Zero-copy array shuffle

distributed/shuffle/_buffer.py

@@ -45,6 +45,7 @@ class ShardsBuffer(Generic[ShardType]):
 
     shards: defaultdict[str, _List[ShardType]]
     sizes: defaultdict[str, int]
+    sizes_detail: defaultdict[str, list[int]]
     concurrency_limit: int
     memory_limiter: ResourceLimiter
     diagnostics: dict[str, float]
@@ -71,6 +72,7 @@ def __init__(
         self._accepts_input = True
         self.shards = defaultdict(_List)
         self.sizes = defaultdict(int)
+        self.sizes_detail = defaultdict(list)
         self._exception = None
         self.concurrency_limit = concurrency_limit
         self._inputs_done = False
@@ -149,7 +151,7 @@ def _continue() -> bool:
                         try:
                             shard = self.shards[part_id].pop()
                             shards.append(shard)
-                            s = sizeof(shard)
+                            s = self.sizes_detail[part_id].pop()
                             size += s
                             self.sizes[part_id] -= s
                         except IndexError:
@@ -159,6 +161,8 @@ def _continue() -> bool:
                                 del self.shards[part_id]
                                 assert not self.sizes[part_id]
                                 del self.sizes[part_id]
+                                assert not self.sizes_detail[part_id]
+                                del self.sizes_detail[part_id]
                 else:
                     shards = self.shards.pop(part_id)
                     size = self.sizes.pop(part_id)
@@ -201,6 +205,7 @@ async def write(self, data: dict[str, ShardType]) -> None:
         async with self._shards_available:
             for worker, shard in data.items():
                 self.shards[worker].append(shard)
+                self.sizes_detail[worker].append(sizes[worker])
                 self.sizes[worker] += sizes[worker]
             self._shards_available.notify()
         await self.memory_limiter.wait_for_available()

distributed/shuffle/_comms.py

@@ -52,7 +52,7 @@ class CommShardsBuffer(ShardsBuffer):
 
     def __init__(
         self,
-        send: Callable[[str, list[tuple[Any, bytes]]], Awaitable[None]],
+        send: Callable[[str, list[tuple[Any, Any]]], Awaitable[None]],
         memory_limiter: ResourceLimiter,
         concurrency_limit: int = 10,
     ):
@@ -63,7 +63,7 @@ def __init__(
         )
         self.send = send
 
-    async def _process(self, address: str, shards: list[tuple[Any, bytes]]) -> None:
+    async def _process(self, address: str, shards: list[tuple[Any, Any]]) -> None:
         """Send one message off to a neighboring worker"""
         with log_errors():
             # Consider boosting total_size a bit here to account for duplication

distributed/shuffle/_core.py

@@ -140,7 +140,7 @@ async def barrier(self, run_ids: Sequence[int]) -> int:
         return self.run_id
 
     async def _send(
-        self, address: str, shards: list[tuple[_T_partition_id, bytes]]
+        self, address: str, shards: list[tuple[_T_partition_id, Any]]
     ) -> None:
         self.raise_if_closed()
         return await self.rpc(address).shuffle_receive(
@@ -150,7 +150,7 @@ async def _send(
         )
 
     async def send(
-        self, address: str, shards: list[tuple[_T_partition_id, bytes]]
+        self, address: str, shards: list[tuple[_T_partition_id, Any]]
     ) -> None:
         retry_count = dask.config.get("distributed.p2p.comm.retry.count")
         retry_delay_min = parse_timedelta(
@@ -186,12 +186,12 @@ def heartbeat(self) -> dict[str, Any]:
         }
 
     async def _write_to_comm(
-        self, data: dict[str, tuple[_T_partition_id, bytes]]
+        self, data: dict[str, tuple[_T_partition_id, Any]]
     ) -> None:
         self.raise_if_closed()
         await self._comm_buffer.write(data)
 
