What is vLLM? Everything You Should Know

If you’ve ever used AI tools like ChatGPT and wondered how they’re able to generate so many prompt responses so quickly, vLLM is a big part of the explanation. It’s a high-performance engine to make large language models (LLMs) run faster and more efficiently. 

This blog effectively summarizes what vLLM is, why it matters, how it works and how developers can use it. Whether you’re a developer looking to accelerate your AI models or simply curious about the inner workings of AI, this guide will give you the low-down and help you start using vLLM in your own projects. Let’s dive in.

What is vLLM?

vLLM is a light-weight, open-source, efficient inference engine designed for large language models (LLMs) such as LLaMA, Mistral, GPT-style models, and is also well-suited for smaller models. It is optimized for high-throughput generation of text with low latency using clever features such as PagedAttention and dynamic batching. In simple terms, vLLM helps you run LLMs faster and more efficiently whether for chatbots, APIs, or real-time applications.

Why Was vLLM Developed?

Transformer models are powerful but can be slow and inefficient when many users try to use them at once. During inference, they often struggle with high memory usage and poor parallel handling. vLLM was developed to fix these issues by making inference faster, lighter on memory, and better at serving multiple users at the same time. 

It uses techniques like PagedAttention and dynamic batching to improve speed and scalability. This makes it ideal for real-world apps like chatbots, APIs, and AI assistants.

Traditional Inference vs vLLM: Feature-by-Feature Comparison

vLLM Architecture and Key Innovations

High Throughput

• High throughput means processing a large number of requests or tokens quickly in a short amount of time. In the case of LLMs, it refers to how many words (tokens) the model can generate per second, especially when handling multiple users or queries at once. 
• vLLM is designed to maximize throughput so you can serve many users efficiently without slowing down.

vLLM introduces several smart techniques to make large language models faster and more efficient. Here are the key ones

PagedAttention

To understand PagedAttention, let’s first walk through two basic concepts: normal attention and the KV cache, then we’ll explain what PagedAttention improves.

What Is Attention?

When an AI model generates text, it needs to “pay attention” to the words it has already written so it knows what to say next.This process is called attention. It’s like writing a story and constantly looking back at the last few lines to keep it flowing naturally.

Example of Attention

Let’s say the model is generating this sentence:

"The quick brown fox jumps over the lazy dog"

• When predicting “quick” → it looks at: [The]
• When predicting “brown” → it looks at: [The, quick]
• When predicting “fox” → it looks at: [The, quick, brown]
• ... and so on.

Problem:

As the sentence gets longer, the model has to look back at more and more words each time, repeating the same work over and over. This makes it slow and inefficient for long text. To solve this, a method called KV cache was introduced.

What Is KV Cache?

To eliminate the need to repeat all the previous work at every decoding step, the model holds key pieces of information from past words in this fast access memory called KV cache ( Key-Value cache). Consider this metaphor: You’re doing a puzzle, and you’ve figured out some of it, so you write down on a sticky note the hints you’ve learned.

Example with KV cache

• The model stores the results for “The”, then “quick”, then “brown”…
• When it reaches “fox”, instead of recomputing everything, it just reads cached data:
  • “Oh, I already know how ‘The’, ‘quick’, and ‘brown’ affect this.”
  • Basically it does calculation once and stores the calculated value, so it won’t do calculation again instead it uses the calculated value for reference.
• So it skips re-reading and saves time.

Benefit: Much faster than normal attention!

Problem:Imagine you’re doing this for 100 users, each writing their own sentence like “The quick brown fox...”. All their KV caches are stored in one big memory block that fills up fast and can’t be reused well.

What Is PagedAttention?

PagedAttention solves these problems by organizing memory-like pages in a notebook instead of one long scroll.Each user’s info is saved in small pages, and the system can:

• Reuse empty pages when users leave
• Add or remove pages on the fly
• Access only the pages it needs, making everything faster and more efficient.

Real-life analogy:Imagine you’re running a library.

• Traditional KV Cache: You put all books from all users on one giant, messy shelf.
• PagedAttention: You give each user their own small shelf, and if they leave, you reuse that shelf space for someone else.

Example with PagedAttention:

• User A is writing: “The quick brown fox...”
• Each word's info is stored on a separate memory page
  • Page 1: “The”
  • Page 2: “quick”
  • Page 3: “brown”
  • Page 4: “fox”
• If user A finishes or disconnects, those pages are freed and reused by User B writing: “The fast grey wolf

Benefits:

• No wasted space
• Can serve many users at once
• Perfect for long conversations or chatbots

What Is Dynamic Batching?

Think of a bus sitting there waiting to pick you up. In the traditional systems, the bus doesn’t leave until enough people have gotten on (fixed batching), losing time if there aren’t enough passengers. Dynamic batching is the equivalent of one smart bus that keeps going, it simply picks up customers as they appear, no need to wait for it to fill up.

Dynamic batching in vLLM is that the AI model receives the latest requests from the user in real time, enqueue and process them together. This makes things fast and efficient, even when there are many users hitting the model at various times.

