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Return a sentinel for unfilled HNSW search slots - #3025
hnsw::search returns made-up neighbors when a query reaches fewer than k elements.
hnswlib's searchKnn returns however many results it found, but get_search_knn_results pops k times. Popping an empty queue is undefined behavior. Debug builds stop at the assert. Release builds return stale entries as if they were results, and the real results end up at the end of the row.
Found results stay at the start of the row, nearest first. Remaining slots get std::numeric_limits<uint64_t>::max() as the id and std::numeric_limits<float>::max() as the distance. The C++, C and Python search docs now say so.
Why a sentinel and not an error
It matches the rest of cuVS. ivf_pq::kOutOfBoundsRecord is std::numeric_limits<IdxT>::max() and is documented as what search returns when it cannot fill top-k. IVF-Flat uses the same value through ivf_common.cuh. CAGRA reports FLT_MAX for unfilled slots, and the Java bindings rely on that distance to detect them.
A short result is not always a caller error. It can happen with k within the index size, for example with deleted elements in a loaded index, or with parts of a graph that the search cannot reach. HNSWShortResults.FewerReachableThanK covers the deleted case. So an up-front k <= size check, as IVF-PQ also has, would not replace this, and I did not add one.
The shortfall is found per query inside an OpenMP parallel loop, where an exception escaping the region terminates the process. Stock hnswlib's Python binding raises because it collects errors across its own thread pool, and because it has to return a full 2D array.
The sentinel changes what callers see only in cases that were already wrong. One follow-up, not changed here: Java's SearchResultsImpl, which HNSW uses, does not skip these slots the way CagraSearchResults does. With the default identity mapping they collapse into one map entry with key -1 and distance FLT_MAX, and a caller-supplied mapping receives -1.
Tests
New cpp/tests/neighbors/hnsw_short_results.cpp, added to NEIGHBORS_HNSW_TEST. It writes an index with stock hnswlib, loads it with hnsw::deserialize and searches with hnsw::search, so it needs no GPU. It is a separate .cpp file because it includes hnswlib, which nvcc warns on and the tests build with -Werror=all-warnings.
Results are from GH200 (aarch64), OMP_NUM_THREADS=4.
How this was built
A full libcuvs build was not run. cpp/src/neighbors/hnsw.cpp and the new test were compiled from this branch against hnswlib 0.7.0 with the repo's patch, and linked ahead of nightly libcuvs 26.12.00a198, which is the parent of the base commit. The base commit (#3006) changes none of the files compiled here. Both compiled with the repo's GCC warning flags (-Wall -Werror). The existing hnsw.cu compiled unchanged with -Werror=all-warnings. clang-format 20.1.8 with the pre-commit arguments reports no changes on the changed C and C++ files.
Not done: x86 and Clang builds, the CMake test target itself, the Python and Java bindings, and CI.
GPU, on GH200: linked ahead of the same nightly, the AnnHNSW cases and the new tests pass (12 tests). Leading valid results for k above the index size, unpatched and then fixed: stock index k=12, n=8: 12, then 8. hnsw::from_cagra with hierarchy NONE and CPU, k=1016, n=1000: 1016, then 1000. Not run on B300.
hnswlib's searchKnn returns fewer than k results when it reaches fewer than k
elements, which happens with k above the index size and also with k within it
(deleted elements, parts of a graph that the search cannot reach). The copy
loop still popped k times. Popping an empty queue is undefined behavior: debug
builds stop at the assert, and release builds return stale ids as if they were
results, with the real results moved towards the end of the row.
Keep the found results at the start of the row, nearest first, and set the
remaining slots to the maximum id and distance. IVF search documents the same
id value for results it cannot fill, and CAGRA reports FLT_MAX for unfilled
slots. An exception is not an option here: the shortfall is found per query
inside an OpenMP region, and it is not always a caller error.
Signed-off-by: Dezhen Lu <dezhen.lu@student.uni-tuebingen.de>
HNSW searches that find fewer than the requested number of neighbors now return available results and fill remaining slots with maximum-value sentinels, instead of failing.
Documentation
Clarified the behavior of unfilled neighbor and distance slots across the C, C++, and Python interfaces.
Tests
Added coverage for short results, including searches with deleted or limited neighbors.
HNSW search now fills remaining neighbor and distance slots with maximum-value sentinels when fewer than k results are available. C, C++, and Python documentation describes these outputs. New tests cover short-result cases and a full-result case.
Changes: HNSW short-result handling
Layer / File(s)
Summary
Handle and document short results cpp/src/neighbors/detail/hnsw.hpp, c/include/cuvs/neighbors/hnsw.h, cpp/include/cuvs/neighbors/hnsw.hpp, python/cuvs/cuvs/neighbors/hnsw/hnsw.pyx
get_search_knn_results fills remaining neighbor IDs and distances with their maximum values when fewer than k results are returned. The search documentation describes these sentinel values.
Test short and full results cpp/tests/neighbors/hnsw_short_results.cpp, cpp/tests/CMakeLists.txt
New tests compare search results with exact L2 results across short-result cases, deleted rows, and a full-result case. The test target now includes the new test file.
No actionable code issue remains from this review. Complete the normal build and test checks before merging.
