State Space Models (SSMs), such as Mamba, have recently demonstrated potential in language understanding tasks, positioning them as competitors to transformer architectures. However, our investigations reveal that the Mamba architecture still has room for further optimization—not only in linear projections but also in state caches, which contribute significantly to memory consumption, particularly after quantizing the former into low bits. After a theoretical analysis of the causes of outliers in states, we propose Decoupled Scale Quantization (DSQ), which mitigates outliers in both the state and channel dimensions by applying separate quantization scales. To preserve the selective ability of quantized Mamba, we introduce Efficient Selectivity Reconstruction (ESR), a novel quantization simulation scheme in block-wise reconstruction that enables fast parallel scan algorithms with the non-linear quantization function. We demonstrate the effectiveness of Q-Mamba across various quantization settings, model sizes, and both generation and zero-shot tasks. In particular, for Mamba2-2.7B with W8A8H4 (8-bit weights and activations, 4-bit state caches) quantization, Q-Mamba achieves a 50% reduction in memory consumption with only a 2.13% average accuracy degradation on zero-shot tasks.

Delta compression methods focus on efficiently serving multiple uniquely fine-tuned models, each tailored to specific tasks and user requirements. These approaches decompose a fine-tuned LLM into a base model and corresponding delta weights, which are compressed using low-rank or low-bit representations to reduce storage costs. However, their effectiveness is highly sensitive to the magnitude of the model deltas—a factor directly influenced by the scale of the training data. We propose the Residual Quantization Tree (RQT), a hierarchical quantization framework that automatically shares low-bit integer weights across similar fine-tuned models. The RQT construction employs a two-phase greedy algorithm: a bottom-up aggregation of models based on weight matrix similarity, and top-down residual quantization, in which each node optimizes the quantization parameters and then delegates residual errors to child nodes. We evaluate RQT on fine-tuned models across mathematics, coding, chatbot, and Chinese LLMs. The results show that RQT achieves an average accuracy degradation of approximately 3% (comparable to previous 4-bit post-training quantization) while maintaining an effective bitwidth of around 2 bits.