at the same step is still normal (divergence follows a step later)
- per-rank pre-reduce norms differ from post-reduce values
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| grad_norm=41211.7 after steps of ~1-2 with unchanged schedule | Switch ALU corruption when NCCL_SHARP is enabled on faulty firmware |
| allreduce result mismatch when verified rank-by-rank | Bit corruption over a link failing CRC-silently |
| occurs only at specific scale/topology (SHARP-enabled fabrics) | comm-hook dtype truncation overflowing fp16 during reduce |
Which systems are affected
- NCCL with SHARP/in-network computing
- InfiniBand fabrics with switch-resident reduction
- mixed bf16/fp32 comm hooks
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: grad_norm=41211.7 after steps of ~1-2 with unchanged schedule
- ✓Verified signal present: allreduce result mismatch when verified rank-by-rank
- ✓Verified signal present: occurs only at specific scale/topology (SHARP-enabled fabrics)
- ✓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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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.
Install the free VS Code extensionNCCL 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 sideRelated failures to investigate next
Root cause
- Switch ALU corruption when NCCL_SHARP is enabled on faulty firmware
- Bit corruption over a link failing CRC-silently
- comm-hook dtype truncation overflowing fp16 during reduce
The fix and how to prevent it
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