DeFT: Decoding with Flash Tree-attention for Efficient Tree-structured LLM Inference
Keywords: LLM inference, attention, memory-efficiency, tree-based decoding
TL;DR: We propose DeFT, a hardware-efficient tree attention algorithm to improve the tree-based decoding (e.g. multi-step reasoning, speculative decoding, etc) efficiency with IO-awareness for shared prefixes and load-balancing..
Abstract: Large language models (LLMs) are increasingly employed for complex tasks that process multiple generation calls in a tree structure with shared prefixes of tokens, including few-shot prompting, multi-step reasoning, speculative decoding, etc. However, existing inference systems for tree-based applications are inefficient due to improper partitioning of queries and KV cache during attention calculation.This leads to two main issues: (1) a lack of memory access (IO) reuse for KV cache of shared prefixes, and (2) poor load balancing.As a result, there is redundant KV cache IO between GPU global memory and shared memory, along with low GPU utilization. To address these challenges, we propose DeFT(Decoding with Flash Tree-Attention), a hardware-efficient attention algorithm with prefix-aware and load-balanced KV cache partitions. DeFT reduces the number of read/write operations of KV cache during attention calculation through **KV-Guided Grouping**, a method that avoids repeatedly loading KV cache of shared prefixes in attention computation. Additionally, we propose **Flattened Tree KV Splitting**, a mechanism that ensures even distribution of the KV cache across partitions with little computation redundancy, enhancing GPU utilization during attention computations. By reducing 73-99$\%$ KV cache IO and nearly 100$\%$ IO for partial results during attention calculation, DeFT achieves up to 2.52/3.82$\times$ speedup in the end-to-end/attention latency across three practical tree-based workloads compared to state-of-the-art attention algorithms.
Primary Area: infrastructure, software libraries, hardware, systems, etc.
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Submission Number: 8454
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