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Fix off-by-one num_segments in matrix::sort_cols_per_row #3010

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Fix off-by-one num_segments in matrix::sort_cols_per_row #3010

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15 changes: 10 additions & 5 deletions cpp/include/raft/matrix/detail/columnWiseSort.cuh
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Original file line number Diff line number Diff line change
Expand Up @@ -220,7 +220,12 @@ void sortColumnsPerRow(const InType* in,
// device Segmented radix sort
// 2^18 column cap to restrict size of workspace ~512 MB
// will give better perf than below deviceWide Sort for even larger dims
int numSegments = n_rows + 1;

// n_rows CSR-style segments aliased onto an n_rows + 1 offsets array,
// with begin = &offsets[0] and end = &offsets[1]. cub's `num_segments`
// is the actual segment count (n_rows).
int numSegments = n_rows;
int numOffsets = n_rows + 1;

// need auxiliary storage: cub sorting + keys (if user not passing) +
// staging for values out + segment partition
Expand All @@ -247,8 +252,8 @@ void sortColumnsPerRow(const InType* in,
// value in KV pair need to be passed in, out buffer is separate
workspaceSize += raft::alignTo(sizeof(OutType) * (size_t)totalElements, memAlignWidth);

// for segment offsets
workspaceSize += raft::alignTo(sizeof(int) * (size_t)numSegments, memAlignWidth);
// for segment offsets (numOffsets = numSegments + 1, see above)
workspaceSize += raft::alignTo(sizeof(int) * (size_t)numOffsets, memAlignWidth);
} else {
size_t workspaceOffset = 0;

Expand All @@ -263,14 +268,14 @@ void sortColumnsPerRow(const InType* in,
workspacePtr = (void*)((size_t)workspacePtr + workspaceOffset);

int* dSegmentOffsets = reinterpret_cast<int*>(workspacePtr);
workspaceOffset = raft::alignTo(sizeof(int) * (size_t)numSegments, memAlignWidth);
workspaceOffset = raft::alignTo(sizeof(int) * (size_t)numOffsets, memAlignWidth);
workspacePtr = (void*)((size_t)workspacePtr + workspaceOffset);

// layout idx
RAFT_CUDA_TRY(layoutIdx(dValuesIn, n_rows, n_columns, stream));

// layout segment lengths - spread out column length
RAFT_CUDA_TRY(layoutSortOffset(dSegmentOffsets, n_columns, numSegments, stream));
RAFT_CUDA_TRY(layoutSortOffset(dSegmentOffsets, n_columns, numOffsets, stream));

RAFT_CUDA_TRY(cub::DeviceSegmentedRadixSort::SortPairs(workspacePtr,
workspaceSize,
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