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Summary

Sort CUDA strings with native PyTorch in eager and compiled execution, replacing cuDF; CPU keeps Arrow.

Description of the change

Strings are compared as integer words of seven bytes each, so stable integer sorts order them. An LSD radix sort orders every string by its first 21 bytes while tracking which strings tie there. Only longer strings that tie on that prefix are ranked by their remaining bytes, by block-pair prefix doubling. In eager mode, tie groups whose strings are byte-identical are skipped, and the sort returns early when no tied string is longer than the prefix. The sorted output is reordered with _permute_entries (#1271). Sorting is stable, with nulls last in both directions. Compiled code has no graph breaks and reuses one graph across input sizes.

Validation

One NVIDIA L4, PyTorch 2.14.1+cu130, cuDF 26.10.02, compile caches disabled. Median of 3 runs; timings are warm, include the sorted-output gather, and exclude compilation. "cuDF" is the previous CUDA path: cuDF argsort plus the same gather. All outputs match cuDF exactly. Inputs are nullable strings: random words up to 127 bytes, the same with one 50 kB string (skew), 50 categories, URLs, a 300-byte shared prefix, 20 distinct 1 kB strings, 1–3 byte strings, and 200 strings sharing a 50 kB string's prefix (outlier ties).

Case	Strings	cuDF, ms	Native eager, ms	Native compiled, ms	Peak MiB: cuDF / eager / compiled
Words	40k	2.52	1.95	0.54	6.5 / 6.1 / 3.1
Words	200k	3.22	2.12	0.71	32.6 / 28.4 / 15.3
Skew	40k	2.50	1.93	0.55	7.1 / 6.7 / 3.1
Skew	200k	3.23	2.10	0.71	34.1 / 29.3 / 15.3
Categories	40k	3.70	1.93	0.53	6.6 / 6.2 / 3.1
Categories	200k	4.67	2.17	0.68	34.0 / 29.3 / 15.3
URLs	40k	14.69	6.84	5.02	27.4 / 19.2 / 24.5
URLs	200k	24.54	7.27	7.44	137.0 / 89.7 / 124.9
Shared prefix	40k	88.52	9.75	5.51	147.6 / 126.7 / 88.7
Shared prefix	200k	167.87	67.69	28.82	740.0 / 633.6 / 445.3
1 kB duplicates	40k	129.01	30.20	16.50	472.0 / 206.9 / 260.8
1 kB duplicates	200k	226.98	147.97	106.25	2361.7 / 1037.0 / 1309.1
Tiny	40k	2.33	0.92	0.53	2.2 / 1.9 / 3.1
Tiny	200k	2.88	0.94	0.64	10.8 / 9.7 / 15.3
Outlier ties	40k	1088.16	14.15	5.37	61.6 / 50.9 / 43.5
Outlier ties	200k	1102.28	15.55	5.69	87.9 / 59.1 / 99.8

Compiled sorting does not see the length bound, so it sizes work from the column's byte count, while eager sorting (used at fit) does.

cuDF peaks combine its Torch allocations and its RMM peak. On CPU, native sorting was 0.1–1.0× Arrow on the same inputs, so CPU keeps Arrow.

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Codecov Report

❌ Patch coverage is 93.08176% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.60%. Comparing base (fc82e92) to head (616f168).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sdm/tensor/_string_sort.py	93.33%	10 Missing ⚠️
sdm/tensor/string.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@                               Coverage Diff                               @@
##           compile/kumotabular-native-string-selection    #1272      +/-   ##
===============================================================================
+ Coverage                                        82.32%   82.60%   +0.27%     
===============================================================================
  Files                                              177      178       +1     
  Lines                                            11586    11794     +208     
===============================================================================
+ Hits                                              9538     9742     +204     
- Misses                                            2048     2052       +4     

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📝 Summary

Summary by CodeRabbit

• Improvements
  • String sorting on CUDA now supports compiled execution when string lengths are known, while other inputs continue to use existing sorting paths.
  • Sorting preserves input order for equal strings and null values, places nulls last, and handles UTF-8 strings and empty strings.
• Tests
  • Added coverage for sorting results and permutation indices across native and PyTorch execution paths, including compiled execution and repeated views.

Adds a native stable sorting path for compiled CUDA string tensors when their maximum length is known. Tests cover sort order, null placement, stable ties, and permutation indices.

Changes: Compiled CUDA string sorting

Layer / File(s)	Summary
Native sort dispatch and validation sdm/tensor/string.py, test/tensor/test_string.py	StringTensor._sort dispatches eligible inputs to _sort_native. The native path packs strings into words, uses stable per-word sorts, and selects entries. CUDA tests check ascending and descending order, UTF-8 byte ordering, nulls last, stable ties, and edge cases.

Priority: Normal

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

Sequence Diagram(s)

sequenceDiagram
  participant StringTensor._sort
  participant StringTensor._sort_native
  participant pack
  participant _select_entries
  StringTensor._sort->>StringTensor._sort_native: Dispatch eligible compiled CUDA input
  StringTensor._sort_native->>pack: Pack strings into words
  pack-->>StringTensor._sort_native: Return packed words
  StringTensor._sort_native->>_select_entries: Select entries using sorted permutation
  _select_entries-->>StringTensor._sort_native: Return sorted entries
  StringTensor._sort_native-->>StringTensor._sort: Return entries and permutation

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Merge Risk: Moderate · up to ef826

Compiled CUDA string sorting can run out of GPU memory when a tensor has one very long string and many short or empty ones, because memory use follows the maximum length bound times the row count. Typical bounded workloads benefit from the faster path. Consider adding a work-based guard or fallback before merge, or explicitly accept the limitation.

Pre-merge checks: 4

Check name	Status	Explanation
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.
Title check		The title accurately identifies the main CUDA string-sorting change. It is somewhat broader because eager CUDA and fallback paths still use cuDF, but it remains clearly related to the changeset.
Description check		The pull request description directly describes the native CUDA string sorting changes, stable ordering, null handling, compilation behavior, fallbacks, and validation results. It matches the changese…

Full details: Description check

Explanation

The pull request description directly describes the native CUDA string sorting changes, stable ordering, null handling, compilation behavior, fallbacks, and validation results. It matches the changeset.

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Inline comments:
Review comments at @sdm/tensor/string.py:
- Around line 528-533: Update the native-sort selection in the condition leading
to _sort_native so a known maximum string length alone does not select a
memory-intensive path; account for estimated aggregate work or skewed string
lengths and use a bounded alternative when dense intermediates would require
excessive allocation.

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• sdm/tensor/string.py
• test/tensor/test_string.py

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