GPU Utilization Optimizations:

Large models need powerful hardware (GPUs) to run. But traditional methods don’t always use the GPU fully, they leave parts of it idle.

vLLM uses smart scheduling, memory reuse, and optimized code to make full use of the GPU’s power, which leads to higher speed, better performance, and less waste.

Quantization:

vLLM currently supports quantized models, with AWQ being the most stable and integrated format. Support for other methods like GPTQ, INT4, INT8, AutoRound, and FP8 is emerging or experimental depending on model and backend.

VLLM Setup for Text Generation Model:

pip install vllm gradio

import gradio as gr
from vllm import LLM, SamplingParams

# Load model once
llm = LLM(model="TinyLlama/TinyLlama-1.1B-Chat-v1.0")

# Set decoding parameters
sampling_params = SamplingParams(
    temperature=0.7,
    top_p=0.95,
    max_tokens=128,
)

# Build prompt using chat-style format
def build_prompt(history, user_input):
    prompt = ""
    for user, bot in history:
        prompt += f"<|user|>\n{user}<|end|>\n<|assistant|>\n{bot}<|end|>\n"
    prompt += f"<|user|>\n{user_input}<|end|>\n<|assistant|>\n"
    return prompt

# Inference function
def chat_fn(user_input, history):
    prompt = build_prompt(history, user_input)
    outputs = llm.generate(prompt, sampling_params)
    reply = outputs[0].outputs[0].text.strip()
    return reply

# Gradio UI
gr.ChatInterface(
    fn=chat_fn,
    title="🦙 TinyLlama Chat (Basic vLLM)",
    description="Running TinyLlama-1.1B with vLLM — simple Gradio interface.",
    chatbot=gr.Chatbot(height=400),
    theme="soft",
    examples=["Hi", "What's the capital of India?", "Tell me a joke"],
).launch()

vLLM is designed to work with a specific type of large language model: decoder-only transformer models.

Model Architecture: Decoder-Only Transformer Models

These models are the most common type used for text generation, like answering questions, writing code, or chatting with users.

What is a Decoder-Only Model?Think of a transformer model as a machine with two main parts:

• An encoder (understands input, like in translation)
• A decoder (generates new text based on input)

Decoder-only models skip the encoder and focus just on generating output from previous text, they're like really smart autocomplete systems.

Example When you type:

“The quick brown fox”

A decoder-only model tries to complete it, predicting something like:

“jumps over the lazy dog”

Popular decoder-only models supported by vLLM:

• LLaMA (Meta)
• GPT-2 / GPT-3 / GPT-NeoX
• Falcon
• Mistral
• OPT
• BLOOM

These models work well with vLLM because they generate text one word (token) at a time and benefit from vLLM’s features like PagedAttention and dynamic batching.

OpenAI Compatibility

vLLM is designed to mimic the same API that OpenAI uses. That means you can run models on vLLM just like you would with OpenAI’s GPT API, using the same code (like openai.ChatCompletion.create()).

Why Is This Useful?

1. Plug-and-Play for Developers:If you already built apps using OpenAI’s API, you can switch to vLLM with almost no code changes.
2. Avoid API Costs:Instead of paying per request to OpenAI, you can run your own model locally or on your own server using vLLM.
3. More Control:You can host your own model, choose which one to use (like LLaMA or Mistral), and have full control over data, privacy, and updates.
4. Scale for Production:vLLM can handle many users at once, making it a great alternative for high-traffic applications.

vLLM OpenAI-Compatible API Setup (With Example Code)

vLLM makes it easy to use your own large language models as a drop-in replacement for OpenAI’s API. Here’s how you can set it up and test it.

Step 1: Start the vLLM Server

Use the vllm serve command to launch your model. This example runs Llama 3.2B Instruct with an API key:

vllm serve meta-llama/Llama-3.2-3B-Instruct --dtype auto --api-key token-abc123

• --dtype auto: Automatically selects the best precision (like float16) for performance.
• --api-key: Adds a simple layer of authentication for your API.

By default, the server runs at: http://localhost:8000/v1

Step 2: Use the OpenAI SDK with vLLM

vLLM follows the OpenAI API format, so you can use the official OpenAI Python client with almost no changes:

from openai import OpenAI
client = OpenAI(
    base_url="http://localhost:8000/v1",  # Make sure it's http, not https
    api_key="token-abc123",               # Same key as used in vllm serve
)
completion = client.chat.completions.create(
    model="meta-llama/Llama-3.2-3B-Instruct",
    messages=[{"role": "user", "content": "Hello!"}]
)
print(completion.choices[0].message)

Conclusion

vLLM makes it easier to serve large language models at scale, handling multiple users efficiently without slowing down. With features like PagedAttention, dynamic batching, and OpenAI-compatible APIs, it solves common problems like high latency and underused hardware.

Whether you're building a chatbot, an AI-powered tool, or a large-scale application, vLLM helps you streamline performance, cut costs, and scale with ease. If you're looking for more control, better speed, and production-ready performance from your LLM deployments, vLLM is well worth exploring.

Frequently Asked Questions?