feature(threading): use explicit/configurable thread pool executor

src/zarr/codecs/_v2.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 from dataclasses import dataclass
 from typing import TYPE_CHECKING
 
@@ -8,7 +9,6 @@
 
 from zarr.abc.codec import ArrayArrayCodec, ArrayBytesCodec
 from zarr.core.buffer import Buffer, NDBuffer, default_buffer_prototype
-from zarr.core.common import to_thread
 from zarr.registry import get_ndbuffer_class
 
 if TYPE_CHECKING:
@@ -30,7 +30,7 @@ async def _decode_single(
     ) -> NDBuffer:
         if self.compressor is not None:
             chunk_numpy_array = ensure_ndarray(
-                await to_thread(self.compressor.decode, chunk_bytes.as_array_like())
+                await asyncio.to_thread(self.compressor.decode, chunk_bytes.as_array_like())
             )
         else:
             chunk_numpy_array = ensure_ndarray(chunk_bytes.as_array_like())
@@ -54,7 +54,7 @@ async def _encode_single(
             ):
                 chunk_numpy_array = chunk_numpy_array.copy(order="A")
             encoded_chunk_bytes = ensure_bytes(
-                await to_thread(self.compressor.encode, chunk_numpy_array)
+                await asyncio.to_thread(self.compressor.encode, chunk_numpy_array)
             )
         else:
             encoded_chunk_bytes = ensure_bytes(chunk_numpy_array)
@@ -80,7 +80,7 @@ async def _decode_single(
         # apply filters in reverse order
         if self.filters is not None:
             for filter in self.filters[::-1]:
-                chunk_ndarray = await to_thread(filter.decode, chunk_ndarray)
+                chunk_ndarray = await asyncio.to_thread(filter.decode, chunk_ndarray)
 
         # ensure correct chunk shape
         if chunk_ndarray.shape != chunk_spec.shape:
@@ -100,7 +100,7 @@ async def _encode_single(
 
         if self.filters is not None:
             for filter in self.filters:
-                chunk_ndarray = await to_thread(filter.encode, chunk_ndarray)
+                chunk_ndarray = await asyncio.to_thread(filter.encode, chunk_ndarray)
 
         return get_ndbuffer_class().from_ndarray_like(chunk_ndarray)
 

src/zarr/codecs/blosc.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 from dataclasses import dataclass, replace
 from enum import Enum
 from functools import cached_property
@@ -10,7 +11,7 @@
 
 from zarr.abc.codec import BytesBytesCodec
 from zarr.core.buffer.cpu import as_numpy_array_wrapper
-from zarr.core.common import JSON, parse_enum, parse_named_configuration, to_thread
+from zarr.core.common import JSON, parse_enum, parse_named_configuration
 from zarr.registry import register_codec
 
 if TYPE_CHECKING:
@@ -169,7 +170,7 @@ async def _decode_single(
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer:
-        return await to_thread(
+        return await asyncio.to_thread(
             as_numpy_array_wrapper, self._blosc_codec.decode, chunk_bytes, chunk_spec.prototype
         )
 
@@ -180,7 +181,7 @@ async def _encode_single(
     ) -> Buffer | None:
         # Since blosc only support host memory, we convert the input and output of the encoding
         # between numpy array and buffer
-        return await to_thread(
+        return await asyncio.to_thread(
             lambda chunk: chunk_spec.prototype.buffer.from_bytes(
                 self._blosc_codec.encode(chunk.as_numpy_array())
             ),

src/zarr/codecs/gzip.py

@@ -1,13 +1,14 @@
 from __future__ import annotations
 
+import asyncio
 from dataclasses import dataclass
 from typing import TYPE_CHECKING
 
 from numcodecs.gzip import GZip
 
 from zarr.abc.codec import BytesBytesCodec
 from zarr.core.buffer.cpu import as_numpy_array_wrapper
-from zarr.core.common import JSON, parse_named_configuration, to_thread
+from zarr.core.common import JSON, parse_named_configuration
 from zarr.registry import register_codec
 
 if TYPE_CHECKING:
@@ -51,7 +52,7 @@ async def _decode_single(
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer:
-        return await to_thread(
+        return await asyncio.to_thread(
             as_numpy_array_wrapper, GZip(self.level).decode, chunk_bytes, chunk_spec.prototype
         )
 
