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CAGRA-ACE: Add Augmentation Control - #2698

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CAGRA-ACE augments every core partition with the second closest neighboring vectors. This patch proposes to add control of the augmentation to balance graph quality and build times.

The build parameter target_alpha defines the augmentation budget as the expected number of spill copies per input vector. ACE derives a boundary-margin threshold from the dataset and duplicates likely cross-partition vectors until the requested ratio is reached.

  • alpha = 0.5 targets one extra copy per two vectors.
  • alpha = 1.0 (default) matches the existing 2× augmented-dataset budget.
  • alpha = 0.0 disables augmentation.
  • ace_max_spills_per_vector controls the number of partitions each vector can spill to. This was set empirically to three. Higher values did not show benefits in graph quality in a sweep of the three datasets shown below.

The proposed boundary-margin threshold method with alpha = 1.0 shows slightly higher graph quality for the tested datasets. The build times are similar for BIGANN-100M and Wiki-10M but increase the build time by ~16% for OpenAI-5M. There seems to be no clear rule/heuristic on guiding the setting. Increase the augmentation budget to alpha = 2.0 does not match the quality achieved with CAGRA. Recovering CAGRA's graph quality seems to require merging which is costly on disk.

Strategy BIGANN-100M build (s) BIGANN-100M recall@10 BIGANN-100M QPS Wiki-10M build (s) Wiki-10M recall@10 Wiki-10M QPS OpenAI-5M build (s) OpenAI-5M recall@10 OpenAI-5M QPS
CAGRA 411.2 0.96948 22,207 73.2 0.98063 4,358 65.2 0.98604 2,768
Regular ACE (fixed 2x) 334.8 0.95904 22,294 178.3 0.96284 3,839 144.4 0.97855 2,548
alpha=0.25 228.5 0.94020 24,216 128.9 0.94291 3,849 100.6 0.96343 2,533
alpha=0.5 265.6 0.95127 24,032 145.8 0.95790 3,914 113.9 0.97256 2,537
alpha=0.7 292.8 0.95476 23,655 155.1 0.96786 3,891 132.0 0.97740 2,554
alpha=0.75 293.5 0.95516 24,378 158.0 0.96948 3,981 163.1 0.97825 2,546
alpha=1.0 326.6 0.95975 23,589 174.4 0.97474 3,966 167.3 0.98126 2,539
alpha=1.25 360.7 0.96298 24,710 189.0 0.97492 3,936 189.4 0.98300 2,557
alpha=1.5 400.4 0.96410 23,295 206.3 0.97688 3,961 206.3 0.98472 2,548
alpha=1.75 432.6 0.96541 24,692 231.1 0.97902 3,975 230.9 0.98549 2,544
alpha=2.0 473.5 0.96670 24,407 247.5 0.97987 3,975 250.7 0.98571 2,527

@julianmi
julianmi requested review from a team as code owners September 29, 2026 08:30
@julianmi julianmi self-assigned this Sep 29, 2026
@julianmi julianmi added improvement Improves an existing functionality non-breaking Introduces a non-breaking change labels Sep 29, 2026
@julianmi
julianmi requested review from a team as code owners October 2, 2026 08:07
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  • cpp/tests/neighbors/ann_hnsw_ace.cuh

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📝 Summary

Summary by CodeRabbit

  • New Features
    • Added the target_alpha setting for ACE graph builds across C++, C, Python, Java, and Rust APIs. It controls the target average number of extra partition copies per vector, defaults to 1.0, and can be set to 0 to disable augmentation. Values above 1 allow multiple spill partitions, up to three per vector; sampling may affect the realized count.
    • Added target_alpha support in CAGRA benchmark settings.
  • Documentation
    • Documented the finite valid range: 0 through min(3, N - 1), where N is the effective partition count.

ACE now exposes target_alpha across its C, C++, Java, Python, and Rust parameter APIs. CAGRA uses it to select boundary-gated spill partitions and updates memory estimates, mappings, and disk output for realized spill copies.

