Move replicated-output dedup to the Dask and Ray frontends

python/cudf_polars/cudf_polars/experimental/rapidsmpf/frontend/dask.py

@@ -408,14 +408,20 @@ def _worker_evaluate(
     mp_ctx: _WorkerContext = getattr(dask_worker, f"_cudf_polars_mp_context_{uid}")
     if mp_ctx.ctx is None or mp_ctx.comm is None or mp_ctx.py_executor is None:
         raise RuntimeError("_setup_worker must be called before _worker_evaluate")
-    return evaluate_on_rank(
+    # Always collect the final metadata so we can suppress duplicated outputs
+    # on non-root ranks. The client concatenates per-rank results, so without
+    # this dedup an output marked ``duplicated=True`` would be N-counted.
+    df, metadata = evaluate_on_rank(
         mp_ctx.ctx,
         mp_ctx.comm,
         mp_ctx.py_executor,
         ir,
         config_options,
-        collect_metadata=collect_metadata,
+        collect_metadata=True,
     )
+    if mp_ctx.comm.rank != 0 and metadata and metadata[-1].duplicated:
+        df = df.clear()
+    return df, metadata if collect_metadata else None
 
 
 def evaluate_pipeline_dask_mode(

python/cudf_polars/cudf_polars/experimental/rapidsmpf/frontend/ray.py

@@ -363,14 +363,26 @@ def evaluate_polars_ir(
         # object store (pickle / Arrow IPC). The DataFrame is already on CPU at
         # this point (to_polars() copies the result off-GPU), so no GPU memory
         # crosses process boundaries.
-        return evaluate_on_rank(
+        # Always collect the final metadata so we can suppress duplicated
+        # outputs on non-root ranks. The client concatenates per-rank results,
+        # so without this dedup an output marked ``duplicated=True`` would be
+        # N-counted.
+        df, metadata = evaluate_on_rank(
             self._ctx,
             self._comm,
             self._py_executor,
             ir,
             config_options,
-            collect_metadata=collect_metadata,
+            collect_metadata=True,
         )
+        if (
+            self._comm is not None
+            and self._comm.rank != 0
+            and metadata
+            and metadata[-1].duplicated
+        ):
+            df = df.clear()
+        return df, metadata if collect_metadata else None
 
     def _run(self, func: Callable[..., T], *args: Any, **kwargs: Any) -> T:
         return func(*args, **kwargs)

python/cudf_polars/cudf_polars/experimental/rapidsmpf/join.py

@@ -205,19 +205,9 @@ async def _collect_small_side_for_broadcast(
             for s_id in range(len(chunks)):
                 inserter.insert(s_id, chunks.pop(0))
         stream = ir_context.get_cuda_stream()
-        gathered = await allgather.extract_concatenated(stream)
-        # When every rank inserted zero chunks, the AllGather has no schema
-        # to infer and returns a 0-column table. Substitute a properly typed
-        # empty table for the small side so downstream joins still match the
-        # expected schema.
-        table = (
-            empty_table_chunk(ir, context, stream).table_view()
-            if gathered.num_columns() == 0 and len(ir.schema) > 0
-            else gathered
-        )
         dfs = [
             DataFrame.from_table(
-                table,
+                await allgather.extract_concatenated(stream),
                 list(ir.schema.keys()),
                 list(ir.schema.values()),
                 stream,

python/cudf_polars/cudf_polars/experimental/rapidsmpf/nodes.py

@@ -732,20 +732,10 @@ async def metadata_drain_node(
     ):
         # Drain metadata channel (we don't need it after this point)
         metadata = await recv_metadata(ch_in, context)
-        send_empty = metadata.duplicated and comm.rank != 0
         if metadata_collector is not None:
             metadata_collector.append(metadata)
 
-        # Forward non-duplicated data messages
         while (msg := await ch_in.recv(context)) is not None:
-            if not send_empty:
-                await ch_out.send(context, msg)
-
-        # Send empty data if needed
-        if send_empty:
-            stream = ir_context.get_cuda_stream()
-            await ch_out.send(
-                context, Message(0, empty_table_chunk(ir, context, stream))
-            )
+            await ch_out.send(context, msg)
 
         await ch_out.drain(context)

python/cudf_polars/cudf_polars/experimental/rapidsmpf/union.py

@@ -24,7 +24,6 @@
 )
 
 if TYPE_CHECKING:
-    from rapidsmpf.communicator.communicator import Communicator
     from rapidsmpf.streaming.core.channel import Channel
     from rapidsmpf.streaming.core.context import Context
 
@@ -35,7 +34,6 @@
 @define_actor()
 async def union_node(
     context: Context,
-    comm: Communicator,
     ir: Union,
     ir_context: IRExecutionContext,
     ch_out: Channel[TableChunk],
@@ -48,9 +46,6 @@ async def union_node(
     ----------
     context
         The rapidsmpf context.
-    comm
-        The communicator. Used to suppress duplicated children's chunks on
-        non-root ranks so they aren't emitted twice cluster-wide.
     ir
         The Union IR node.
     ir_context
@@ -66,19 +61,14 @@ async def union_node(
         # Merge and forward metadata.
         # Union loses partitioning/ordering info since sources may differ.
         # TODO: Warn users that Union does NOT preserve order?
+        total_local_count = 0
+        duplicated = True
         metadata = await gather_in_task_group(
             *(recv_metadata(ch, context) for ch in chs_in)
         )
-        # When a child has duplicated=True, every rank has produced the same
-        # data and only rank 0 should forward it -- otherwise the downstream
-        # client-side concat would over-count by `nranks - 1` for each
-        # duplicated chunk.
-        skip = tuple(meta.duplicated and comm.rank != 0 for meta in metadata)
-        total_local_count = sum(
-            0 if drop else meta.local_count
-            for meta, drop in zip(metadata, skip, strict=True)
-        )
-        duplicated = all(meta.duplicated for meta in metadata)
+        for meta in metadata:
+            total_local_count += meta.local_count
+            duplicated = duplicated and meta.duplicated
         await send_metadata(
             ch_out,
             context,
@@ -89,22 +79,21 @@ async def union_node(
         )
 
