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QJL: 1-Bit Quantized JL Transform for KV Cache Quantization with Zero Overhead

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arxiv 2406.03482 v2 pith:EZMF6WOR submitted 2024-06-05 cs.LG cs.AIcs.CLcs.PF

QJL: 1-Bit Quantized JL Transform for KV Cache Quantization with Zero Overhead

classification cs.LG cs.AIcs.CLcs.PF
keywords quantizationcachememoryestimatoroverheadtransformapproachbits
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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Serving LLMs requires substantial memory due to the storage requirements of Key-Value (KV) embeddings in the KV cache, which grows with sequence length. An effective approach to compress KV cache is quantization. However, traditional quantization methods face significant memory overhead due to the need to store quantization constants (at least a zero point and a scale) in full precision per data block. Depending on the block size, this overhead can add 1 or 2 bits per quantized number. We introduce QJL, a new quantization approach that consists of a Johnson-Lindenstrauss (JL) transform followed by sign-bit quantization. In contrast to existing methods, QJL eliminates memory overheads by removing the need for storing quantization constants. We propose an asymmetric estimator for the inner product of two vectors and demonstrate that applying QJL to one vector and a standard JL transform without quantization to the other provides an unbiased estimator with minimal distortion. We have developed an efficient implementation of the QJL sketch and its corresponding inner product estimator, incorporating a lightweight CUDA kernel for optimized computation. When applied across various LLMs and NLP tasks to quantize the KV cache to only 3 bits, QJL demonstrates a more than fivefold reduction in KV cache memory usage without compromising accuracy, all while achieving faster runtime. Codes are available at \url{https://github.com/amirzandieh/QJL}.

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Cited by 9 Pith papers

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  4. Statistical Inference and Quality Measures of KV Cache Quantisations Inspired by TurboQuant

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