Add General HNSW GRAPH_ONLY Support by julianmi · Pull Request #2677 · NVIDIA/cuvs

julianmi · 2026-09-23T09:08:59Z

This patch proposes to extend the HNSW graph only format to all build algorithms called during hnsw::build. Thus, a user can select to build the HNSW graph only independent of the memory requirements and the chosen build method. This minimizes the network transfer required when moving the index to another system with the local dataset. Builds on top of #2148 which focuses on CAGRA-ACE support.

CC @tfeher

coderabbitai · 2026-10-09T09:18:33Z

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Reviewing files that changed from the base of the PR and between fc2487f and f8aef05.

📒 Files selected for processing: 3

cpp/src/neighbors/detail/hnsw.hpp
cpp/src/neighbors/hnsw_layered_gather.cu
examples/cpp/src/hnsw_ace_layered_example.cu

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📝 Summary

Summary by CodeRabbit

New Features
- Layered HNSW graph-only output now supports regular CAGRA, in-memory ACE, and disk-backed ACE sources, with options to persist graphs after building or loading.
- The C++ example now demonstrates regular, in-memory ACE, and disk-backed ACE builds.
Bug Fixes
- Improved compatibility when reading serialized half-precision indexes using legacy dtype metadata.
Documentation
- Clarified dataset requirements, artifact ownership, output locations, and behavior when a destination already exists.

Graph-only HNSW conversion now supports regular CAGRA, in-memory ACE, and disk-backed ACE sources. The changes add layered artifact creation and serialization, broaden source and lifecycle tests, update the example and API documentation, and accept a legacy half-precision dtype descriptor.

Changes: Graph-only HNSW artifacts

Layer / File(s)	Summary
Graph-only API contract `cpp/include/cuvs/neighbors/hnsw.hpp`, `fern/pages/cpp_api/cpp-api-neighbors-hnsw.md`	Documentation describes supported sources, dataset requirements, row mappings, artifact paths, and ownership.
Graph-only artifact creation `cpp/src/neighbors/detail/hnsw.hpp`, `cpp/src/neighbors/detail/hnsw_layered_gather.hpp`, `cpp/src/neighbors/hnsw_layered_gather.cu`, `cpp/CMakeLists.txt`	The layered builder handles in-memory and disk-backed graph topology. Device vectors are gathered in bounded batches. Conversion manages temporary artifacts for in-memory sources and writes disk-backed output beside the source graph.
Loaded graph serialization `cpp/src/neighbors/detail/hnsw.hpp`	Loaded graph-only indexes serialize their existing topology and construction dtype.
Artifact lifecycle and source coverage `cpp/tests/neighbors/ann_hnsw_ace*`, `cpp/tests/CMakeLists.txt`, `examples/cpp/src/hnsw_ace_layered_example.cu`	Tests cover source variants, gathering, artifact cleanup, serialization round trips, and construction dtype retention. The example accepts a build method and output directory.

Legacy half dtype validation

Layer / File(s)	Summary
Half dtype compatibility `cpp/src/util/serialize_validation.hpp`, `cpp/tests/util/serialize_validation_test.cpp`, `cpp/tests/CMakeLists.txt`	Half-precision validation accepts the legacy `<e2` descriptor. Tests cover legacy and standard half descriptors, invalid descriptors, and unchanged float and double validation.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Feature

Suggested reviewers: tfeher, bdice, imotov

Merge Risk: 🟡 Moderate · up to f8aef

Previously written IVF-RaBitQ indexes may be misread, and HNSW artifact publication may miss a close error. Resolve or explicitly accept these risks before merging.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 8.11% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 74 functions across 13 files.	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 and concisely describes the main change: extending HNSW GRAPH_ONLY support beyond the existing build path.
Description check		The description directly explains the proposed generalization of HNSW graph-only output and its benefit for reducing index transfer requirements.
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.

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Actionable comments posted: 3

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Inline comments:
Review comments at @cpp/src/neighbors/detail/hnsw.hpp:
- Line 2737: Close `fd` before renaming `output.temporary_path()` to `path`,
ensuring any close-time write errors are detected before the artifact is
published.
- Around line 2387-2392: Document the existing-artifact precondition in the
disk-backed GRAPH_ONLY from_cagra flow near artifact_path: publication fails if
hnsw_index.cuvs already exists, so repeated direct conversion requires a fresh
graph directory or is unsupported.

