Annotation-less P2P shuffling

distributed/exceptions.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+
+class Reschedule(Exception):
+    """Reschedule this task
+
+    Raising this exception will stop the current execution of the task and ask
+    the scheduler to reschedule this task, possibly on a different machine.
+
+    This does not guarantee that the task will move onto a different machine.
+    The scheduler will proceed through its normal heuristics to determine the
+    optimal machine to accept this task.  The machine will likely change if the
+    load across the cluster has significantly changed since first scheduling
+    the task.
+    """

distributed/shuffle/_merge.py

@@ -136,13 +136,15 @@ def merge_transfer(
     id: ShuffleId,
     input_partition: int,
     npartitions: int,
+    parts_out: set[int],
 ):
     return shuffle_transfer(
         input=input,
         id=id,
         input_partition=input_partition,
         npartitions=npartitions,
         column=_HASH_COLUMN_NAME,
+        parts_out=parts_out,
     )
 
 
@@ -340,6 +342,7 @@ def _construct_graph(self) -> dict[tuple | str, tuple]:
                 token_left,
                 i,
                 self.npartitions,
+                self.parts_out,
             )
         for i in range(self.n_partitions_right):
             transfer_keys_right.append((name_right, i))
@@ -349,6 +352,7 @@ def _construct_graph(self) -> dict[tuple | str, tuple]:
                 token_right,
                 i,
                 self.npartitions,
+                self.parts_out,
             )
 
         _barrier_key_left = barrier_key(ShuffleId(token_left))

distributed/shuffle/_rechunk.py

@@ -8,6 +8,7 @@
 from dask.base import tokenize
 from dask.highlevelgraph import HighLevelGraph, MaterializedLayer
 
+from distributed.exceptions import Reschedule
 from distributed.shuffle._shuffle import (
     ShuffleId,
     ShuffleType,
@@ -57,6 +58,8 @@ def rechunk_unpack(
         return _get_worker_extension().get_output_partition(
             id, barrier_run_id, output_chunk
         )
+    except Reschedule as e:
+        raise e
     except Exception as e:
         raise RuntimeError(f"rechunk_unpack failed during shuffle {id}") from e
 
@@ -70,12 +73,6 @@ def rechunk_p2p(x: da.Array, chunks: ChunkedAxes) -> da.Array:
         # Special case for empty array, as the algorithm below does not behave correctly
         return da.empty(x.shape, chunks=chunks, dtype=x.dtype)
 
-    if dask.config.get("optimization.fuse.active") is not False:
-        raise RuntimeError(
-            "P2P rechunking requires the fuse optimization to be turned off. "
-            "Set the 'optimization.fuse.active' config to False to deactivate."
-        )
-
     dsk: dict = {}
     token = tokenize(x, chunks)
     _barrier_key = barrier_key(ShuffleId(token))

distributed/shuffle/_scheduler_extension.py

@@ -6,7 +6,9 @@
 import logging
 from collections import defaultdict
 from dataclasses import dataclass
-from typing import TYPE_CHECKING, Any, ClassVar
+from functools import partial
+from itertools import product
+from typing import TYPE_CHECKING, Any, Callable, ClassVar, Iterable, Sequence
 
 from distributed.diagnostics.plugin import SchedulerPlugin
 from distributed.shuffle._rechunk import ChunkedAxes, NIndex
@@ -105,6 +107,7 @@ def __init__(self, scheduler: Scheduler):
                 "shuffle_barrier": self.barrier,
                 "shuffle_get": self.get,
                 "shuffle_get_or_create": self.get_or_create,
+                "shuffle_restrict_task": self.restrict_task,
             }
         )
         self.heartbeats = defaultdict(lambda: defaultdict(dict))
@@ -122,14 +125,22 @@ async def barrier(self, id: ShuffleId, run_id: int) -> None:
             msg=msg, workers=list(shuffle.participating_workers)
         )
 
