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Fix L2 expanded distance clamp for cutlass and legacy fused backend - #2841
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| Check name | Status | Explanation |
|---|---|---|
| Linked Issues check |  |
Issue [#2094] reports a flaky spectral clustering test and gives no additional coding acceptance criteria. The PR removes the tolerance-based clamp that can convert small positive expanded L2 distance… |
| Out of Scope Changes check | |
The changes in cpp/src/distance/detail/distance_ops/l2_exp.cuh and cpp/src/distance/detail/distance.cuh support the L2 distance behavior used by the flaky spectral clustering test in [#2094]. The … |
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No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0… |
| Title check | |
The title clearly and concisely describes the main change: fixing the L2 expanded-distance clamp in the Cutlass and legacy fused backends. |
| Description check | |
The description is directly related to the changeset. It explains the clamp behavior, the spectral clustering issue, the diagonal correction, affected backends, and validation results. |
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Review comments at @cpp/src/distance/detail/distance_ops/l2_exp.cuh:
- Around line 26-28: Update both expanded-L2 clamps in the distance computation
and accumulation path so they replace only negative values with zero and
preserve NaN values; locate the clamps by their outVal and acc[i][j]
assignments. Keep the existing square-root behavior unchanged.
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| template <typename OutT, typename AccT, typename FinOpT, typename IdxT> | ||
| RAFT_KERNEL set_self_distances_kernel(OutT* out, | ||
| IdxT diagonal_size, | ||
| IdxT leading_dimension, | ||
| FinOpT fin_op) | ||
| { | ||
| IdxT idx = static_cast<IdxT>(blockIdx.x) * blockDim.x + threadIdx.x; | ||
| if (idx < diagonal_size) { out[std::size_t(idx) * leading_dimension + idx] = fin_op(AccT{0}, 0); } | ||
| } |
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I was trying to use alternative primitives like raft::matrix::set_diagonal, but that requires a temporary allocation and copy, whereas we should just be able to write in place. I also looked into raft::map, raft::map_offset, and raft::unary_op but they require that we write to every element of out.
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Super small comment and a suggestion: should we also add a small regression through public brute_force::build/search for #1632? The new pairwise and top-1 tests cover the clamp, but they don’t exercise the brute-force paths in fused_l2_knn.cuh and knn_brute_force.cuh.
Close equal-norm points with known neighbor ordering, both L2 variants, and row-major searches with k=15 and k=65 should cover both paths. An unexpanded double reference would help keep the expected distances independent of the expanded calculation. What do you think?
| metric == DistanceType::L2Expanded ? T(2) * scale * scale : std::sqrt(T(2)) * scale; | ||
| for (int64_t i = 0; i < n; i++) { | ||
| for (int64_t j = 0; j < n; j++) { | ||
| EXPECT_NEAR(h_d[i * n + j], i == j ? T(0) : expected, T(1e-3) * expected) |
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Could we add a focused regression for the new exact-zero diagonal guarantee, using dense rows and EXPECT_EQ on the diagonal for both row-major and column-major self-comparisons? This test catches the original positive-distance collapse, but it doesn't specifically protect the new diagonal pass, right?
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Thanks for the review! I added all the new tests here f3aefcb
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Resolves #2094. Resolves #1632
This PR resolves the spectral clustering flaky test on ARM A100 cuda 12.2. It also solves the brute force returning wrong distances issue from #1632.
When root-causing the issue, I found that running:
our spectral embedding + sklearn kmeans works
our spectral embedding + our kmeans doens't work
It turns out that switching the backend of our kmeans from cutlass to the unfused path worked fine. The reason is that the cutlass path clamps distances to zero when the distances are below a certain threshold and the norms are equal. This can cause genuine small positive distances to be zero and in our case cause all points to be assigned to one cluster during spectral clustering.
We should remove the clamping and only clamp if the distance is negative (since L2 distance can't be negative). This is consistent with how the other backends work. For instance, unfused and the new cutile 1nn path.
I was able to reproduce locally on ARM A100 cuda 12.2 with and without this PR:
Current main: fails around 5-8% of the time
With this fix: passes 1000/1000 times.
This PR achieves perfect ARI score of 1 for all the gtests. This wasn't generally the case before.
Summary of changes
Remove equal-norm based clamping
For easily known self-neighbors like same X and Y inputs for pairwise_distances, set the diagonal to zero.
Add regression test (from flaky SpectralClustering scenario) from Make the L2 expanded self-distance clamp relative to the norms #2794
When comparing self-neighbors (same vector against same vector) the distance will not be zero (because we are no longer clamping in this PR). Doing the sqrt on a small number will increase the error. We can loosen the threshold on the go test. We still expect the correct indices to be returned.
However, because we aren't aggressively clamping the distances, we can expect improved relative ordering between very close neighbors, which solves [BUG] Brute Force returns wrong distances #1632.
The recommendation should be if users absolutely need self-neighbors to be zero, then they may run a processing step themselves to refine the distances from the set of returned neighbors (i.e. set same neighbors to be zero).
The changes in this PR affects the L2SqrtExpanded metric and codepaths for Kmeans (cutlass & legacy fused backend only), and brute_force. Other indexes like CAGRA and IVF-PQ during search time use their own distance kernels and not this path.
Aligns the L2SqrtExpanded clamp to be the same as the other kmeans backends like cutile and unfused.