How vLLM Implements Decoding Algorithms | HackerNoon

Table of Links

Abstract and 1 Introduction

2 Background and 2.1 Transformer-Based Large Language Models

2.2 LLM Service & Autoregressive Generation

2.3 Batching Techniques for LLMs

3 Memory Challenges in LLM Serving

3.1 Memory Management in Existing Systems

4 Method and 4.1 PagedAttention

4.2 KV Cache Manager

4.3 Decoding with PagedAttention and vLLM

4.4 Application to Other Decoding Scenarios

4.5 Scheduling and Preemption

4.6 Distributed Execution

5 Implementation

6 Evaluation and 6.1 Experimental Setup

6.2 Basic Sampling

6.3 Parallel Sampling and Beam Search

6.4 Shared prefix

6.5 Chatbot

7 Ablation Studies

8 Discussion

9 Related Work

10 Conclusion, Acknowledgement and References

5 Implementation

vLLM is an end-to-end serving system with a FastAPI [15] frontend and a GPU-based inference engine. The frontend extends the OpenAI API [34] interface, allowing users to customize sampling parameters for each request, such as the maximum sequence length and the beam width 𝑘. The vLLM engine is written in 8.5K lines of Python and 2K lines of C++/CUDA code. We develop control-related components including the scheduler and the block manager in Python while developing custom CUDA kernels for key operations such as PagedAttention. For the model executor, we implement popular LLMs such as GPT [5], OPT [62], and LLaMA [52] using

PyTorch [39] and Transformers [58]. We use NCCL [32] for tensor communication across the distributed GPU workers.

5.1 Kernel-level Optimization

Since PagedAttention introduces memory access patterns that are not efficiently supported by existing systems, we develop several GPU kernels for optimizing it. (1) Fused reshape and block write. In every Transformer layer, the new KV cache are split into blocks, reshaped to a memory layout optimized for block read, then saved at positions specified by the block table. To minimize kernel launch overheads, we fuse them into a single kernel. (2) Fusing block read and attention. We adapt the attention kernel in FasterTransformer [31] to read KV cache according to the block table and perform attention operations on the fly. To ensure coalesced memory access, we assign a GPU warp to read each block. Moreover, we add support for variable sequence lengths within a request batch. (3) Fused block copy. Block copy operations, issued by the copy-on-write mechanism, may operate on discontinuous blocks. This can lead to numerous invocations of small data movements if we use the cudaMemcpyAsync API. To mitigate the overhead, we implement a kernel that batches the copy operations for different blocks into a single kernel launch.

5.2 Supporting Various Decoding Algorithms

vLLM implements various decoding algorithms using three key methods: fork, append, and free. The fork method creates a new sequence from an existing one. The append method appends a new token to the sequence. Finally, the free method deletes the sequence. For instance, in parallel sampling, vLLM creates multiple output sequences from the single input sequence using the fork method. It then adds new tokens to these sequences in every iteration with append, and deletes sequences that meet a stopping condition using free. The same strategy is also applied in beam search and prefix sharing by vLLM. We believe future decoding algorithms can also be supported by combining these methods.