Select batched memcpy flags by copy size

[bdice]

Description

Closes #23674.

Use cudaMemcpyFlagPreferOverlapWithCompute for a cudaMemcpyBatchAsync batch only when every copy is 128 KiB or less. If any copy is larger, use cudaMemcpyFlagDefault for the entire batch.

The choice of 128 KiB was determined by benchmarking on several devices (GB300, GB10, RTX PRO A6000) and minimizing the time cost modeled with latency and bandwidth.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between bd37009 and baae178.

📒 Files selected for processing (2)

• cpp/include/cudf/detail/utilities/cuda_memcpy.hpp
• cpp/src/utilities/cuda_memcpy.cu

🚧 Files skipped from review as they are similar to previous changes (2)

• cpp/include/cudf/detail/utilities/cuda_memcpy.hpp
• cpp/src/utilities/cuda_memcpy.cu

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved asynchronous batch memory-copy behavior by selecting copy preferences based on transfer size.
  • Transfers up to 128 KiB prioritize overlap with computation, while batches containing larger transfers use the default CUDA behavior.
  • Improved consistency when processing batches containing transfers of different sizes.

• Documentation

  • Updated memory-copy documentation to accurately describe size-based batch behavior.
  • Updated copyright attribution.

Walkthrough

The batch copy implementation selects CUDA flags by transfer size. Copies up to 128 KiB prefer overlap with compute. Batches containing larger copies use default flags. The API documentation reflects this behavior.

Changes

CUDA batch copy flags

Layer / File(s)	Summary
Batch copy flag selection cpp/include/cudf/detail/utilities/cuda_memcpy.hpp, cpp/src/utilities/cuda_memcpy.cu	Added the 128 KiB threshold. Batch copies use cudaMemcpyFlagPreferOverlapWithCompute for smaller transfers and cudaMemcpyFlagDefault when any transfer exceeds the threshold. Updated the documentation and copyright attribution.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to baae1

This localized change selects memcpy flags based on copy size; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested labels: bug, libcudf, non-breaking

Suggested reviewers: matt711, qbacpey, pointkernel

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: selecting batched memcpy flags based on copy size.
Description check	✅ Passed	The description directly explains the threshold, flag selection, issue closure, benchmarking basis, and documentation and test updates.
Linked Issues check	✅ Passed	The implementation meets issue [#23674] by using overlap-preferred copying only up to 128 KiB and default flags for larger batches.
Out of Scope Changes check	✅ Passed	The code, documentation, and copyright updates support the linked issue and PR objectives without unrelated changes.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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Actionable comments posted: 1

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Inline comments:
In `@cpp/src/utilities/cuda_memcpy.cu`:
- Around line 95-102: Add direct CUDA 13+ tests for memcpy_batch_async using a
non-default stream, covering all-small, 128*1024, 128*1024+1, and mixed-size
batches; verify the mixed case uses cudaMemcpyFlagDefault. Add benchmarks
covering small-only and large-containing batches, measuring execution time
rather than asserting performance because the flag is only a CUDA hint.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between ba94204 and 375cda0.

📒 Files selected for processing (2)

• cpp/include/cudf/detail/utilities/cuda_memcpy.hpp
• cpp/src/utilities/cuda_memcpy.cu

Included review availability: Your plan includes up to 12 reviews per rolling hour; 11 remain after this review.

[vyasr]

Looks fine to me. The CI failure seems to either be a compiler bug but maybe there is some case where the host compiler sees this flag but not the device compiler? I'm not sure from a quick look unfortunately.

[bdice]

@vyasr It was a couple subtle things (variable unused in CUDA 12, narrowing conversion in CUDA 13). Both are fixed.

[mhaseeb123]

Approving with one optional simplification

[bdice]

/merge

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