Recent advancements in Large Language Models (LLMs) have showcased their impressive problem-solving abilities in various fields. These LLMs, which can have billions of parameters, are trained on massive text collections.
Studies indicate that in LLM inference, memory bandwidth is the main limiting factor for generative tasks, rather than the CPU. This means that the speed at which parameters can be loaded and stored in memory-bound situations becomes the primary barrier to latency. However, advancements in memory bandwidth technology have not kept pace with computation, leading to a phenomenon called the Memory Wall.
Quantization is an effective technique that involves storing model parameters with less accuracy than the standard 16 or 32 bits used during training. However, achieving good quantization performance, especially with lower bit precision and modest models, has remained challenging.
A new study from UC Berkeley delves into low-bit precision quantization and highlights the limitations of current methods. Based on their findings, the researchers introduce SqueezeLLM, a post-training quantization framework that combines a Dense-and-Sparse decomposition technique with a unique sensitivity-based non-uniform quantization strategy. These methods enable ultra-low-bit precision quantization while maintaining competitive model performance, significantly reducing model sizes and inference time costs. In fact, their method improves the perplexity of the LLaMA-7B model at 3-bit precision from 28.26 with uniform quantization to 7.75 on the C4 dataset.
Through extensive testing on benchmark datasets, the researchers demonstrate that SqueezeLLM consistently outperforms existing quantization approaches across different bit precisions for language modeling tasks using LLaMA-7B, 13B, and 30B models.
One of the challenges in low-bit precision quantization for LLMs is dealing with outliers in weight matrices. These outliers affect non-uniform quantization by skewing the allocation of bits towards extremely high or low values. To overcome this, the researchers propose a simple method that splits the model weights into dense and sparse components. By isolating the extreme values, the range of the central region narrows, resulting in better quantization precision. The sparse data can be stored in full precision using efficient sparse storage methods, minimizing overhead.
The researchers validate the efficacy of their framework by applying SqueezeLLM to different models, such as Vicuna-7B and 13B, and comparing the results with other state-of-the-art approaches. SqueezeLLM consistently outperforms GPTQ and AWQ on multiple benchmarks, with the 4-bit quantized model performing just as well as the baseline model.
Furthermore, the proposed method achieves significant latency reductions and improves quantization performance when running on A6000 GPUs. The researchers report speedups of up to 2.3 compared to baseline FP16 inference for LLaMA-7B and 13B models, as well as up to 4x faster latency than GPTQ, highlighting the efficiency of their quantization performance and inference.
Overall, the UC Berkeley study presents an innovative post-training quantization framework, SqueezeLLM, which addresses the challenges of low-bit precision quantization for LLMs. The framework combines a Dense-and-Sparse decomposition technique with a sensitivity-based non-uniform quantization strategy to achieve ultra-low-bit precision while maintaining competitive model performance. The results demonstrate the effectiveness of SqueezeLLM in reducing model sizes, inference time costs, and latency while preserving the quality of generated output.
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