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Toward RL on mixed-precision, mixed-accelerator samplers

Blog post from Modal

Post Details
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Date Published
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3,626
Company Posts That Month
8
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English
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Summary

Researchers demonstrate that large-model reinforcement learning can use heterogeneous, mixed-precision sampling fleets—including B200/B300 NVFP4, H100/H200 FP8 or BF16, and A100/RTX PRO 6000 BF16 accelerators—while training a BF16 model asynchronously on B200 GPUs. In agentic coding training of Qwen3.6-35B-A3B on repository repair tasks, uncorrected GRPO training collapsed even with homogeneous samplers because sampler–trainer mismatch grew over time, causing rising KL divergence, declining rewards, and repeated tool-use failures. The authors stabilize learning through masked importance sampling to reweight off-policy tokens, score centering to remove gradient drift, and top-p mask replay to ensure the sampler and trainer share the same candidate-token support. This combination achieved SWE-bench Pro V2 performance comparable to homogeneous BF16 B200 sampling despite an average sampler–trainer mismatch 4.7 times larger, while reducing sampler cost per update by about 15%. The findings argue that stability depends less on minimizing numerical mismatch than on keeping it bounded through off-policy correction, potentially allowing RL systems to independently optimize training and inference hardware, precision, kernels, scaling, and serving strategies.

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