Review comments at @cpp/src/neighbors/ivf_rabitq/gpu_index/ivf_gpu.cu:
- Around line 194-205: Preserve compatibility in the `ivf_gpu.cu` serialization
path: the previous writer persisted `short_data_` in word-major order, but this
loader interprets codes as vector-major and silently changes existing indexes.
Keep reading and writing the established word-major layout, or add format
versioning and decode both layouts; ensure a multi-row, multi-word index from
the previous writer retains its codes and search results.

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📥 Commits

Reviewing files that changed from the base of the PR and between f751cad and aceb742.

📒 Files selected for processing (17)

cpp/CMakeLists.txt
cpp/include/cuvs/neighbors/hnsw.hpp
cpp/src/neighbors/detail/hnsw.hpp
cpp/src/neighbors/detail/hnsw_layered_gather.hpp
cpp/src/neighbors/hnsw_layered_gather.cu
cpp/src/neighbors/ivf_rabitq/gpu_index/ivf_gpu.cu
cpp/src/util/serialize_validation.hpp
cpp/tests/CMakeLists.txt
cpp/tests/neighbors/ann_hnsw_ace.cuh
cpp/tests/neighbors/ann_hnsw_ace/test_float_uint32_t.cu
cpp/tests/neighbors/ann_hnsw_ace/test_half_uint32_t.cu
cpp/tests/neighbors/ann_hnsw_ace/test_int8_t_uint32_t.cu
cpp/tests/neighbors/ann_hnsw_ace/test_uint8_t_uint32_t.cu
cpp/tests/neighbors/ann_ivf_rabitq/test_float_int64_t.cu
cpp/tests/util/serialize_validation_test.cpp
examples/cpp/src/hnsw_ace_layered_example.cu
fern/pages/cpp_api/cpp-api-neighbors-hnsw.md

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tfeher

Thanks Julian for the PR, it is nice to have the ability to save the graph without the dataset, without requiring a specific build method.

Some of the comments assumes that I know what happens under the hood with file handling (or what was historically happening). Let's revise both the user facing and developer docstrings, to make it easier for uninitiated users/developers.

Overall it looks good, see a list a smaller issues below.

tfeher · 2026-10-14T20:36:36Z

+ * The returned handle references a graph artifact. Save it with `serialize`, then load it with
+ * the two-filename `deserialize` overload and an original-order dataset before searching.
+ * Temporary artifacts from in-memory builds are removed when the handle is destroyed.
+ *


The example below does not use GRAPH_ONLY output format, therefore it is not clear why this comment is necessary here. Since the docstring is already long, I would prefer not to discuss this detail here.

Btw what does it mean "The returned handle references a graph artifact"? Does this refer to the index created by hnsw::build()? Does it mean that the graph is already serialized when we create the HNSW graph from CAGRA?

tfeher · 2026-10-14T20:40:17Z

+ *
+ * Disk-backed `GRAPH_ONLY` conversion creates `hnsw_index.cuvs` beside the CAGRA graph and fails
+ * if that file already exists. Before repeating conversion, move the previous artifact elsewhere
+ * or use a fresh graph directory.


Is the directory controlled by ace.build_dir?

Yes for disk-backed ACE. Direct conversion derives it from the existing graph. I've updated the comment.

tfeher · 2026-10-14T20:45:50Z

+ * Serialize to a filename outside the handle's temporary directory to preserve the artifact
+ * beyond the handle's lifetime. Serializing to its current `file_path()` does not transfer
+ * ownership.


If I set ace.build_dir="/my/ace/dir", does this count as a temporary dir that would be removed when the index is destroyed?

No, only the ACE intermediate files that we expect not to be transferred.I've updated the doc.

tfeher · 2026-10-14T21:03:38Z

+namespace cuvs::neighbors::hnsw::detail {
+// Gather only selected device rows to a compact host matrix using bounded device staging.
+// Row IDs must be valid source rows; host_output must hold rows * row_bytes bytes.
+void gather_layered_hnsw_vectors(raft::resources const& res,


Is this HNSW specific? A general gather should ideally be located together with the other raft::matrix::gather methods.

Please document the arguments, specifically the expectations on size and whether they are device or host resident..