@@ -60,7 +61,7 @@ async def _encode_single(
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer | None:
-        return await to_thread(
+        return await asyncio.to_thread(
             as_numpy_array_wrapper, GZip(self.level).encode, chunk_bytes, chunk_spec.prototype
         )
 

src/zarr/codecs/zstd.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 from dataclasses import dataclass
 from functools import cached_property
 from importlib.metadata import version
@@ -9,7 +10,7 @@
 
 from zarr.abc.codec import BytesBytesCodec
 from zarr.core.buffer.cpu import as_numpy_array_wrapper
-from zarr.core.common import JSON, parse_named_configuration, to_thread
+from zarr.core.common import JSON, parse_named_configuration
 from zarr.registry import register_codec
 
 if TYPE_CHECKING:
@@ -73,7 +74,7 @@ async def _decode_single(
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer:
-        return await to_thread(
+        return await asyncio.to_thread(
             as_numpy_array_wrapper, self._zstd_codec.decode, chunk_bytes, chunk_spec.prototype
         )
 
@@ -82,7 +83,7 @@ async def _encode_single(
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer | None:
-        return await to_thread(
+        return await asyncio.to_thread(
             as_numpy_array_wrapper, self._zstd_codec.encode, chunk_bytes, chunk_spec.prototype
         )
 

src/zarr/core/common.py

@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import asyncio
-import contextvars
 import functools
 import operator
 from collections.abc import Iterable, Mapping
@@ -10,7 +9,6 @@
     TYPE_CHECKING,
     Any,
     Literal,
-    ParamSpec,
     TypeVar,
     cast,
     overload,
@@ -60,17 +58,6 @@ async def run(item: tuple[Any]) -> V:
         return await asyncio.gather(*[asyncio.ensure_future(run(item)) for item in items])
 
 
-P = ParamSpec("P")
-U = TypeVar("U")
-
-
-async def to_thread(func: Callable[P, U], /, *args: P.args, **kwargs: P.kwargs) -> U:
-    loop = asyncio.get_running_loop()
-    ctx = contextvars.copy_context()
-    func_call = functools.partial(ctx.run, func, *args, **kwargs)
-    return await loop.run_in_executor(None, func_call)
-
-
 E = TypeVar("E", bound=Enum)
 
 

src/zarr/core/config.py

@@ -44,6 +44,7 @@ def reset(self) -> None:
             "default_zarr_version": 3,
             "array": {"order": "C"},
             "async": {"concurrency": 10, "timeout": None},
+            "threading": {"max_workers": None},
             "json_indent": 2,
             "codec_pipeline": {
                 "path": "zarr.codecs.pipeline.BatchedCodecPipeline",

src/zarr/core/sync.py

@@ -1,8 +1,10 @@
 from __future__ import annotations
 
 import asyncio
+import atexit
+import logging
 import threading
-from concurrent.futures import wait
+from concurrent.futures import ThreadPoolExecutor, wait
 from typing import TYPE_CHECKING, TypeVar
 
 from typing_extensions import ParamSpec
@@ -13,6 +15,9 @@
     from collections.abc import AsyncIterator, Coroutine
     from typing import Any
 
+logger = logging.getLogger(__name__)
+
+
 P = ParamSpec("P")
 T = TypeVar("T")
 
@@ -23,7 +28,7 @@
     None
 ]  # global event loop for any non-async instance
 _lock: threading.Lock | None = None  # global lock placeholder
-get_running_loop = asyncio.get_running_loop
+_executor: ThreadPoolExecutor | None = None  # global executor placeholder
 
 
 class SyncError(Exception):
@@ -41,6 +46,51 @@ def _get_lock() -> threading.Lock:
     return _lock
 