Security architecture risk: Low · up to ceb1b
The change makes short search results deterministic without adding an entrypoint or increasing caller authority. Callers must recognize sentinel entries before using neighbor IDs as dataset indices; compatibility with external consumers has not been verified.
Retained concerns
No architecture-level concerns identified.
Security review details
Security Blast Radius
inferred — The directly affected scope is the output buffers of existing HNSW search callers when fewer than k neighbors are found. Propagation into other services or privileged consumers is not established by the inspected evidence.
Trust Boundaries and Controls
observed — The Python path retains its trained-index check, query-type checks, output type and shape checks, and existing C API dispatch. Its change is documentation only, not a new route around those controls.
Resilience and Maintainability Implications
inferred — Copying only found results removes the previous empty-queue access on short searches and avoids introducing a shortfall exception inside the parallel region. This improves failure containment without changing index ownership.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
Check name
Status
Explanation
Resolution
Docstring Coverage
Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
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 describes the sentinel values for unfilled HNSW search slots.
Description check
The description explains the HNSW short-result fix, documentation changes, tests, and test limitations.
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 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 2 functions across 4 files. (2 skipped: 2 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
Finishing Touches
Generate unit tests (beta)
Create a new PR
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Failed pre-merge checks:
- Docstring Coverage:
Write docstrings for the functions missing them to satisfy the coverage
threshold.
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Reviewing files that changed from the base of the PR and between b3be92a and ceb1b16.
Files selected for processing: 6
c/include/cuvs/neighbors/hnsw.h
cpp/include/cuvs/neighbors/hnsw.hpp
cpp/src/neighbors/detail/hnsw.hpp
cpp/tests/CMakeLists.txt
cpp/tests/neighbors/hnsw_short_results.cpp
python/cuvs/cuvs/neighbors/hnsw/hnsw.pyx
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What
hnsw::searchreturns made-up neighbors when a query reaches fewer than k elements.hnswlib's
searchKnnreturns however many results it found, butget_search_knn_resultspops k times. Popping an empty queue is undefined behavior. Debug builds stop at theassert. Release builds return stale entries as if they were results, and the real results end up at the end of the row.Index of 8 elements, k = 12:
1 1 1 1 1 6 5 3 4 0 2 770.9 70.9 70.9 70.9 70.9 133.9 151.1 180.7 210.3 264.8 428.9 464.61 6 5 3 4 0 2 7then 4 ×UINT64_MAX70.9 133.9 151.1 180.7 210.3 264.8 428.9 464.6then 4 ×FLT_MAXFix
Found results stay at the start of the row, nearest first. Remaining slots get
std::numeric_limits<uint64_t>::max()as the id andstd::numeric_limits<float>::max()as the distance. The C++, C and Python search docs now say so.Why a sentinel and not an error
ivf_pq::kOutOfBoundsRecordisstd::numeric_limits<IdxT>::max()and is documented as whatsearchreturns when it cannot fill top-k. IVF-Flat uses the same value throughivf_common.cuh. CAGRA reportsFLT_MAXfor unfilled slots, and the Java bindings rely on that distance to detect them.HNSWShortResults.FewerReachableThanKcovers the deleted case. So an up-frontk <= sizecheck, as IVF-PQ also has, would not replace this, and I did not add one.The sentinel changes what callers see only in cases that were already wrong. One follow-up, not changed here: Java's
SearchResultsImpl, which HNSW uses, does not skip these slots the wayCagraSearchResultsdoes. With the default identity mapping they collapse into one map entry with key-1and distanceFLT_MAX, and a caller-supplied mapping receives-1.Tests
New
cpp/tests/neighbors/hnsw_short_results.cpp, added toNEIGHBORS_HNSW_TEST. It writes an index with stock hnswlib, loads it withhnsw::deserializeand searches withhnsw::search, so it needs no GPU. It is a separate.cppfile because it includes hnswlib, whichnvccwarns on and the tests build with-Werror=all-warnings.KLargerThanIndexKLargerThanIndexBaseLayerOnlyFewerReachableThanK(k = 6, 8 elements, 4 deleted)FullResultsAreUnchanged(k = 8, 8 elements)Results are from GH200 (aarch64),
OMP_NUM_THREADS=4.How this was built
A full libcuvs build was not run.
cpp/src/neighbors/hnsw.cppand the new test were compiled from this branch against hnswlib 0.7.0 with the repo's patch, and linked ahead of nightly libcuvs 26.12.00a198, which is the parent of the base commit. The base commit (#3006) changes none of the files compiled here. Both compiled with the repo's GCC warning flags (-Wall -Werror). The existinghnsw.cucompiled unchanged with-Werror=all-warnings.clang-format20.1.8 with the pre-commit arguments reports no changes on the changed C and C++ files.Not done: x86 and Clang builds, the CMake test target itself, the Python and Java bindings, and CI.
GPU, on GH200: linked ahead of the same nightly, the
AnnHNSWcases and the new tests pass (12 tests). Leading valid results for k above the index size, unpatched and then fixed: stock index k=12, n=8: 12, then 8.hnsw::from_cagrawith hierarchy NONE and CPU, k=1016, n=1000: 1016, then 1000. Not run on B300.