Where you round matters: aligning three BF16 regions with inference stopped LLM RL from collapsing
AspireAI community · arXiv (Yijing Ke, Tianle Cai, Ruosong Wang)
Reinforcement learning for large language models usually runs inference and training in separate systems, and even with identical weights and BF16 tensors the implementation differences can push an intended on-policy update off-policy. This paper asks a narrower question than the usual fix: instead of aligning the entire inference and training stacks, what happens if you align only where rounding occurs? The authors keep selected intermediates in FP32 until their BF16 outputs in three regions - RoPE, SwiGLU and residual RMSNorm - and leave the inference engine, the matmul and attention kernels, and the learning objective untouched. Across four Qwen-family models, aligning all three regions prevented the collapse seen under the default BF16 baseline, and aligning the RoPE path alone was enough in the runs studied. The honest caveat is the one the authors state themselves: the RoPE-only result holds 'in the studied runs', with remaining numerical discrepancies left in place - so this is a promising narrow lever, not a general guarantee about BF16 stability.
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