 
+def _get_executor() -> ThreadPoolExecutor:
+    """Return Zarr Thread Pool Executor
+
+    The executor is allocated on first use.
+    """
+    global _executor
+    if not _executor:
+        max_workers = config.get("threading.max_workers", None)
+        print(max_workers)
+        # if max_workers is not None and max_workers > 0:
+        #     raise ValueError(max_workers)
+        _executor = ThreadPoolExecutor(max_workers=max_workers, thread_name_prefix="zarr_pool")
+        _get_loop().set_default_executor(_executor)
+    return _executor
+
+
+def cleanup_resources() -> None:
+    global _executor
+    if _executor:
+        _executor.shutdown(wait=True, cancel_futures=True)
+    _executor = None
+
+    if loop[0] is not None:
+        with _get_lock():
+            # Stop the event loop safely
+            loop[0].call_soon_threadsafe(loop[0].stop)  # Stop loop from another thread
+            if iothread[0] is not None:
+                iothread[0].join(timeout=0.2)  # Add a timeout to avoid hanging
+
+                if iothread[0].is_alive():
+                    logger.warning(
+                        "Thread did not finish cleanly; forcefully closing the event loop."
+                    )
+
+            # Forcefully close the event loop to release resources
+            loop[0].close()
+
+            # dereference the loop and iothread
+            loop[0] = None
+            iothread[0] = None
+
+
+atexit.register(cleanup_resources)
+
+
 async def _runner(coro: Coroutine[Any, Any, T]) -> T | BaseException:
     """
     Await a coroutine and return the result of running it. If awaiting the coroutine raises an
@@ -105,10 +155,10 @@ def _get_loop() -> asyncio.AbstractEventLoop:
             if loop[0] is None:
                 new_loop = asyncio.new_event_loop()
                 loop[0] = new_loop
-                th = threading.Thread(target=new_loop.run_forever, name="zarrIO")
-                th.daemon = True
-                th.start()
-                iothread[0] = th
+                iothread[0] = threading.Thread(target=new_loop.run_forever, name="zarr_io")
+                assert iothread[0] is not None
+                iothread[0].daemon = True
+                iothread[0].start()
     assert loop[0] is not None
     return loop[0]
 

src/zarr/storage/local.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 import io
 import os
 import shutil
@@ -8,7 +9,7 @@
 
 from zarr.abc.store import ByteRangeRequest, Store
 from zarr.core.buffer import Buffer
-from zarr.core.common import concurrent_map, to_thread
+from zarr.core.common import concurrent_map
 
 if TYPE_CHECKING:
     from collections.abc import AsyncGenerator, Iterable
@@ -134,7 +135,7 @@ async def get(
         path = self.root / key
 
         try:
-            return await to_thread(_get, path, prototype, byte_range)
+            return await asyncio.to_thread(_get, path, prototype, byte_range)
         except (FileNotFoundError, IsADirectoryError, NotADirectoryError):
             return None
 
@@ -159,7 +160,7 @@ async def get_partial_values(
             assert isinstance(key, str)
             path = self.root / key
             args.append((_get, path, prototype, byte_range))
-        return await concurrent_map(args, to_thread, limit=None)  # TODO: fix limit
+        return await concurrent_map(args, asyncio.to_thread, limit=None)  # TODO: fix limit
 
     async def set(self, key: str, value: Buffer) -> None:
         return await self._set(key, value)
@@ -178,7 +179,7 @@ async def _set(self, key: str, value: Buffer, exclusive: bool = False) -> None:
         if not isinstance(value, Buffer):
             raise TypeError("LocalStore.set(): `value` must a Buffer instance")
         path = self.root / key
-        await to_thread(_put, path, value, start=None, exclusive=exclusive)
+        await asyncio.to_thread(_put, path, value, start=None, exclusive=exclusive)
 
     async def set_partial_values(
         self, key_start_values: Iterable[tuple[str, int, bytes | bytearray | memoryview]]
@@ -189,19 +190,19 @@ async def set_partial_values(
             assert isinstance(key, str)
             path = self.root / key
             args.append((_put, path, value, start))
-        await concurrent_map(args, to_thread, limit=None)  # TODO: fix limit
+        await concurrent_map(args, asyncio.to_thread, limit=None)  # TODO: fix limit
 
     async def delete(self, key: str) -> None:
         self._check_writable()
         path = self.root / key
         if path.is_dir():  # TODO: support deleting directories? shutil.rmtree?
             shutil.rmtree(path)
         else:
-            await to_thread(path.unlink, True)  # Q: we may want to raise if path is missing
+            await asyncio.to_thread(path.unlink, True)  # Q: we may want to raise if path is missing
 
     async def exists(self, key: str) -> bool:
         path = self.root / key
-        return await to_thread(path.is_file)
+        return await asyncio.to_thread(path.is_file)
 
     async def list(self) -> AsyncGenerator[str, None]:
         """Retrieve all keys in the store.

tests/v3/test_config.py

@@ -42,6 +42,7 @@ def test_config_defaults_set() -> None:
             "default_zarr_version": 3,
             "array": {"order": "C"},
             "async": {"concurrency": 10, "timeout": None},
+            "threading": {"max_workers": None},
             "json_indent": 2,
             "codec_pipeline": {
                 "path": "zarr.codecs.pipeline.BatchedCodecPipeline",