Reserve the memory into_packed_data() allocates

[madsbk]

When a table_chunk is not already packed, into_packed_data() calls cudf::pack() straight into br->device_mr(). The allocation is roughly the size of the table and nothing reserved it, so the memory system under-counts and backpressure cannot see it coming.

It now takes a MemoryReservation instead of a BufferResource, and into_packed_data_cost() reports what it will allocate. That is zero when the chunk is already packed, since the packed data is then moved out rather than serialized, so the sole caller reserves the right amount on every path rather than over-reserving on the spilled one.

Both non-packed constructors already compute cudf::packed_size() into data_alloc_size_, so the cost needs no extra work at call time.

Breaking change

into_packed_data() takes a MemoryReservation rather than a BufferResource, in C++ and in Python. The BufferResource overload is gone rather than kept alongside, since every other allocating method on table_chunk already takes a reservation and leaving it would leave a blocking path that is easier to reach than the correct one.

Callers pair it with into_packed_data_cost():

chunk, extra = await make_table_chunks_available_or_wait(
    context, chunk, reserve_extra=chunk.into_packed_data_cost(), net_memory_delta=0
)
packed = chunk.into_packed_data(extra)

Note

The cudf::chunked_pack() TODO on the same line is still open. A bounce buffer would bound the allocation instead of needing room for a whole second copy.

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• cpp/libcudf_streaming/tests/streaming/test_table_chunk.cpp

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📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Added a method to report the device memory required to pack a table chunk.
  • Already-packed chunks now report zero additional memory requirements.
• API Updates
  • Packing now requires an explicit device-memory reservation.
  • Insufficient reservations are rejected with a documented error.
  • Updated C++ and Python interfaces to support reservation-based packing.
• Bug Fixes
  • Packing allocations are now tracked against reserved device memory, improving memory accounting and reliability.

Walkthrough

table_chunk packing now reports its device-memory cost and requires a MemoryReservation. C++ and Python APIs, the allgather path, and tests now use reservation-tracked packing.

Changes

Table chunk packing

Layer / File(s)	Summary
C++ packing contract and validation cpp/libcudf_streaming/include/cudf_streaming/table_chunk.hpp, cpp/libcudf_streaming/src/table_chunk.cpp, cpp/libcudf_streaming/tests/streaming/test_table_chunk.cpp	The C++ API reports packing cost, consumes a device-memory reservation, and tests exact consumption and insufficient capacity.
Python binding and reservation bridge python/cudf_streaming/cudf_streaming/table_chunk.pxd, python/cudf_streaming/cudf_streaming/table_chunk.pyi, python/cudf_streaming/cudf_streaming/table_chunk.pyx, python/cudf_streaming/cudf_streaming/tests/test_table_chunk.py	The Python API exposes into_packed_data_cost() and passes MemoryReservation directly to C++. Tests reserve the reported cost.
Allgather packing reservation python/cudf_polars/cudf_polars/streaming/actor_graph/collectives/allgather.py	Inserter.insert reserves the chunk packing cost before conversion.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to d87cc

The change adds explicit memory reservation for table packing and updates the represented C++ and Python callers. No actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: bdice, vyasr

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 5.88% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: memory allocated by into_packed_data() is now reserved.
Description check	✅ Passed	The description accurately explains the reservation API change, the new cost method, the breaking change, and the remaining TODO.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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🧪 Generate unit tests (beta)

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