How to use
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FAISS - Efficient Similarity Search
Facebook AI's library for billion-scale vector similarity search.
When to use FAISS
Use FAISS when:
- Need fast similarity search on large vector datasets (millions/billions)
- GPU acceleration required
- Pure vector similarity (no metadata filtering needed)
- High throughput, low latency critical
- Offline/batch processing of embeddings
Metrics:
- 31,700+ GitHub stars
- Meta/Facebook AI Research
- Handles billions of vectors
- C++ with Python bindings
Use alternatives instead:
- Chroma/Pinecone: Need metadata filtering
- Weaviate: Need full database features
- Annoy: Simpler, fewer features
Quick start
Installation
bash
# CPU only
pip install faiss-cpu
# GPU support
pip install faiss-gpuBasic usage
python
import faiss
import numpy as np
# Create sample data (1000 vectors, 128 dimensions)
d = 128
nb = 1000
vectors = np.random.random((nb, d)).astype('float32')
# Create index
index = faiss.IndexFlatL2(d) # L2 distance
index.add(vectors) # Add vectors
# Search
k = 5 # Find 5 nearest neighbors
query = np.random.random((1, d)).astype('float32')
distances, indices = index.search(query, k)
print(f"Nearest neighbors: {indices}")
print(f"Distances: {distances}")Index types
1. Flat (exact search)
python
# L2 (Euclidean) distance
index = faiss.IndexFlatL2(d)
# Inner product (cosine similarity if normalized)
index = faiss.IndexFlatIP(d)
# Slowest, most accurate2. IVF (inverted file) - Fast approximate
python
# Create quantizer
quantizer = faiss.IndexFlatL2(d)
# IVF index with 100 clusters
nlist = 100
index = faiss.IndexIVFFlat(quantizer, d, nlist)
# Train on data
index.train(vectors)
# Add vectors
index.add(vectors)
# Search (nprobe = clusters to search)
index.nprobe = 10
distances, indices = index.search(query, k)3. HNSW (Hierarchical NSW) - Best quality/speed
python
# HNSW index
M = 32 # Number of connections per layer
index = faiss.IndexHNSWFlat(d, M)
# No training needed
index.add(vectors)
# Search
distances, indices = index.search(query, k)4. Product Quantization - Memory efficient
python
# PQ reduces memory by 16-32×
m = 8 # Number of subquantizers
nbits = 8
index = faiss.IndexPQ(d, m, nbits)
# Train and add
index.train(vectors)
index.add(vectors)Save and load
python
# Save index
faiss.write_index(index, "large.index")
# Load index
index = faiss.read_index("large.index")
# Continue using
distances, indices = index.search(query, k)GPU acceleration
python
# Single GPU
res = faiss.StandardGpuResources()
index_cpu = faiss.IndexFlatL2(d)
index_gpu = faiss.index_cpu_to_gpu(res, 0, index_cpu) # GPU 0
# Multi-GPU
index_gpu = faiss.index_cpu_to_all_gpus(index_cpu)
# 10-100× faster than CPULangChain integration
python
from langchain_community.vectorstores import FAISS
from langchain_openai import OpenAIEmbeddings
# Create FAISS vector store
vectorstore = FAISS.from_documents(docs, OpenAIEmbeddings())
# Save
vectorstore.save_local("faiss_index")
# Load
vectorstore = FAISS.load_local(
"faiss_index",
OpenAIEmbeddings(),
allow_dangerous_deserialization=True
)
# Search
results = vectorstore.similarity_search("query", k=5)LlamaIndex integration
python
from llama_index.vector_stores.faiss import FaissVectorStore
import faiss
# Create FAISS index
d = 1536
faiss_index = faiss.IndexFlatL2(d)
vector_store = FaissVectorStore(faiss_index=faiss_index)Best practices
- Choose right index type - Flat for <10K, IVF for 10K-1M, HNSW for quality
- Normalize for cosine - Use IndexFlatIP with normalized vectors
- Use GPU for large datasets - 10-100× faster
- Save trained indices - Training is expensive
- Tune nprobe/ef_search - Balance speed/accuracy
- Monitor memory - PQ for large datasets
- Batch queries - Better GPU utilization
Performance
| Index Type | Build Time | Search Time | Memory | Accuracy |
|---|---|---|---|---|
| Flat | Fast | Slow | High | 100% |
| IVF | Medium | Fast | Medium | 95-99% |
| HNSW | Slow | Fastest | High | 99% |
| PQ | Medium | Fast | Low | 90-95% |
Resources
- GitHub: https://github.com/facebookresearch/faiss ⭐ 31,700+
- Wiki: https://github.com/facebookresearch/faiss/wiki
- License: MIT