+    def restrict_task(self, id: ShuffleId, run_id: int, key: str, worker: str) -> dict:
+        shuffle = self.states[id]
+        if shuffle.run_id != run_id:
+            return {"status": "error", "message": "Stale shuffle"}
+        ts = self.scheduler.tasks[key]
+        self._set_restriction(ts, worker)
+        return {"status": "OK"}
+
     def heartbeat(self, ws: WorkerState, data: dict) -> None:
         for shuffle_id, d in data.items():
             if shuffle_id in self.shuffle_ids():
                 self.heartbeats[shuffle_id][ws.address].update(d)
 
     def get(self, id: ShuffleId, worker: str) -> dict[str, Any]:
         if exception := self.erred_shuffles.get(id):
-            return {"status": "ERROR", "message": str(exception)}
+            return {"status": "error", "message": str(exception)}
         state = self.states[id]
         state.participating_workers.add(worker)
         return state.to_msg()
@@ -144,6 +155,11 @@ def get_or_create(
         try:
             return self.get(id, worker)
         except KeyError:
+            # FIXME: The current implementation relies on the barrier task to be
+            # known by its name. If the name has been mangled, we cannot guarantee
+            # that the shuffle works as intended and should fail instead.
+            self._raise_if_barrier_unknown(id)
+
             state: ShuffleState
             if type == ShuffleType.DATAFRAME:
                 state = self._create_dataframe_shuffle_state(id, spec)
@@ -155,33 +171,33 @@ def get_or_create(
             state.participating_workers.add(worker)
             return state.to_msg()
 
+    def _raise_if_barrier_unknown(self, id: ShuffleId) -> None:
+        key = barrier_key(id)
+        try:
+            self.scheduler.tasks[key]
+        except KeyError:
+            raise RuntimeError(
+                f"Barrier task with key {key!r} does not exist. This may be caused by "
+                "task fusion during graph generation. Please let us know that you ran "
+                "into this by leaving a comment at distributed#7816."
+            )
+
     def _create_dataframe_shuffle_state(
         self, id: ShuffleId, spec: dict[str, Any]
     ) -> DataFrameShuffleState:
         schema = spec["schema"]
         column = spec["column"]
         npartitions = spec["npartitions"]
+        parts_out = spec["parts_out"]
         assert schema is not None
         assert column is not None
         assert npartitions is not None
+        assert parts_out is not None
 
-        workers = list(self.scheduler.workers)
-        output_workers = set()
-
-        name = barrier_key(id)
-        mapping = {}
+        pick_worker = partial(get_worker_for_range_sharding, npartitions)
 
-        for ts in self.scheduler.tasks[name].dependents:
-            part = get_partition_id(ts)
-            if ts.worker_restrictions:
-                output_worker = list(ts.worker_restrictions)[0]
-            else:
-                output_worker = get_worker_for_range_sharding(
-                    part, workers, npartitions
-                )
-            mapping[part] = output_worker
-            output_workers.add(output_worker)
-            self.scheduler.set_restrictions({ts.key: {output_worker}})
+        mapping = self._pin_output_workers(id, parts_out, pick_worker)
+        output_workers = set(mapping.values())
 
         return DataFrameShuffleState(
             id=id,
@@ -193,6 +209,52 @@ def _create_dataframe_shuffle_state(
             participating_workers=output_workers.copy(),
         )
 
+    def _pin_output_workers(
+        self,
+        id: ShuffleId,
+        output_partitions: Iterable[Any],
+        pick: Callable[[Any, Sequence[str]], str],
+    ) -> dict[Any, str]:
+        """Pin the outputs of a P2P shuffle to specific workers.
+
+        Parameters
+        ----------
+        id: ID of the shuffle to pin
+        output_partitions: Output partition IDs to pin
+        pick: Function that picks a worker given a partition ID and sequence of worker
+
+        .. note:
+            This function assumes that the barrier task and the output tasks share
+            the same worker restrictions.
+        """
+        mapping = {}
+        barrier = self.scheduler.tasks[barrier_key(id)]
+
+        if barrier.worker_restrictions:
+            workers = list(barrier.worker_restrictions)
+        else:
+            workers = list(self.scheduler.workers)
+
+        for partition in output_partitions:
+            worker = pick(partition, workers)
+            mapping[partition] = worker
+
+        for dt in barrier.dependents:
+            try:
+                partition = dt.annotations["shuffle"]
+            except KeyError:
+                continue
+
+            if dt.worker_restrictions:
+                worker = pick(partition, list(dt.worker_restrictions))
+                mapping[partition] = worker
+            else:
+                worker = mapping[partition]
+
+            self._set_restriction(dt, worker)
+
+        return mapping
+
     def _create_array_rechunk_state(
         self, id: ShuffleId, spec: dict[str, Any]
     ) -> ArrayRechunkState:
@@ -201,21 +263,9 @@ def _create_array_rechunk_state(
         assert old is not None
         assert new is not None
 
