CAGRA-ACE: Support Direct HNSW Index Creation During Build

[julianmi]

A hnsw::build using CAGRA-ACE on-disk builds a CAGRA index on disk and uses hnsw::from_cagra to build an HNSW index from the ACE intermediate files. This patch adds support to directly create an HNSW index during ACE builds. It streams partitioned graph data into HNSW format with transactional publication, KvikIO-based I/O, and GPU hierarchy support. This speeds up the end-to-end runtime by removing a disk read of the full CAGRA graph. An exemplary BIGANN-100M build on an L40S GPU shows 8.14% end-to-end speedup with direct POSIX I/O on a consumer NVMe.

The direct CAGRA-ACE disk path does not produce a byte-identical HNSW index. Mainline assigns hierarchy levels with hnswlib’s stateful RNG after ACE reordering; the fused path uses deterministic per-ID levels and partitioned hierarchy construction. This changes maximum levels, internal IDs, and neighbor lists while preserving a valid hnswlib layout. The maximum absolute recall difference was 0.0087 for three exemplary datasets:

ef	Wiki main	Wiki fused	OpenAI main	OpenAI fused	LAION main	LAION fused
24	0.81716	0.82577	0.86655	0.86689	0.91345	0.92028
48	0.89432	0.90063	0.92568	0.92503	0.94778	0.95093
96	0.93976	0.94562	0.96076	0.95935	0.96731	0.96825
192	0.96633	0.97075	0.97996	0.97929	0.97622	0.97677
384	0.98310	0.98494	0.99035	0.98989	0.98121	0.98088
768	0.99190	0.99257	0.99550	0.99518	0.98314	0.98317

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• conda/recipes/libcuvs/recipe.yaml

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📝 Summary

Summary by CodeRabbit

• New Features

  • Added partitioned, disk-backed HNSW builds that produce serialized indexes for loading and searching.
  • Build planning accounts for host and device memory limits and reports an error when the minimum required memory cannot fit.
  • Added support for building and searching indexes from read-only memory-mapped datasets.
  • Memory-triggered builds create a build directory automatically, and temporary staging files are cleaned up after failed builds.

• Documentation

  • Updated the C++ example with information about build-directory contents, storage requirements, and cleanup after failed builds.

The pull request adds memory planning, external staging, and serialization for partitioned HNSW builds. Eligible builds can use the external builder. The change also updates build selection, tests, and example guidance.

Changes: External HNSW Build

Layer / File(s)	Summary
Partition and memory planning cpp/src/neighbors/detail/cagra/ace_external_plan.hpp, cpp/src/neighbors/detail/hnsw.hpp	The planner estimates partitions, buffers, I/O, and phase memory requirements. It selects partitions within configured budgets and rejects plans that exceed them. HNSW host-memory estimates also inform build selection.
External file format and workspace cpp/src/neighbors/detail/hnsw/external_format.hpp, cpp/src/neighbors/detail/hnsw/external_workspace.hpp	Stage files use validated headers, checked sizes, and positional I/O. The workspace manages private staging files, manifests, cleanup, and no-replace publication.
Shared HNSW serialization and loading cpp/src/neighbors/detail/hnsw/serialize_layout.hpp, cpp/src/neighbors/detail/hnsw/index_impl.hpp, cpp/src/neighbors/detail/hnsw.hpp	Shared helpers derive hierarchy layout and write HNSW headers, base rows, and upper links. The existing serializer uses these helpers, and the extracted index implementation provides lazy disk loading.
Partitioned external build cpp/src/neighbors/detail/hnsw/external_build.cuh, cpp/src/neighbors/detail/hnsw/external_translate.*, cpp/CMakeLists.txt	The builder stages rows by partition, builds partition graphs with CAGRA, translates local graph IDs, writes HNSW layers, and publishes the index. The GPU translation unit is included when BUILD_CAGRA_HNSWLIB is enabled.
Build routing and validation cpp/src/neighbors/detail/hnsw.hpp, cpp/tests/CMakeLists.txt, cpp/tests/neighbors/*hnsw*, python/cuvs/cuvs/tests/test_hnsw_ace.py, examples/cpp/src/hnsw_ace_example.cu, conda/recipes/libcuvs/recipe.yaml	Eligible builds use the partitioned path; other cases retain CAGRA build and conversion. Tests cover planner limits, staging, serialization, and search. The example comments describe the build directory and staging cleanup. The test package adds libkvikio as a host and runtime dependency.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Feature

Suggested reviewers: tarang-jain, divyegala

Merge Risk: ⚪ Minimal · up to 84f4b

The test package includes the KvikIO dependency needed by its linked tests at build and runtime. No actionable merge risk is evident.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 16.82% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 107 functions across 11 files. (1 skipped…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly identifies the main change: direct HNSW index creation during CAGRA-ACE builds.
Description check		The description directly explains the direct HNSW build path, transactional publication, I/O changes, performance impact, and recall differences.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 16.82% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 107 functions across 11 files. (1 skipped: 1 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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Actionable comments posted: 1

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Inline comments:
Review comments at @cpp/src/neighbors/detail/hnsw.hpp:
- Around line 2844-2852: Update the automatic ACE fallback path near
`cagra_ace_selected` so builds triggered by insufficient memory use a unique
temporary `build_dir` instead of the default `/tmp/ace_build`. Preserve any
explicitly supplied `ace_params` directory, and ensure the generated directory
is unique across existing and concurrent builds before publishing the index.

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📥 Commits

Reviewing files that changed from the base of the PR and between f751cad and 5cd656e.

📒 Files selected for processing (15)

• cpp/CMakeLists.txt
• cpp/src/neighbors/detail/cagra/ace_external_plan.hpp
• cpp/src/neighbors/detail/hnsw.hpp
• cpp/src/neighbors/detail/hnsw/external_build.cuh
• cpp/src/neighbors/detail/hnsw/external_format.hpp
• cpp/src/neighbors/detail/hnsw/external_translate.cu
• cpp/src/neighbors/detail/hnsw/external_translate.hpp
• cpp/src/neighbors/detail/hnsw/external_workspace.hpp
• cpp/src/neighbors/detail/hnsw/index_impl.hpp
• cpp/src/neighbors/detail/hnsw/serialize_layout.hpp
• cpp/tests/CMakeLists.txt
• cpp/tests/neighbors/ann_hnsw_ace.cuh
• cpp/tests/neighbors/hnsw_external_io.cu
• examples/cpp/src/hnsw_ace_example.cu
• python/cuvs/cuvs/tests/test_hnsw_ace.py

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