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K-selection Determinism: break a k-selection tie - #3158
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📝 Summary
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| Check name | Status | Explanation | Resolution |
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| Docstring Coverage |  |
Docstring coverage is 44.44% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files. (1 skipped: 1… | Write docstrings for the functions missing them to satisfy the coverage threshold. |
✅ Passed checks (4 passed)
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Check name Status Explanation Description check The description clearly explains the deterministic k-selection goal and the payload-index tie-breaking change. Title check The title clearly identifies the main change: breaking ties in k-selection to improve determinism. 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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Docstring coverage is 44.44% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files. (1 skipped: 1 unsupported.)
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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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The tie break was a run-time flag. Every warp-sort kernel then held both comparisons in its unrolled bitonic networks, and shuffled the index on each step even with the flag off. The kernels of every select_k caller grew, and the default path did extra work. - `bitonic` and `warp_sort` take a `TieBreak` template parameter, false by default. With false, the generated code is the same as before the change. - `warp_sort_distributed_ext` takes the same parameter. The new class `warp_sort_distributed_ext_stable` sets it to true. - `SelectAlgo::kWarpDistributedShmStable` uses that class, for float and half keys only. Other key types fail with an error, so their instantiations do not grow. - The `tie_break` arguments of the kernels and of `select_k_impl` are removed.
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Inline comments:
Review comments at @cpp/include/raft/matrix/detail/select_warpsort.cuh:
- Around line 622-637: Update the threshold filtering used by
warp_sort_distributed_ext_stable so it tracks the threshold entry’s index
alongside k_th_ and uses four-argument is_ordered when TieBreak is enabled.
Define the sentinel index ordering so inserted kDummy entries cannot outrank
real results when their keys compare equal, including ascending twiddled
NaN-like keys.
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`SelectAlgo::kWarpDistributedShmStable` must return the same elements for the
same input, whatever the input order. Two defects broke that.
`warp_sort_distributed_ext::add` filtered a candidate on its key alone. When
the queue was full, it rejected a candidate whose key equalled the k-th key,
even when its index should win the tie, so the selected set depended on the
input order. With `TieBreak`, the queue now keeps the index of the k-th element,
`k_th_idx_`, and the filter compares (key, index) with
`is_ordered<Ascending>(val, k_th_, idx, k_th_idx_)`.
An empty slot carried index 0, so a real key that twiddles to the same bits as
`kDummy` could lose the tie to an empty slot. An empty slot now carries
`kDummyIdx`, which loses every tie: the largest `IdxT` for an ascending
selection and the smallest for a descending one. The queue start-up, the buffer
reset and the padding in `block_kernel` use it. `block_kernel` trims `k` to
`len`, so an empty slot never reaches the output.
Without `TieBreak` the generated code does not change: every new step is under
`if constexpr (TieBreak)`, and `kDummyIdx` is `IdxT{}`.
`cpp/tests/matrix/select_k_stable.cu` compares the selector with a CPU sort by
key, then by index, for two input orders of the same pairs, and checks that
real keys equal to the empty key are selected.
I think this should be fixed |
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Inline comments:
Review comments at @cpp/tests/matrix/select_k_stable.cu:
- Line 72: Remove the sorting of `out` and the corresponding sorting of
`expected_ids` in the stable selection test, then compare the returned ID
sequence directly with the key-and-ID reference order so differing output orders
are detected.
- Line 147: Update the stable_select call in the test to set k to ids.size(), so
it requests no more outputs than there are inputs while preserving the existing
output capacity.
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| std::vector<int64_t> out(k); | ||
| raft::copy(out.data(), out_ids.data_handle(), k, stream); | ||
| raft::resource::sync_stream(handle, stream); | ||
| std::sort(out.begin(), out.end()); |
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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win
Compare the returned ID order.
When equal keys select the same IDs in different orders, std::sort(out) makes both runs pass. The test therefore misses an output-order difference in the new deterministic selection mode. Remove this sort and the corresponding sort in expected_ids, then compare the returned sequence with the key-and-ID reference order. (raw.githubusercontent.com)
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Review comment at @cpp/tests/matrix/select_k_stable.cu at line 72:
Remove the sorting of `out` and the corresponding sorting of `expected_ids` in
the stable selection test, then compare the returned ID sequence directly with
the key-and-ID reference order so differing output orders are detected.
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| for (bool select_min : {true, false}) { | ||
| float key = from_bits(select_min ? 0x7FFFFFFFu : 0xFFFFFFFFu); | ||
| std::vector<float> keys(ids.size(), key); | ||
| auto got = stable_select(handle, keys, ids, 8, select_min); |
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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win
Request no more outputs than inputs.
This call requests eight outputs from five inputs. The kernel trims its write count to five, but stable_select copies and sorts all eight output positions. Set k to ids.size(). A capacity of eight still creates internal empty slots without reading unwritten output. (raw.githubusercontent.com)
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Review comment at @cpp/tests/matrix/select_k_stable.cu at line 147:
Update the stable_select call in the test to set k to ids.size(), so it requests
no more outputs than there are inputs while preserving the existing output
capacity.
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The PR carefully avoids changing the existing logic, so don't worry about the impact on performance (existing algorithms stay unaffected).
LGTM except one small thing.
| uint32_t buf_len_; // 0 <= buf_len_ < WarpSize | ||
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| T k_th_; | ||
| IdxT k_th_idx_; // read only with TieBreak |
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I'm not sure if the compiler will notice this member is unused and not allocate registers for it.
Maybe, in the spirit of the rest of this PR, guard it by the TieBreak too?
| IdxT k_th_idx_; // read only with TieBreak | |
| [[no_unique_address]] std::conditional_t<TieBreak, | |
| IdxT, | |
| cuda::std::monostate> k_th_idx_; |
Without TieBreak, k_th_idx_ is a cuda::std::monostate marked [[no_unique_address]], so the default selectors keep no extra state.
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LGTM, thanks for the updates!
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NVIDIA/raft#3158 added 18 new instantiations of select-k kernels and this grows the cuvs binary size by 6 MB Authors: - Artem M. Chirkin (https://github.com/achirkin) Approvers: - James Lamb (https://github.com/jameslamb) URL: #2942
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.