llm_infer_engine: Implementation and Performance Analysis of a Modular LLM Inference Engine

llm_infer_engine is a modular LLM inference engine implemented in C++. It supports paged attention, continuous batching, and OpenAI-compatible APIs, aiming to provide developers with a concise and easy-to-understand inference engine implementation. It is suitable for scenarios such as learning inference engine principles and lightweight customization. Although its performance is not as good as mature solutions like vLLM, its modular design is its prominent advantage.

In the field of LLM inference engines, mature solutions like vLLM dominate, but developers have a demand for concise and modular implementations. The design goal of llm_infer_engine is to provide a concise and modular inference engine, written in C++ to balance performance and code clarity. Currently, it supports the Qwen2.5-7B-Instruct model.

1. Modular layer architecture: Encapsulates Transformer components into independent modules to lower the barrier of understanding;
2. Paged attention: Referencing vLLM, divides KV cache into fixed pages to improve memory efficiency, support sharing and dynamic expansion (default KV cache size is 2GB);
3. Continuous batching: Dynamically adds/removes requests to improve GPU utilization;
4. OpenAI-compatible API: Implemented via FastAPI/Uvicorn, supports chat completion interface and streaming output.

Single-request metrics: TTFT is approximately 975ms, TPOT about 152ms, end-to-end latency around 8819ms. Concurrent testing: When batch size is 8, throughput is 0.13 req/s with latency 54.7s; when batch size is 1, throughput is 0.05 req/s with latency 150s. Key findings: Batch size has a significant impact on performance; when concurrency exceeds the batch size, the system can adjust flexibly, and the latency distribution is stable.

Applicable scenarios: Education (learning inference engine principles), lightweight deployment (resource-constrained environments), custom development (easy to modify due to modularity), prototype verification (quickly validate optimization ideas). Limitations: Only supports Qwen2.5-7B-Instruct; performance is not as good as vLLM; lacks advanced features like multi-GPU support and quantization; insufficient ecosystem integration.

llm_infer_engine is a concise implementation that demonstrates core technologies like paged attention and continuous batching, making it an excellent material for learning inference engines. For production environments, it is recommended to use mature solutions like vLLM. We look forward to the project adding more model support, optimizing performance, and improving features in the future.