Faiss: Efficient Similarity Search and Clustering of Dense Vectors

Pankaj Pandey
3 min readJun 14, 2023
Faiss: Efficient Similarity Search and Clustering of Dense Vectors

Faiss is a powerful library designed for efficient similarity search and clustering of dense vectors. It offers various algorithms for searching in sets of vectors, even when the data size exceeds the available RAM. Developed primarily at Meta’s Fundamental AI Research group, Faiss provides complete wrappers for Python/numpy and supports GPU implementations for faster performance. This blog post explores the key features of Faiss and demonstrates its usage with code examples.

Key Points:

  1. Faiss is a library for efficient similarity search and clustering of dense vectors.
  2. It supports various algorithms for searching in sets of vectors.
  3. Faiss can handle data sizes that do not fit in RAM.
  4. It provides complete Python/numpy wrappers and GPU implementations.
  5. The library is written in C++ with a focus on performance and scalability.

Understanding Faiss Methods for Similarity Search

Faiss assumes that instances are represented as vectors and can be compared using L2 (Euclidean) distances or dot products. Similarity is determined by the vectors with the lowest L2 distance or the highest dot product with a query vector. Faiss also supports cosine similarity for normalized vectors. Some methods in Faiss use compressed representations of vectors, while others employ indexing structures like HNSW and NSG to improve search efficiency.


Performing Similarity Search with Faiss

Here’s a code example demonstrating how to perform a similarity search using Faiss:

import faiss
import numpy as np

# Generate random data
data = np.random.rand(1000, 128).astype('float32')

# Create an index
index = faiss.IndexFlatL2(128)

# Add data to the index

# Perform a similarity search
query_vector = np.random.rand(1, 128).astype('float32')
k = 5 # Number of nearest neighbors to retrieve
distances, indices = index.search(query_vector, k)

# Print the results
print("Nearest neighbors:")
for i in range(k):
print(f"Neighbor {i+1}: Index {indices[0][i]}, Distance {distances[0][i]}")

Installing Faiss

Faiss can be easily installed using precompiled libraries for Anaconda in Python or PIP. The library has minimal dependencies and requires only a BLAS implementation. CUDA can be used for optional GPU support. Installation instructions and details can be found in the project’s INSTALL.md file.

Understanding How Faiss Works

Faiss revolves around index types that store sets of vectors and provide search functions based on L2 and/or dot product vector comparison. Different index types offer trade-offs in terms of search time, search quality, memory usage per index vector, training time and the need for external data for unsupervised training. Faiss’s GPU implementation is known for its fast exact and approximate nearest neighbor search, Lloyd’s k-means and small k-selection algorithms.

Exploring the Full Documentation of Faiss

For comprehensive documentation on Faiss, including tutorials, FAQs and troubleshooting tips, refer to the project’s wiki page. The doxygen documentation provides per-class information extracted from code comments. If you’re interested in reproducing research results using Faiss, the benchmarks README and link_and_code README offer valuable insights.


Faiss is an indispensable library for efficient similarity search and clustering of dense vectors. Its wide range of algorithms, support for large-scale data and integration with Python/numpy make it a go-to choice for various AI and machine learning tasks. Whether you’re working with CPU or GPU, Faiss provides optimized solutions for high-performance vector operations. Explore the Faiss documentation, try out the code examples and join the vibrant Faiss community to leverage its full potential.

Legal Notice

Faiss is MIT-licensed. Please refer to the LICENSE file in the top-level directory for more details. Meta Platforms, Inc. holds the copyright for Faiss. For Terms of Use and Privacy Policy specific to this project, kindly refer to the relevant documentation.

For More information, please visit GitHub — facebookresearch/faiss: A library for efficient similarity search and clustering of dense vectors.



Pankaj Pandey

Expert in software technologies with proficiency in multiple languages, experienced in Generative AI, NLP, Bigdata, and application development.