The Mamba in the Llama: Distilling and Accelerating Hybrid Models
Paper · 2024 · Junxiong Wang; Daniele Paliotta; Avner May; Alexander M. Rush; and Tri Dao
Summary
Transformer LLMs decode slowly on long sequences because of quadratic attention cost and a growing KV cache. Linear RNNs such as Mamba run inference far faster, but pretraining them at scale is expensive. The authors instead convert existing pretrained chat Transformers (Zephyr-7B, Llama-3 8B) into hybrid Mamba/Mamba2 models.
Each attention head becomes a Mamba head whose input, B and C projections reuse the attention value, key and query weights. Only the A matrix and the step size ∆ are new parameters, and the MLP blocks stay frozen. Attention layers are removed in stages, first down to 50% and then to 25%. The models are then trained with distillation-based SFT and DPO on about 20B tokens, which fits academic-scale GPU budgets. The authors also design a hardware-aware speculative decoding algorithm and kernel for Mamba and hybrid models.
The distilled hybrids roughly match their teachers on chat benchmarks. They beat Mamba models trained from scratch on trillions of tokens. This suggests that knowledge in Transformers can be transferred cheaply into architectures that are faster at inference.
Key points
- The best model, distilled from Llama3-8B-Instruct with 25% attention layers, reaches a 29.61 length-controlled win rate on AlpacaEval 2 (vs. GPT-4) and scores 7.35 on MT-Bench.
- It matches or exceeds Mamba 7B (trained on 1.2T tokens) and NVIDIA Hybrid Mamba2 (trained on 3.5T tokens) on tasks such as MMLU and TruthfulQA.
- Initializing from attention weights is critical. In the Zephyr 50%-attention hybrid, random initialization drops MT-Bench from 6.69 to 1.04 and raises LAMBADA perplexity from 6.20 to 55.01. Removing the Mamba blocks entirely is worse still, with perplexity of 151.98.
- The speculative decoding method exceeds 300 tokens/s for a Mamba 7B model, and the distilled model handles needle-in-a-haystack retrieval almost perfectly at 20× the distillation length.
- Code and checkpoints are public in the GitHub repos jxiw/MambaInLlama and itsdaniele/speculative_mamba.
Fields, topics and concepts
📚 Shelves 4The field of science this work belongs to (OpenAlex fields).Computer Science · 19,503 worksArtificial Intelligence · 18,805 worksComputer Vision and Pattern Recognition · 3,621 worksHardware and Architecture · 183 works
Cite
Cite as Junxiong Wang; Daniele Paliotta; Avner May; Alexander M. Rush; and Tri Dao. 2024. The Mamba in the Llama: Distilling and Accelerating Hybrid Models. arXiv:2408.15237.
@misc{rush2024mamba,
title = {{The Mamba in the Llama: Distilling and Accelerating Hybrid Models}},
author = {Junxiong Wang; Daniele Paliotta; Avner May; Alexander M. Rush; and Tri Dao},
year = {2024},
doi = {10.48550/arxiv.2408.15237},
eprint = {2408.15237},
archivePrefix = {arXiv},
url = {https://doi.org/10.48550/arxiv.2408.15237}
}References
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