ChunkAttention: Efficient Self-Attention with Prefix-Aware KV Cache and Two-Phase Partition

Abstract

Self-attention is an essential component of large language models (LLM) but a significant source of inference latency for long sequences.In multi-tenant LLM serving scenarios, the compute and memory operation cost of selfattention can be optimized by using the probability that multiple LLM requests have shared system prompts in prefixes.In this paper, we introduce ChunkAttention, a prefix-aware self-attention module that can detect matching prompt prefixes across multiple requests and share their key/value tensors in memory at runtime to improve the memory utilization of KV cache.This is achieved by breaking monolithic key/value tensors into smaller chunks and structuring them into the auxiliary prefix tree.Consequently, on top of the prefix-tree based KV cache, we design an efficient selfattention kernel, where a two-phase partition algorithm is implemented to improve the data locality during self-attention computation in the presence of shared system prompts.Experiments show that ChunkAttention can speed up the self-attention kernel by 3.2-4.8compared to the start-of-the-art implementation, with the length of the system prompt ranging from 1024 to 4096. 1

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