How to Use vllm: A Comprehensive Guide in 2024

What is vLLM?

In recent times, large language models (LLMs) have transformed various fields. However, deploying these models in real-world applications can be challenging due to their significant computational requirements. This is where vLLM comes into play. vLLM stands for Virtual Large Language Model and is an open-source library designed to efficiently support LLM inferencing and model serving.

vLLM was first introduced in a paper titled “Efficient Memory Management for Large Language Model Serving with PagedAttention” authored by Kwon et al. Developed at UC Berkeley, vLLM is built to handle high-throughput and memory-constrained workloads using an advanced algorithm called PagedAttention.

vllm virtual large language model
vLLM virtual large language model

What is the Core Idea of vLLM?

The main concept behind vLLM is to optimize memory management and inference speed for large language models. The key innovation is the PagedAttention algorithm, which efficiently handles attention keys and values in non-contiguous memory spaces. This approach minimizes memory fragmentation and enhances resource utilization.


The attention mechanism in LLMs enables them to focus on relevant parts of the input sequence while generating output. Traditional systems store key-value (KV) pairs in contiguous memory spaces, leading to inefficient memory management. PagedAttention partitions the KV cache into KV block tables, allowing flexible management of KV vectors across layers and attention heads. This results in optimized memory usage and reduced redundant duplication.

Continuous Batching and Quantization

vLLM also incorporates continuous batching to maximize hardware utilization and reduce idle time. It uses quantization techniques like FP16 to optimize memory usage by representing the KV cache in reduced precision, leading to smaller memory footprints and faster computations. Additionally, vLLM uses optimized CUDA kernels for maximum performance.

Use Cases of vLLM (Virtual Large Language Model)

vLLM’s efficient operation of LLMs opens numerous practical applications. Here are some compelling scenarios that highlight vLLM’s potential:

Revolutionizing Chatbots and Virtual Assistants

With its efficient serving support, vLLM can enhance chatbots and virtual assistants to hold nuanced conversations, understand complex requests, and respond with human-like empathy. By enabling faster response times and lower latency, vLLM ensures smoother interactions. Additionally, vLLM empowers chatbots to access and process vast amounts of information, allowing them to provide comprehensive and informative answers. This combination of speed, knowledge, and adaptability can transform chatbots into invaluable tools for customer service, technical support, and even emotional counseling.

Democratizing Code Generation and Programming Assistance

The field of software development is constantly evolving, and keeping pace with the latest technologies can be challenging. vLLM can act as a valuable companion for programmers of all experience levels. By leveraging its code-understanding capabilities, vLLM can suggest code completions, identify potential errors, and recommend alternative solutions to coding problems. This can significantly reduce development time and improve code quality. vLLM’s ability to generate documentation can also alleviate a major pain point for developers by automatically generating clear and concise documentation based on the written code.

Challenges of LLM

LLMs have shown their worth in tasks like text generation, summarization, language translation, and more. However, deploying these models with traditional LLM inference approaches suffers from several limitations:

High Memory Footprint

LLMs require large amounts of memory to store their parameters and intermediate activations, making them challenging to deploy in resource-constrained environments.

Limited Throughput

Traditional implementations struggle to handle high volumes of concurrent inference requests, hindering scalability and responsiveness. This affects the performance of LLMs on production servers and limits their efficiency with GPUs.

Computational Cost

The intense load of matrix calculations involved in LLM inference can be expensive, especially on large models. High memory requirements and low throughput further add to the computational costs.

Benefits of vLLM

vLLM offers several benefits over traditional LLM serving methods:

Higher Throughput

vLLM can achieve up to 24x higher throughput than HuggingFace Transformers, the most popular LLM library. This allows you to serve more users with fewer resources.

Lower Memory Usage

vLLM requires significantly less memory compared to traditional LLM serving methods, making it ready to deploy on platforms with limited hardware.

OpenAI-Compatible API

vLLM provides an OpenAI-compatible API, making it easy to integrate with existing LLM applications.

Seamless Integration with Hugging Face Models

vLLM can be used with various models from Hugging Face, making it a versatile tool for LLM serving.

How to Use vLLM?

vLLM is easy to use. Here is a step-by-step guide to getting started with vLLM in Python:


First, create a new conda environment and install vLLM with CUDA support:

# (Recommended) Create a new conda environment. conda create -n myenv python=3.9 -y conda activate myenv # Install vLLM with CUDA 12.1. pip install vllm

Offline Inference

To perform offline inference, import the vLLM module and initialize the vLLM engine with a specific built-in LLM model. The models are downloaded from Hugging Face by default.

from vllm import LLM, SamplingParams # Define input sequence and set sampling parameters. prompts = ["The future of humanity is"] sampling_params = SamplingParams(temperature=0.8, top_p=0.95) llm = LLM(model="meta-llama/Meta-Llama-3-8B-Instruct") # Generate the output/response. responses = llm.generate(prompts, sampling_params) print(f"Prompt: {responses[0].prompt!r} Generated text: {responses[0].outputs[0].text!r}")

Online Serving

To use vLLM for online serving, you can utilize OpenAI’s completions and APIs. Start the server with Python:

python -m vllm.entrypoints.openai.api_server --model NousResearch/Meta-Llama-3-8B-Instruct --dtype auto --api-key token-abc123

Call the server using the official OpenAI Python client library or any other HTTP client:

from openai import OpenAI client = OpenAI( base_url="http://localhost:8000/v1", api_key="token-abc123" ) completion = model="NousResearch/Meta-Llama-3-8B-Instruct", messages=[ {"role": "user", "content": "Hello!"} ] ) print(completion.choices[0].message)

Here is the official Documentation of vLLM.


vLLM represents a significant advancement in the field of large language models, addressing the challenges of high memory consumption and computational cost associated with traditional LLM serving methods. By leveraging the PagedAttention algorithm and other optimization techniques, vLLM offers higher throughput, lower memory usage, and seamless integration with popular models. Whether you are developing chatbots, programming assistants, or any other application that relies on LLMs, vLLM provides a robust and efficient solution.


What is vLLM for?

vLLM stands for Virtual Large Language Model and is an open-source library that supports efficient inferencing and model serving for LLMs.

Why is vLLM faster?

vLLM employs dynamic memory management, continuous batching, and optimized CUDA kernels to enhance performance and reduce memory usage.

Is OpenAI compatible with vLLM?

Yes, vLLM provides an OpenAI-compatible API, making it easy to integrate with existing OpenAI applications.

Who made vLLM?

vLLM was developed by UC Berkeley in 2023 to optimize ML workloads for high-throughput and memory-constrained environments.

How do you optimize inference speed using batching with vLLM?

By continuously batching incoming requests, vLLM maximizes hardware utilization and minimizes idle time, leading to faster inference speeds.

Where does vLLM store models?

vLLM automatically downloads models (if not already downloaded) and stores them in your Hugging Face cache directory.

What are the benefits of using vLLM?

vLLM offers higher throughput, lower memory usage, OpenAI-compatible API, and seamless integration with Hugging Face models, making it an efficient tool for LLM serving.

Can vLLM be used for both offline and online inference?

Yes, vLLM supports both offline and online inference, making it versatile for various applications.