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Preserve row order for ordered multi-rank Union - #24685
aryansri05 wants to merge 8 commits into
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On multi-rank streaming engines, an order-preserving Union (pl.concat) forwarded each child's local chunks in channel order on every rank, so the global result interleaved children across ranks. When maintain_order is set and the union runs on more than one rank, assign every chunk a global position from the AllGathered per-rank local_count estimates, route chunks to their owning rank with one SparseAlltoall per child, and emit them in position order. Chunks beyond the estimate go to the owner of the last estimated chunk, which keeps destinations non-decreasing and so preserves order. Unordered, single-rank, and all-duplicated unions keep the existing pass-through. Closes NVIDIA#22376
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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| Docstring Coverage |  |
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Check name Status Explanation Title check The title "Preserve row order for ordered multi-rank Union" matches the main change: the PR makes order-preserving Union(pl.concatwithmaintain_order) keep child order across ranks. It is spec…Description check The description is directly related to the changeset. It explains the multi-rank ordering bug in Union/pl.concat, the AllGather and routing approach, the ID reservation for collective nodes, the d…Linked Issues check [ #22376] requires that multi-rankUnion/pl.concatundermaintain_orderemit all rows of child i before child i+1. At head 065a228,union.pyimplements this in_ordered_union, gated by `_need…Out of Scope Changes check The changes in common.pyaddUnionto the collective-ID reservation. This supports the ordered-union exchanges and does not add unrelated behavior. Theunion.pychanges implement the ordered rou…Full details: Title check
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Explanation
The title "Preserve row order for ordered multi-rank Union" matches the main change: the PR makes order-preserving
Union(pl.concatwithmaintain_order) keep child order across ranks. It is specific and concise, and it names the affected operation and condition.
Full details: Description check
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Explanation
The description is directly related to the changeset. It explains the multi-rank ordering bug in
Union/pl.concat, the AllGather and routing approach, the ID reservation for collective nodes, the decoupled read and emit tasks, and the test results. It matches the raw summary and the PR objectives.
Full details: Linked Issues check
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Explanation
[
#22376] requires that multi-rankUnion/pl.concatundermaintain_orderemit all rows of child i before child i+1. At head 065a228,union.pyimplements this in_ordered_union, gated by_needs_ordered_exchange(maintain_order, more than one child, more than one rank, not all children duplicated). It runs one AllGather of per-(child, rank)local_countvalues, builds an_OrderedUnionPlanthat assigns global positions withpartition_range/partition_owner, and routes chunks with oneSparseAlltoallper child (_exchange_child). Overflow chunks go to the owner of the last estimated chunk, so destinations stay non-decreasing. Duplicated children count and emit only on rank 0. Reading and exchanging run inread_and_exchange, separate fromemit, which emits children in order. Each exchange is finished in afinally. Thecommon.pydiff reserves one collective ID per child forUnion, with ID 0 shared by the AllGather and child 0's exchange. Unordered, single-rank, and all-duplicated unions use_passthrough_union. Tests cover routing, inaccurate estimates, empty and uneven children, duplicated children, collective IDs, theover()deadlock case, and the#22405reproducer, and the PR reports passing runs fortest_dataframescan_concaton all engines. The tail ofunion.py(the graph-generation wiring that passescollective_ids) was truncated in the read, so that wiring was not directly confirmed in this review. The earlier PASS conclusion still holds.
Full details: Out of Scope Changes check
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Explanation
The changes in
common.pyaddUnionto the collective-ID reservation. This supports the ordered-union exchanges and does not add unrelated behavior. Theunion.pychanges implement the ordered routing, the emit/read split, and the pass-through path, all of which serve#22376. The test changes intest_union.py,test_collective_ids.py,test_dataframescan.py, andtest_rolling.pyverify ordering, routing, collective IDs, the xfail removal fortest_dataframescan_concat, and the#22405regression. No unrelated change is demonstrated. The truncated tail ofunion.pywas not read, so its wiring was not checked line by line.
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Review comments at
@python/cudf_polars/cudf_polars/streaming/actor_graph/collectives/common.py:
- Around line 236-242: Update ordered-Union ID allocation in
ReserveOpIDs.__enter__ so it does not reserve an ID for every child and exceed
the collective pool limit. Skip reservations when needs_ordered_exchange selects
_passthrough_union; for active exchanges, bound or replace per-child ID
allocation while preserving ordered output, and reuse a SparseAlltoall ID only
if its contract guarantees safe reuse after all ranks finish insertion and
extraction.
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The over(...)-in-filter fallback reproducer from NVIDIA#22405 depends only on the ordered multi-rank Union, so check its row order on every engine with fallback_mode="silent". Also drop the outdated note that the multi-rank fallback doesn't preserve order; test_over_in_filter_unsupported stays xfailed on Dask/Ray because the warning is raised on the workers.
rjzamora
added
bug
Reading and exchanging the children of an ordered multi-rank Union now runs separately from emitting the output. Before, a downstream actor that waits on a collective before draining its input (e.g. a non-scalar over() sampling chunks under dynamic planning) could block one rank on sending while the other ranks waited for it to join the next child's exchange. The per-child exchanges alternate between two collective IDs, so an ordered Union reserves 3 IDs no matter how many children it has.
