K-selection Determinism: break a k-selection tie

[mnorris11]

We have a GPU use case that requires full determinism between runs given the same input. I.e. when provided a 100M dataset with 5M queries (made up numbers), we need it to return the same results if we run it again with same vectors. I will also open one in cuVS.

RAFT PR: #3158
cuVS PR: NVIDIA/cuvs#2696
Faiss PR: facebookresearch/faiss#5682

Description: The bitonic sort compares keys only. Lane arrival therefore decides which of two equidistant candidates survives k-selection. Two runs over the same data can return different neighbours. This change breaks an equal-key tie on the payload index. The tie direction follows the sort direction, as cmp2 does in the faiss heaps.

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Reviewing files that changed from the base of the PR and between fd6eb8c and bac82df.

📒 Files selected for processing (4)

• cpp/include/raft/matrix/detail/select_k-inl.cuh
• cpp/include/raft/matrix/detail/select_warpsort.cuh
• cpp/include/raft/matrix/select_k_types.hpp
• cpp/include/raft/util/bitonic_sort.cuh

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📝 Summary

Summary by CodeRabbit

• New Features
  • Added an optional stable selection mode that breaks ties between equal keys using payload indices. Tie order follows the selected ascending or descending direction.
  • Tie-breaking remains disabled by default; existing sorting behavior is unchanged unless enabled.

Walkthrough

Bitonic sort and warp-sort can use payload values or indices to break equal-key ties. The tie-break direction follows ascending or descending order. Selection adds a stable distributed shared-memory algorithm option. Tie-breaking remains disabled by default.

Changes

Equal-key ordering

Layer / File(s)	Summary
Bitonic payload tie-break cpp/include/raft/util/bitonic_sort.cuh	Bitonic sort can use the first payload to break equal-key ties in merge and warp-level comparisons. The constructor defaults tie-breaking to false.
Warp-sort index tie-break cpp/include/raft/matrix/detail/select_warpsort.cuh	Warp-sort queues can compare indices for equal keys. The tie-break option passes through queue sorts, merges, block kernels, and both selection passes. select_k and select_k_impl default the option to false.
Stable selection option cpp/include/raft/matrix/select_k_types.hpp, cpp/include/raft/matrix/detail/select_k-inl.cuh	SelectAlgo adds kWarpDistributedShmStable. Its dispatch calls distributed shared-memory warp-sort with stable ordering enabled.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: achirkin, bdice

Merge Risk: 🔵 Low · up to bac82

The change adds an opt-in stable k-selection mode and leaves default behavior unchanged. It looks mergeable, but the equal-key tie-breaking paths deserve owner attention because the supplied context contains no tests for them.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (3 skipped: 3 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly describes the main change: deterministic k-selection through equal-key tie breaking.
Description check	✅ Passed	The description explains the nondeterminism problem, the payload-index tie-breaking approach, and the intended deterministic behavior. It is directly related to the changeset.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (3 skipped: 3 unsupported.)

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Actionable comments posted: 3

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Inline comments:
Review comments at @cpp/include/raft/matrix/detail/select_warpsort.cuh:
- Around line 118-119: Update the threshold checks in warp_sort_filtered::add,
warp_sort_distributed::add, and warp_sort_distributed_ext::add to compare each
candidate’s key and index against the threshold key and index using the same
ascending/descending tie-break ordering. Preserve key/payload association across
all three sorting paths.
- Around line 118-119: Update the tie-breaking comparison in the select_warpsort
comparator so real entries sort before dummy slots when keys tie, including when
a real key equals kDummy. Track dummy validity separately if needed, and
preserve the association between each real key and its payload index.

Review comments at @cpp/include/raft/util/bitonic_sort.cuh:
- Line 245: Update the comparison used by merge so equal keys retain the
documented key-only behavior and key-sorted halves remain a valid precondition;
do not apply the new payload tie-break there. Preserve the tie-break behavior in
sorting paths where it is required, and keep key/payload associations intact.

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📒 Files selected for processing (2)

• cpp/include/raft/matrix/detail/select_warpsort.cuh
• cpp/include/raft/util/bitonic_sort.cuh

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[cjnolet]

/ok to test bac82df

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/ok to test bac82df

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