Changes: ACE spill augmentation

Layer / File(s) Summary
Expose target_alpha in ACE parameter APIs
c/include/cuvs/neighbors/*, cpp/include/cuvs/neighbors/cagra.hpp, java/cuvs-java/src/main/java/com/nvidia/cuvs/*AceParams.java, python/cuvs/cuvs/neighbors/*, rust/cuvs-sys/src/bindings.rs, fern/pages/*api/*
C, C++, Java, Python, and Rust parameter APIs add target_alpha, defaulting to 1.0. The documentation describes zero as disabling augmentation, the valid range, and the sampling-based difference between the target and realized spill count.
Forward target_alpha into ACE builds
c/src/neighbors/*, cpp/bench/ann/src/cuvs/cuvs_ann_bench_param_parser.h, cpp/src/neighbors/detail/hnsw.hpp, java/cuvs-java/src/main/java22/com/nvidia/cuvs/internal/*, python/cuvs/cuvs/neighbors/*
C and language bindings copy target_alpha into native build parameters. The benchmark parser reads ACE-prefixed configuration, and HNSW forwards the value to CAGRA.
Select and assign boundary-gated spill labels
cpp/src/neighbors/detail/cagra/cagra_build.cuh
CAGRA samples boundary-margin scores to set a threshold from target_alpha. It assigns each vector a core partition and up to three eligible spill partitions.
Size and process realized spill copies
cpp/src/neighbors/detail/cagra/cagra_build.cuh
Memory estimates account for expansion and spill-label slots. In-memory and disk paths map valid spill copies, reorder disk data, and size augmented storage from the realized spill count.
Validate parameter behavior and resource bounds
cpp/tests/neighbors/ann_cagra/test_float_uint32_t.cu, cpp/tests/CMakeLists.txt, cpp/tests/neighbors/ann_hnsw_ace.cuh, python/cuvs/cuvs/tests/test_*_ace.py
Tests cover parameter defaults, invalid values, in-memory and disk builds, search outputs, spill-count limits, and labeling memory calculations.

Priority: ➖ Normal

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

Change: Feature

Suggested reviewers: cjnolet, viclafargue, imotov

Merge Risk: 🔵 Low · up to 3bfb5

Previously compiled applications that directly allocate ACE parameter structs may encounter a layout incompatibility; creator-API users are unaffected, and rebuilding or migrating offers a bounded workaround. Resolve or explicitly accept this compatibility risk before merging.

Pre-merge checks: Passed 4 · Failed 1

Failed Failed checks (1 warning)

Check name Status Explanation Resolution
Docstring Coverage Warning Docstring coverage is 51.16% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 43 functions across 15 files. (1 skipped:… Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed Passed checks (4 passed)
Check name Status Explanation
Description check Passed The description clearly explains the new target_alpha parameter, its behavior, supported values, and the observed build-quality tradeoffs. It directly matches the changeset.
Title check Passed The title clearly identifies the primary change: adding augmentation control to CAGRA-ACE through the target_alpha parameter.
Linked Issues check Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check Passed Check skipped because no linked issues were found for this pull request.
Full details: Docstring Coverage

Explanation

Docstring coverage is 51.16% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 43 functions across 15 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 4

🧹 Nitpick comments (1)
python/cuvs/cuvs/tests/test_cagra_ace.py (1)

25-32: 🎯 Functional Correctness | 🔵 Trivial | ⚡ Quick win

Exercise target_alpha through ACE builds.

The current tests only verify constructor storage. Add build assertions for target_alpha=-1, float("nan"), and 3.5 to ensure cagra.build rejects invalid values, plus a target_alpha=0.0 case to verify that augmentation can be disabled. Mirror these cases in test_hnsw_ace.py; HNSW forwards target_alpha into the CAGRA ACE build path, so this can detect a forwarding regression that the round-trip test cannot.

This is recommended coverage, not an explicit project testing-rule violation.

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Review comment at @python/cuvs/cuvs/tests/test_cagra_ace.py around lines 25 -
32:
Extend test_ace_target_alpha_params to exercise target_alpha through
cagra.build, asserting rejection for -1, NaN, and 3.5 and successful build with
0.0 to verify augmentation can be disabled. Add equivalent build-path coverage
in test_hnsw_ace.py to verify HNSW forwards target_alpha.