         seq_num_offset = 0
-        for ch_in, drop in zip(chs_in, skip, strict=True):
+        for ch_in in chs_in:
             num_ch_chunks = 0
             while (msg := await ch_in.recv(context)) is not None:
-                if not drop:
-                    await ch_out.send(
-                        context,
-                        Message(
-                            msg.sequence_number + seq_num_offset,
-                            TableChunk.from_message(
-                                msg, br=context.br()
-                            ).make_available_and_spill(
-                                context.br(), allow_overbooking=True
-                            ),
+                num_ch_chunks += 1
+                await ch_out.send(
+                    context,
+                    Message(
+                        msg.sequence_number + seq_num_offset,
+                        TableChunk.from_message(
+                            msg, br=context.br()
+                        ).make_available_and_spill(
+                            context.br(), allow_overbooking=True
                         ),
-                    )
-                    num_ch_chunks += 1
+                    ),
+                )
             seq_num_offset += num_ch_chunks
 
         await ch_out.drain(context)
@@ -127,7 +116,6 @@ def _(
     nodes[ir] = [
         union_node(
             rec.state["context"],
-            rec.state["comm"],
             ir,
             rec.state["ir_context"],
             channels[ir].reserve_input_slot(),

python/cudf_polars/cudf_polars/experimental/select.py

@@ -431,17 +431,13 @@ def _(
         )
         named_expr = expr.NamedExpr(ir.exprs[0].name or "len", lit_expr)
 
-        # Use Empty as the input so the streaming network's metadata flows
-        # `duplicated=True` end-to-end. Without that, every rank emits the
-        # literal once and the client concatenates N copies.
-        input_ir: IR = Empty({})
         new_node = Select(
             {named_expr.name: named_expr.value.dtype},
             [named_expr],
             should_broadcast=True,
-            df=input_ir,
+            df=child,
         )
-        partition_info[input_ir] = partition_info[new_node] = PartitionInfo(count=1)
+        partition_info[new_node] = PartitionInfo(count=1)
         return new_node, partition_info
 
     if not any(

python/cudf_polars/tests/experimental/test_spmd.py

@@ -294,6 +294,25 @@ def test_run(spmd_engine: SPMDEngine) -> None:
     assert result == [os.getpid()]
 
 
+def test_sort_slice_over_union_of_duplicated_streams(
+    spmd_engine: SPMDEngine,
+) -> None:
+    """Sort+head over a concat of two group-by branches returns the global result on every rank."""
+    lf1 = (
+        pl.LazyFrame({"name": ["alice"], "score": [1.0]})
+        .group_by("name")
+        .agg(pl.col("score").sum())
+    )
+    lf2 = (
+        pl.LazyFrame({"name": ["bob"], "score": [2.0]})
+        .group_by("name")
+        .agg(pl.col("score").sum())
+    )
+    lf = pl.concat([lf1, lf2]).sort("score").head(10)
+    result = lf.collect(engine=spmd_engine)
+    assert sorted(result["name"].to_list()) == ["alice", "bob"]
+
+
 def test_reset_keeps_comm_alive(comm: Communicator) -> None:
     """``_reset`` must not rebuild the communicator."""
     with SPMDEngine(

python/cudf_polars/tests/experimental/test_union.py

@@ -17,3 +17,19 @@ def test_union_shared_fanout_no_deadlock(streaming_engine):
     project = df.select("key", "val")
     q = pl.concat([gb, project])
     assert_gpu_result_equal(q, engine=streaming_engine, check_row_order=False)
+
+
+def test_sort_slice_over_union_of_duplicated_streams(streaming_engine):
+    # Sort+head over a concat of two group-by branches.
+    lf1 = (
+        pl.LazyFrame({"name": ["alice"], "score": [1.0]})
+        .group_by("name")
+        .agg(pl.col("score").sum())
+    )
+    lf2 = (
+        pl.LazyFrame({"name": ["bob"], "score": [2.0]})
+        .group_by("name")
+        .agg(pl.col("score").sum())
+    )
+    q = pl.concat([lf1, lf2]).sort("score").head(10)
+    assert_gpu_result_equal(q, engine=streaming_engine, check_row_order=False)

python/cudf_polars/tests/test_groupby.py

@@ -484,13 +484,8 @@ def test_groupby_sum_decimal_null_group(engine: pl.GPUEngine) -> None:
 
 
 @pytest.mark.xfail(
-    raises=(AssertionError, pl.exceptions.SchemaError),
-    reason=(
-        "https://github.com/rapidsai/cudf/issues/19610 — in-memory engine "
-        "fails with AssertionError (wrong values); multi-rank streaming "
-        "fails earlier with SchemaError (literal agg yields a divergent "
-        "schema after cross-rank concat)."
-    ),
+    raises=AssertionError,
+    reason="https://github.com/rapidsai/cudf/issues/19610",
 )
 def test_groupby_literal_agg(engine: pl.GPUEngine):
     df = pl.LazyFrame({"c0": [True, False]})