Faiss

Faiss is a C++ library with Python/NumPy bindings for efficient similarity search and clustering of dense vectors. Use this skill when a task involves nearest-neighbor search, vector indexing, clustering, quantization, binary search, index files, or Faiss CPU/GPU integration.

Start with the environment gate

1. Identify the distribution and backend required by the task. Use faiss-cpu for the CPU baseline. Use a documented CUDA/ROCm/Metal/cuVS/SVS package or source build only when the requested capability needs it.

2. Run the bundled read-only probe before making backend claims. The backend-specific checker is bundled with the accelerated route:

   python path/to/faiss/sub-skills/accelerated-and-interoperable/scripts/check_backend.py --json
   

   It reports the imported version, compile options, available GPU symbols, device count, optional module status, NumPy availability, and tool/runtime signals. A visible GPU does not turn a CPU build into a GPU build.

3. Normalize float inputs to contiguous float32 arrays of shape (n, d). Binary indexes instead use packed uint8 rows with a bit dimension that is divisible by eight. Establish the metric and preprocessing before selecting an index.

4. Validate an approximate or compressed index against an exact Flat baseline on a bounded fixture before tuning recall, memory, or latency.

For public installation, prefer the documented Conda packages: faiss-cpu for CPU, faiss-gpu for CUDA, and faiss-gpu-cuvs for the cuVS variant. A source build uses CMake and can enable Python, CUDA/ROCm, C API, Metal, cuVS, or SVS independently; do not enable extras merely because they exist.

Route by task

• Index selection and search covers Flat, IVF, HNSW/graph search, metrics, factory strings, result contracts, nprobe, efSearch, range search, and CPU search tuning.
• Training and compression covers train/add/code lifecycles, IVF-PQ, PQ, SQ, residual/additive/RaBitQ, transforms, fast scan, reconstruction, and binary/Hamming indexes.
• Composition and filtering covers ID maps, selectors, per-search parameters, transforms, refinement, child ownership, direct maps, shards, replicas, and advanced IVF inspection.
• Persistence and evaluation covers byte/file serialization, cloning, mmap/on-disk storage, merge, clustering, exact ground truth, recall/precision, and operating points.
• Accelerated and interoperable covers CUDA and multi-GPU transfer, cuVS/ROCm/Metal/SVS gates, C++/C APIs, Torch buffers, FFI, dynamic-library diagnosis, and RPC boundaries.

If a workflow spans branches, keep the core CPU baseline in the first relevant route, then follow its explicit sibling link for composition, persistence, compression, or backend-specific work. Do not treat a benchmark result as validated until its data, metric, exact baseline, and backend are recorded.

Minimal CPU check

import numpy as np
import faiss

xb = np.ascontiguousarray(np.random.default_rng(0).random((100, 16), dtype="float32"))
index = faiss.IndexFlatL2(xb.shape[1])
index.add(xb)
D, I = index.search(xb[:2], 4)
assert D.shape == I.shape == (2, 4)
print(faiss.__version__, index.ntotal, faiss.get_compile_options())

Read cross-cutting troubleshooting for installation/import failures, dtype and shape errors, missing optional backends, unsafe index files, and thread/runtime problems. Read repository provenance before deciding whether this graph is stale for a changed Faiss checkout.