  • 🪄 Fix CodeRabbit comments on this PR
🤖 Prompt to fix review comments
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @c/include/cuvs/neighbors/cagra.h:
- Around line 202-206: Document the ABI migration caused by adding target_alpha
to both cuvsAceParams and cuvsHnswAceParams. Add deprecation warnings stating
that callers must recompile with the current headers and that directly allocated
structs must use the complete current layout; recommend cuvsAceParamsCreate and
cuvsHnswAceParamsCreate to allocate and initialize it. Add matching guidance to
the corresponding Fern pages.
- Around line 202-206: Update the `target_alpha` documentation in the C header
and Fern C API page to state that the value must be finite and within [0, min(3,
N - 1)], where N is the effective partition count used by the build. Preserve
the existing description of zero and the default.

Review comments at @cpp/src/neighbors/detail/cagra/cagra_build.cuh:
- Line 318: Update the n_partitions guard and labeler so target_alpha == 0
bypasses augmentation-specific scoring and centroid-distance allocation, labels
only core partitions, and fills spill slots with the invalid sentinel. For
augmented builds, avoid the quadratic n_partitions-by-n_partitions allocation or
reject unsupported counts before disk-artifact creation, preserving supported
disk builds.
- Around line 343-353: Update ace_check_use_disk_mode and
ace_validate_disk_mode_partitions to include the live
n_partitions-by-n_partitions float centroid_distances_dev allocation as
non-scalable GPU overhead; re-evaluate this cost after any partition adjustment,
since increasing the partition count cannot reduce it.

---

Nitpick comments:
Review comments at @python/cuvs/cuvs/tests/test_cagra_ace.py:
- Around line 25-32: Extend test_ace_target_alpha_params to exercise
target_alpha through cagra.build, asserting rejection for -1, NaN, and 3.5 and
successful build with 0.0 to verify augmentation can be disabled. Add equivalent
build-path coverage in test_hnsw_ace.py to verify HNSW forwards target_alpha.

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📥 Commits

Reviewing files that changed from the base of the PR and between 24b0b17 and dcabdb4.

📒 Files selected for processing (26)
  • c/include/cuvs/neighbors/cagra.h
  • c/include/cuvs/neighbors/hnsw.h
  • c/src/neighbors/cagra.cpp
  • c/src/neighbors/hnsw.cpp
  • cpp/bench/ann/src/cuvs/cuvs_ann_bench_param_parser.h
  • cpp/include/cuvs/neighbors/cagra.hpp
  • cpp/src/neighbors/detail/cagra/cagra_build.cuh
  • cpp/src/neighbors/detail/hnsw.hpp
  • fern/pages/c_api/c-api-neighbors-cagra.md
  • fern/pages/c_api/c-api-neighbors-hnsw.md
  • fern/pages/cpp_api/cpp-api-neighbors-cagra.md
  • fern/pages/java_api/java-api-com-nvidia-cuvs-cuvsaceparams.md
  • fern/pages/java_api/java-api-com-nvidia-cuvs-hnswaceparams.md
  • fern/pages/python_api/python-api-neighbors-cagra.md
  • fern/pages/python_api/python-api-neighbors-hnsw.md
  • java/cuvs-java/src/main/java/com/nvidia/cuvs/CuVSAceParams.java
  • java/cuvs-java/src/main/java/com/nvidia/cuvs/HnswAceParams.java
  • java/cuvs-java/src/main/java22/com/nvidia/cuvs/internal/CagraIndexImpl.java
  • java/cuvs-java/src/main/java22/com/nvidia/cuvs/internal/HnswIndexImpl.java
  • python/cuvs/cuvs/neighbors/cagra/cagra.pxd
  • python/cuvs/cuvs/neighbors/cagra/cagra.pyx
  • python/cuvs/cuvs/neighbors/hnsw/hnsw.pxd
  • python/cuvs/cuvs/neighbors/hnsw/hnsw.pyx
  • python/cuvs/cuvs/tests/test_cagra_ace.py
  • python/cuvs/cuvs/tests/test_hnsw_ace.py
  • rust/cuvs-sys/src/bindings.rs

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Comment thread c/include/cuvs/neighbors/cagra.h
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