-    async def _write_to_disk(self, data: dict[NDIndex, bytes]) -> None:
+    async def _write_to_disk(self, data: dict[NDIndex, Any]) -> None:
         self.raise_if_closed()
         await self._disk_buffer.write(
             {"_".join(str(i) for i in k): v for k, v in data.items()}
@@ -239,7 +239,7 @@ def _read_from_disk(self, id: NDIndex) -> list[Any]:  # TODO: Typing
         self.raise_if_closed()
         return self._disk_buffer.read("_".join(str(i) for i in id))
 
-    async def receive(self, data: list[tuple[_T_partition_id, bytes]]) -> None:
+    async def receive(self, data: list[tuple[_T_partition_id, Any]]) -> None:
         await self._receive(data)
 
     async def _ensure_output_worker(self, i: _T_partition_id, key: str) -> None:
@@ -259,7 +259,7 @@ def _get_assigned_worker(self, i: _T_partition_id) -> str:
         """Get the address of the worker assigned to the output partition"""
 
     @abc.abstractmethod
-    async def _receive(self, data: list[tuple[_T_partition_id, bytes]]) -> None:
+    async def _receive(self, data: list[tuple[_T_partition_id, Any]]) -> None:
         """Receive shards belonging to output partitions of this shuffle run"""
 
     def add_partition(
@@ -275,7 +275,7 @@ def add_partition(
     @abc.abstractmethod
     def _shard_partition(
         self, data: _T_partition_type, partition_id: _T_partition_id
-    ) -> dict[str, tuple[_T_partition_id, bytes]]:
+    ) -> dict[str, tuple[_T_partition_id, Any]]:
         """Shard an input partition by the assigned output workers"""
 
     def get_output_partition(
@@ -299,7 +299,7 @@ def read(self, path: Path) -> tuple[Any, int]:
         """Read shards from disk"""
 
     @abc.abstractmethod
-    def deserialize(self, buffer: bytes) -> Any:
+    def deserialize(self, buffer: Any) -> Any:
         """Deserialize shards"""
 
 

distributed/shuffle/_disk.py

@@ -4,12 +4,15 @@
 import pathlib
 import shutil
 import threading
-from collections.abc import Generator
+from collections.abc import Callable, Generator, Iterable
 from contextlib import contextmanager
-from typing import Any, Callable
+from typing import Any
+
+from toolz import concat
 
 from distributed.shuffle._buffer import ShardsBuffer
 from distributed.shuffle._limiter import ResourceLimiter
+from distributed.shuffle._pickle import pickle_bytelist
 from distributed.utils import Deadline, log_errors
 
 
@@ -135,7 +138,7 @@ def __init__(
         self._read = read
         self._directory_lock = ReadWriteLock()
 
-    async def _process(self, id: str, shards: list[bytes]) -> None:
+    async def _process(self, id: str, shards: list[Any]) -> None:
         """Write one buffer to file
 
         This function was built to offload the disk IO, but since then we've
@@ -157,11 +160,18 @@ async def _process(self, id: str, shards: list[bytes]) -> None:
                 with self._directory_lock.read():
                     if self._closed:
                         raise RuntimeError("Already closed")
-                    with open(
-                        self.directory / str(id), mode="ab", buffering=100_000_000
-                    ) as f:
-                        for shard in shards:
-                            f.write(shard)
+
+                    frames: Iterable[bytes | bytearray | memoryview]
+
+                    if isinstance(shards[0], bytes):
+                        # Manually serialized dataframes
+                        frames = shards
+                    else:
+                        # Unserialized numpy arrays
+                        frames = concat(pickle_bytelist(shard) for shard in shards)
+
+                    with open(self.directory / str(id), mode="ab") as f:
+                        f.writelines(frames)
 
     def read(self, id: str) -> Any:
         """Read a complete file back into memory"""

distributed/shuffle/_memory.py

@@ -11,17 +11,15 @@
 
 
 class MemoryShardsBuffer(ShardsBuffer):
-    _deserialize: Callable[[bytes], Any]
-    _shards: defaultdict[str, deque[bytes]]
+    _deserialize: Callable[[Any], Any]
+    _shards: defaultdict[str, deque[Any]]
 
