lancedb-gatefixer[bot]
9e8f1c1a6d
## Summary - expose `memory_limit` and `num_workers` on the Python FTS configuration for local builds - forward both build-only settings to the Lance inverted-index builder - add an end-to-end regression proving the configured memory budget reaches the native build ## Root cause LanceDB 0.26.1 pinned Lance 1.0.1. That Lance version used an FTS partition-merge path whose retained data made memory grow with merge progress on very large indexes. Upstream Lance [#5754](https://github.com/lance-format/lance/pull/5754) changed partition merging to stream its inputs, reducing peak memory by about 25%. Lance [#6174](https://github.com/lance-format/lance/pull/6174) then removed the old merge phase, compressed posting lists during construction, reduced indexing memory by about 60%, and introduced a total build `memory_limit` for bounded workers. Current `main` pins Lance 11.0.0-beta.3, which contains those architectural fixes. This PR does not duplicate or claim the upstream leak fix; it addresses the remaining Python API gap. ## This repair LanceDB Python did not expose the native FTS builder resource controls. `memory_limit` now sets the total local-build budget in MiB, divided among effective workers, and `num_workers` controls build parallelism. Both are build-only settings and do not affect remote builds or persisted index configuration. ## Validation - `cargo check --quiet --features remote --tests --examples` - `cargo fmt --all` - `uv run --project python --extra tests --extra dev ruff check .` - `uv run --project python --extra tests --extra dev ruff format --check python/python/lancedb/index.py python/python/tests/test_fts.py` - `uv run --project python --extra tests pytest python/tests/test_fts.py -q` (51 passed) Fixes #2923 <!-- lance-gatekeeper-fix:v1 agent=a1ceedf74531e0212cb6f1ebf9390a26 generation=1 --> --------- Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>
LanceDB Python SDK
A Python library for LanceDB.
Installation
pip install lancedb
Pre-Haswell x86_64 hosts: lancedb-compat
The default lancedb wheel targets x86-64-haswell (AVX2 + FMA + F16C) for full performance on modern hardware. Pre-Haswell hosts — Intel Sandy Bridge / Ivy Bridge / Westmere; AMD Bulldozer / Piledriver / Steamroller — don't have AVX2 and crash with Illegal instruction at import lancedb.
For those hosts, install the lancedb-compat package instead:
pip install lancedb-compat
Same Python API (import lancedb works as usual). The compat wheel is compiled at the x86-64-v2 baseline (Nehalem-class) and uses runtime SIMD dispatch in the embedded lance crate to pick the right kernel tier (scalar / AVX / AVX+FMA / AVX2+FMA / AVX-512) at load time, so it still goes fast on modern hardware while running cleanly on the pre-Haswell silicon. Use lance.simd_info() from Python to verify which tier was selected.
lancedb and lancedb-compat install to the same lancedb/ namespace and conflict at install time. Pick one. To switch, pip uninstall lancedb first, then pip install lancedb-compat (or vice-versa).
If you need a custom baseline (or lancedb-compat isn't yet published for your platform), build from source with the override:
RUSTFLAGS="-C target-cpu=x86-64-v2" maturin build --release
pip install ./target/wheels/lancedb-*.whl
Preview Releases
Stable releases are created about every 2 weeks. For the latest features and bug fixes, you can install the preview release. These releases receive the same level of testing as stable releases, but are not guaranteed to be available for more than 6 months after they are released. Once your application is stable, we recommend switching to stable releases.
pip install --pre --extra-index-url https://pypi.fury.io/lancedb/ lancedb
Threading in CPU-limited containers
LanceDB uses separate pools for compute work and storage I/O. On a container with two visible CPUs, current releases intentionally use one compute worker by default; no manual configuration is needed. If every query logs an I/O core reservation warning on a two-CPU container, upgrade from LanceDB 0.21.1 or earlier.
The two commonly tuned environment variables control different resources:
LANCE_CPU_THREADSoverrides the number of compute workers. One worker is the appropriate setting for a two-CPU container when an explicit override is needed.LANCE_IO_THREADScontrols concurrent storage operations, not reserved CPU cores. Its default can be greater than the number of CPUs because I/O workers spend much of their time waiting for storage.
Keep the defaults unless measurements show that the workload benefits from an override. See the Lance threading model for the current defaults and tuning guidance.
Usage
Basic Example
import lancedb
db = lancedb.connect('<PATH_TO_LANCEDB_DATASET>')
table = db.open_table('my_table')
results = table.search([0.1, 0.3]).limit(20).to_list()
print(results)
Development
See CONTRIBUTING.md for information on how to contribute to LanceDB.