-        workers = list(self.scheduler.workers)
-        output_workers = set()
-
-        name = barrier_key(id)
-        mapping = {}
-
-        for ts in self.scheduler.tasks[name].dependents:
-            part = get_partition_id(ts)
-            if ts.worker_restrictions:
-                output_worker = list(ts.worker_restrictions)[0]
-            else:
-                output_worker = get_worker_for_hash_sharding(part, workers)
-            mapping[part] = output_worker
-            output_workers.add(output_worker)
-            self.scheduler.set_restrictions({ts.key: {output_worker}})
+        parts_out = product(*(range(len(c)) for c in new))
+        mapping = self._pin_output_workers(id, parts_out, get_worker_for_hash_sharding)
+        output_workers = set(mapping.values())
 
         return ArrayRechunkState(
             id=id,
@@ -227,6 +277,22 @@ def _create_array_rechunk_state(
             participating_workers=output_workers.copy(),
         )
 
+    def _set_restriction(self, ts: TaskState, worker: str) -> None:
+        if "shuffle_original_restrictions" in ts.annotations:
+            # This may occur if multiple barriers share the same output task,
+            # e.g. in a hash join.
+            return
+        ts.annotations["shuffle_original_restrictions"] = ts.worker_restrictions.copy()
+        self.scheduler.set_restrictions({ts.key: {worker}})
+
+    def _unset_restriction(self, ts: TaskState) -> None:
+        # shuffle_original_restrictions is only set if the task was first scheduled
+        # on the wrong worker
+        if "shuffle_original_restrictions" not in ts.annotations:
+            return
+        original_restrictions = ts.annotations.pop("shuffle_original_restrictions")
+        self.scheduler.set_restrictions({ts.key: original_restrictions})
+
     def remove_worker(self, scheduler: Scheduler, worker: str) -> None:
         from time import time
 
@@ -247,7 +313,7 @@ def remove_worker(self, scheduler: Scheduler, worker: str) -> None:
                 for dt in barrier_task.dependents:
                     if worker not in dt.worker_restrictions:
                         continue
-                    dt.worker_restrictions.clear()
+                    self._unset_restriction(dt)
                     recs.update({dt.key: "waiting"})
                 # TODO: Do we need to handle other states?
 
@@ -293,34 +359,27 @@ def _clean_on_scheduler(self, id: ShuffleId) -> None:
         with contextlib.suppress(KeyError):
             del self.heartbeats[id]
 
+        barrier_task = self.scheduler.tasks[barrier_key(id)]
+        for dt in barrier_task.dependents:
+            self._unset_restriction(dt)
+
     def restart(self, scheduler: Scheduler) -> None:
         self.states.clear()
         self.heartbeats.clear()
         self.erred_shuffles.clear()
 
 
-def get_partition_id(ts: TaskState) -> Any:
-    """Get the output partition ID of this task state."""
-    try:
-        return ts.annotations["shuffle"]
-    except KeyError:
-        raise RuntimeError(
-            f"{ts} has lost its ``shuffle`` annotation. This may be caused by "
-            "unintended optimization during graph generation. "
-            "Please report this problem on GitHub and link it to "
-            "the tracking issue at https://github.com/dask/distributed/issues/7716."
-        )
-
-
 def get_worker_for_range_sharding(
-    output_partition: int, workers: list[str], npartitions: int
+    npartitions: int, output_partition: int, workers: Sequence[str]
 ) -> str:
     """Get address of target worker for this output partition using range sharding"""
     i = len(workers) * output_partition // npartitions
     return workers[i]
 
 
-def get_worker_for_hash_sharding(output_partition: NIndex, workers: list[str]) -> str:
+def get_worker_for_hash_sharding(
+    output_partition: NIndex, workers: Sequence[str]
+) -> str:
     """Get address of target worker for this output partition using hash sharding"""
     i = hash(output_partition) % len(workers)
     return workers[i]