-    def __init__(self, deserialize: Callable[[bytes], Any]) -> None:
-        super().__init__(
-            memory_limiter=ResourceLimiter(None),
-        )
+    def __init__(self, deserialize: Callable[[Any], Any]) -> None:
+        super().__init__(memory_limiter=ResourceLimiter(None))
         self._deserialize = deserialize
         self._shards = defaultdict(deque)
 
-    async def _process(self, id: str, shards: list[bytes]) -> None:
+    async def _process(self, id: str, shards: list[Any]) -> None:
         # TODO: This can be greatly simplified, there's no need for
         # background threads at all.
         with log_errors():

distributed/shuffle/_pickle.py

@@ -0,0 +1,42 @@
+from __future__ import annotations
+
+import pickle
+from collections.abc import Iterator
+from typing import Any
+
+from distributed.protocol.utils import pack_frames_prelude, unpack_frames
+
+
+def pickle_bytelist(obj: object) -> list[bytes | memoryview]:
+    """Variant of :func:`serialize_bytelist`, that doesn't support compression, locally
+    defined classes, or any of its other fancy features but runs 10x faster for numpy
+    arrays
+
+    See Also
+    --------
+    serialize_bytelist
+    unpickle_bytestream
+    """
+    frames: list = []
+    pik = pickle.dumps(
+        obj, protocol=5, buffer_callback=lambda pb: frames.append(pb.raw())
+    )
+    frames.insert(0, pik)
+    frames.insert(0, pack_frames_prelude(frames))
+    return frames
+
+
+def unpickle_bytestream(b: bytes | bytearray | memoryview) -> Iterator[Any]:
+    """Unpickle the concatenated output of multiple calls to :func:`pickle_bytelist`
+
+    See Also
+    --------
+    pickle_bytelist
+    deserialize_bytes
+    """
+    while True:
+        pik, *buffers, remainder = unpack_frames(b, remainder=True)
+        yield pickle.loads(pik, buffers=buffers)
+        if remainder.nbytes == 0:
+            break
+        b = remainder

distributed/shuffle/_rechunk.py

@@ -96,8 +96,8 @@
 
 from __future__ import annotations
 
+import mmap
 import os
-import pickle
 from collections import defaultdict
 from collections.abc import Callable, Sequence
 from concurrent.futures import ThreadPoolExecutor
@@ -123,6 +123,7 @@
     handle_unpack_errors,
 )
 from distributed.shuffle._limiter import ResourceLimiter
+from distributed.shuffle._pickle import unpickle_bytestream
 from distributed.shuffle._scheduler_plugin import ShuffleSchedulerPlugin
 from distributed.shuffle._shuffle import barrier_key, shuffle_barrier
 from distributed.shuffle._worker_plugin import ShuffleWorkerPlugin
@@ -281,6 +282,9 @@ def convert_chunk(shards: list[list[tuple[NDIndex, np.ndarray]]]) -> np.ndarray:
     for index, shard in indexed.items():
         rec_cat_arg[tuple(index)] = shard
     arrs = rec_cat_arg.tolist()
+
+    # This may block for several seconds, as it physically reads the memory-mapped
+    # buffers from disk
     return concatenate3(arrs)
 
 
@@ -364,72 +368,84 @@ def __init__(
         self.worker_for = worker_for
         self.split_axes = split_axes(old, new)
 
-    async def _receive(self, data: list[tuple[NDIndex, bytes]]) -> None:
+    async def _receive(
+        self,
+        data: list[tuple[NDIndex, list[tuple[NDIndex, tuple[NDIndex, np.ndarray]]]]],
+    ) -> None:
         self.raise_if_closed()
 