Good catch, thanks. I think we can use raft::matrix::gather. I think we should keep a HNSW-specific helper here to handle the batched transfers.

tfeher · 2026-10-14T21:04:35Z

+
+namespace cuvs::neighbors::hnsw::detail {
+namespace {
+__global__ void gather_rows(const uint8_t* source,


Couldn't we use an existing raft::matrix::gather for this?

tfeher · 2026-10-14T21:07:49Z


 namespace cuvs::neighbors::hnsw {

+enum class LayeredSource {


Could you describe what the different options mean? (what is regular, attached, explicit, standard?)

tfeher · 2026-10-14T21:12:03Z

 // need to be transferred to the search server, which typically has the dataset locally.
 //
-// This example demonstrates how to build a graph-only HNSW artifact with ACE:
+// Run with an optional backend (regular by default) and output directory.


What is a backend?

Is "build method" clearer?

tfeher · 2026-10-14T21:30:54Z

  }
 }

+// Reconstruct the graph-only artifact from a loaded index without retaining its source file.


What does it mean "without retaining its source file"?
Does a loaded index have a source file that we track?

The source file is closed and its path is not stored with the index. Meaning, the original graph can be deleted, moved, etc. I've added more details in the comment.

tfeher · 2026-10-14T22:08:46Z

+}
+
+template <typename T, typename CagraIndexT>
+auto serialize_to_layered_hnswlib(


The function names are somewhat confusing: here we not only serialize to a different format, but we actually build a hierarchy. Could you clarify in a docstring what this function does?

Right. I've renamed it to build_and_serialize_layered_hnsw and added a docstring. Is that clearer?

…d source

Add general HNSW GRAPH_ONLY support

00ff546

julianmi requested review from a team as code owners September 23, 2026 09:09

julianmi self-assigned this Sep 23, 2026

julianmi added improvement Improves an existing functionality non-breaking Introduces a non-breaking change labels Sep 23, 2026

julianmi added 3 commits September 23, 2026 14:47

Get the stream from the reference

31bcbd5

Merge branch 'main' into hnsw-graph-only

624f560

Merge branch 'main' into hnsw-graph-only

fbbb364

github-project-automation Bot added this to Unstructured Data Processing Oct 1, 2026

julianmi added 3 commits October 2, 2026 10:07

Merge branch 'main' into hnsw-graph-only

c35c85c

Merge branch 'main' into hnsw-graph-only

0c2d445

Merge branch 'main' into hnsw-graph-only

aceb742

coderabbitai Bot reviewed Oct 9, 2026

View reviewed changes

Comment thread cpp/src/neighbors/detail/hnsw.hpp

Comment thread cpp/src/neighbors/detail/hnsw.hpp

Comment thread cpp/src/neighbors/ivf_rabitq/gpu_index/ivf_gpu.cu Outdated

julianmi added 2 commits October 9, 2026 11:47

Document disk-backed HNSW graph-only conversion artifact precondition

270364d

Remove unrelated RaBitQ changes from HNSW graph-only proposal

cbf4dd9

tfeher requested changes Oct 14, 2026

View reviewed changes

julianmi added 12 commits October 15, 2026 16:16

Clarify GRAPH_ONLY build behavior and consolidate its usage example

79d4dae

Explain how disk-backed HNSW conversion selects its output directory

2336b04

Distinguish temporary HNSW output from persistent ACE build directories

103c236

Document HNSW gather residency sizes and transfer contract

128596d

Reuse RAFT matrix gather for HNSW device vector staging

303cda4

Explain the HNSW layered test source variants

016bf54

Describe graph build method choices in the HNSW example

d862e5e

Clarify serialization of loaded HNSW graphs without source files

0a2ad75

Name and document HNSW hierarchy construction before serialization

b36b15e

Clarify host vector requirements for disk-backed GRAPH_ONLY conversion

bf96e23

Correct the persistent graph artifact name in the HNSW example

1b9663d

Align the layered HNSW NVTX range with hierarchy construction

361b8ca

julianmi added 6 commits October 15, 2026 16:32

Describe artifact-backed HNSW serialization independently of its buil…

7eebd15

…d source

Merge upstream main after the CAGRA-ACE header refactor

fc2487f

Reuse RAFT transfer helpers in the layered HNSW gather

c2e8fe4

Share layered HNSW descriptor encoding between graph writers

ad46b4c

Share the float and quantized HNSW example load and search flow

3bbca2f

Merge remote-tracking branch 'upstream/main' into hnsw-graph-only

f8aef05

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