- Keep ReserveOpIDs independent of the rank count: add Union to the collective node types and reserve one collective ID per child. The ordered path uses ID 0 for the chunk-count AllGather and child 0's exchange, and ID i for child i's exchange. - Always finish a child's exchange, even if reading or packing fails or the task is cancelled. - Mark the NVIDIA#22405 concat regression test with pytest.mark.spmd. - Add the type annotations mypy needs in test_union.py.
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Thanks for the review @rjzamora! Pushed changes for all the comments and merged in |
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rjzamora
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Thank you for iterating on this @aryansri05 - It's looking good to me.
I think I only have one remaining concern related to ChunkStore cleanup.
| sequence_number, | ||
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| await gather_in_task_group(read_and_exchange(), emit()) |
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Can we move creation of the temporary ChunkStores into read_and_exchange, track them, and wrap this gather_in_task_group call in a try/finally that clears them? This would cover partially filled, queued, and partially emitted buffers if either task fails.
I haven’t tested this locally, but a rough sketch might look like:
buffers: list[ChunkStore] = []
exchanged: asyncio.Queue[list[ChunkStore]] = asyncio.Queue()
async def read_and_exchange() -> None:
for child, (ch_in, meta) in enumerate(
zip(chs_in, metadata, strict=True)
):
owned = [ChunkStore(context) for _ in range(comm.nranks)]
buffers.extend(owned)
await _exchange_child(
context,
comm,
plan,
child,
ch_in,
collective_ids[child],
owned=owned,
drop=meta.duplicated and comm.rank != 0,
)
exchanged.put_nowait(owned)
# Keep the existing emit() function unchanged.
try:
await gather_in_task_group(read_and_exchange(), emit())
finally:
for store in buffers:
store.clear()This would also require _exchange_child to accept owned rather than create and return its own stores. Its existing exchange-finalization try/finally can stay unchanged.
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Done, thanks for the sketch! _exchange_child now takes owned, read_and_exchange creates the stores and tracks them in buffers, and the gather_in_task_group call is wrapped in a try/finally that clears them.
Create the per-child ChunkStores in read_and_exchange and track them, so that partially filled, queued or partially emitted stores are cleared if either the reader or the emitter task fails. _exchange_child now fills stores it is given instead of creating its own.
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/ok to test c180d05 |
Description
Closes #22376.
On multi-rank streaming engines, an order-preserving
Union(pl.concat) forwarded each child's local chunks on every rank, so the result interleaved children across ranks ([A_r0, B_r0, A_r1, B_r1]instead of[A, B]).Following the approach discussed in #22376, when
maintain_orderis set and the union runs on more than one rank:local_countin a single collective.partition_range/partition_owner.SparseAlltoallper child; each rank emits its chunks of childifully beforei + 1.local_countis off.Reading/exchanging is decoupled from emitting. One task reads and exchanges each child in turn and never waits on the output channel; a second task emits children in order as their exchanges finish (exchanged chunks sit in spillable
ChunkStores until then). Each exchange is finished in afinally, so a failure while reading or packing doesn't leave it unfinished. An earlier revision did both in one loop, which deadlocked when the union fed something that waits on a collective before reading all of its input, e.g. a non-scalarover()sampling under dynamic planning: one rank blocked sending to its downstream while the other was still waiting for it to join the next child's exchange.Unionis a collective node and reserves one ID per child: the ordered path uses ID 0 for the AllGather and then child 0's exchange (the same sharingOverdoes), and IDifor childi's exchange. Unordered (pl.union), single-rank and all-duplicated unions keep the existing pass-through and don't use their IDs.Trade-off: output is balanced by position, so with balanced inputs roughly
(P - 1) / Pof the data moves between ranks. That's inherent to giving each rank a contiguous slice of the concatenated output.Testing
Kaggle, 2× Tesla T4, nightly
cudf-polars-cu1226.12 with Polars 2.0, Python 3.13 (dask/rayrun 2 ranks), branch merged withmain@ 761d410.test_dataframescan_concat: fails ondask/rayonmain@ 8d80134, passes on all engines with this PR (xfail removed).test_union.py: routing-plan unit tests (including random cases with wronglocal_countestimates), uneven and empty children, a duplicated child,pl.union, andconcatfeedingshift().over()(the deadlock case above). The order tests (and the [BUG]over(...)inside.filter()fallback reorders rows on multi-rank streaming engines #22405 one below) are markedspmdso they also run in multi-rank SPMD (rrun) jobs.test_collective_ids.py: a union reserves one ID per child.concat→shift(1).over("g"),max_rows_per_partition=10,RayEngine(num_ranks=2)): hung on the previous revision (killed after 240 s) and finishes with the correct result now.test_union.py,test_collective_ids.py,test_dataframescan.py,test_rolling.py: 384 passed, 1 skipped, 6 xfailed, 0 failed.tests/streaming: 2470 passed, 35 skipped, 6 xfailed, 6 xpassed, 0 failed.over(...)inside.filter()fallback reorders rows on multi-rank streaming engines #22405 (concat+filter(pl.len().over("k") == 2)onRayEngine(num_ranks=2)) fails onmain@ 8d80134 with a value mismatch on columnkand passes with this PR; covered by the newtest_over_in_filter_after_concat_preserves_order(all 4 engines).Before / after
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