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For mixture-of-experts models, deduplicating expert reads only paid off past five to eight tokens per pass

AspireAI community · arXiv
Speculative decoding verifies several drafted tokens in one pass, and on mixture-of-experts models consecutive tokens often pick the same experts, so a deduplicating engine reads each selected expert once rather than once per token. The authors model a step's time as the bytes each kernel requests over its effective bandwidth plus fixed time, derive when speculation beats plain decoding under geometric acceptance, and show the best draft length is a monotone staircase in the acceptance rate. After modifying llama.cpp Metal, MLX-LM and llama.cpp CUDA to deduplicate, they measured that the per-slot kernel has the higher effective bandwidth, at ratios of 0.32 to 0.75, which means deduplication is faster only from five to eight tokens per pass, or never. A prediction computed after the measurements placed the crossover within one pass size in 10 of 12 engine, model and depth configurations, and on DeepSeek-V2-Lite the predicted best draft length was within one of the measured optimum at all 22 acceptance values.
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