-        filtered = []
+        # Repartition shards and filter out already received ones
+        shards = defaultdict(list)
         for d in data:
-            id, payload = d
-            if id in self.received:
+            id1, payload = d
+            if id1 in self.received:
                 continue
-            filtered.append(payload)
-            self.received.add(id)
+            self.received.add(id1)
+            for id2, shard in payload:
+                shards[id2].append(shard)
             self.total_recvd += sizeof(d)
         del data
-        if not filtered:
+        if not shards:
             return
+
         try:
-            shards = await self.offload(self._repartition_shards, filtered)
-            del filtered
             await self._write_to_disk(shards)
         except Exception as e:
             self._exception = e
             raise
 
-    def _repartition_shards(self, data: list[bytes]) -> dict[NDIndex, bytes]:
-        repartitioned: defaultdict[
-            NDIndex, list[tuple[NDIndex, np.ndarray]]
-        ] = defaultdict(list)
-        for buffer in data:
-            for id, shard in pickle.loads(buffer):
-                repartitioned[id].append(shard)
-        return {k: pickle.dumps(v) for k, v in repartitioned.items()}
-
     def _shard_partition(
-        self, data: np.ndarray, partition_id: NDIndex, **kwargs: Any
-    ) -> dict[str, tuple[NDIndex, bytes]]:
+        self, data: np.ndarray, partition_id: NDIndex
+    ) -> dict[str, tuple[NDIndex, Any]]:
         out: dict[str, list[tuple[NDIndex, tuple[NDIndex, np.ndarray]]]] = defaultdict(
             list
         )
-        from itertools import product
-
         ndsplits = product(*(axis[i] for axis, i in zip(self.split_axes, partition_id)))
 
         for ndsplit in ndsplits:
             chunk_index, shard_index, ndslice = zip(*ndsplit)
+
+            shard = data[ndslice]
+            # Don't wait until all shards have been transferred over the network
+            # before data can be released
+            if shard.base is not None:
+                shard = shard.copy()
+
             out[self.worker_for[chunk_index]].append(
-                (chunk_index, (shard_index, data[ndslice]))
+                (chunk_index, (shard_index, shard))
             )
-        return {k: (partition_id, pickle.dumps(v)) for k, v in out.items()}
+        return {k: (partition_id, v) for k, v in out.items()}
 
     def _get_output_partition(
         self, partition_id: NDIndex, key: str, **kwargs: Any
     ) -> np.ndarray:
+        # Quickly read metadata from disk.
+        # This is a bunch of seek()'s interleaved with short reads.
         data = self._read_from_disk(partition_id)
-        return convert_chunk(data)
-
-    def deserialize(self, buffer: bytes) -> Any:
-        result = pickle.loads(buffer)
-        return result
-
-    def read(self, path: Path) -> tuple[Any, int]:
-        shards: list[list[tuple[NDIndex, np.ndarray]]] = []
-        with path.open(mode="rb") as f:
-            size = f.seek(0, os.SEEK_END)
-            f.seek(0)
-            while f.tell() < size:
-                shards.append(pickle.load(f))
-        return shards, size
+        # Copy the memory-mapped buffers from disk into memory.
+        # This is where we'll spend most time.
+        with self._disk_buffer.time("read"):
+            return convert_chunk(data)
+
+    def deserialize(self, buffer: Any) -> Any:
+        return buffer
+
+    def read(self, path: Path) -> tuple[list[list[tuple[NDIndex, np.ndarray]]], int]:
+        """Open a memory-mapped file descriptor to disk, read all metadata, and unpickle
+        all arrays. This is a fast sequence of short reads interleaved with seeks.
+        Do not read in memory the actual data; the arrays' buffers will point to the
+        memory-mapped area.
+
+        The file descriptor will be automatically closed by the kernel when all the
+        returned arrays are dereferenced, which will happen after the call to
+        concatenate3.
+        """
+        with path.open(mode="r+b") as fh:
+            buffer = memoryview(mmap.mmap(fh.fileno(), 0))
+
+        # The file descriptor has *not* been closed!
+        shards = list(unpickle_bytestream(buffer))
+        return shards, buffer.nbytes
 
     def _get_assigned_worker(self, id: NDIndex) -> str:
         return self.worker_for[id]