concurrency limit 64 -> 10 (#3578)

* concurrency limit 64 -> 10

* update reference to concurrency limit in docs

* update tests

* remove release note

Author: d-v-b

changes/3526.feature.md

@@ -175,13 +175,18 @@ Coming soon.
 
 ## Parallel computing and synchronization
 
-Zarr is designed to support parallel computing and enables concurrent reads and writes to arrays. This section covers how to optimize Zarr's concurrency settings for different parallel computing scenarios.
+Zarr is designed to support parallel computing and enables concurrent reads and writes to arrays.
+This section covers how to optimize Zarr's concurrency settings for different parallel computing
+scenarios.
 
 ### Concurrent I/O operations
 
-Zarr uses asynchronous I/O internally to enable concurrent reads and writes across multiple chunks. The level of concurrency is controlled by the `async.concurrency` configuration setting, which determines the maximum number of concurrent I/O operations.
+Zarr uses asynchronous I/O internally to enable concurrent reads and writes across multiple chunks.
+The level of concurrency is controlled by the `async.concurrency` configuration setting, which
+determines the maximum number of concurrent I/O operations.
 
-The default value is 64, which provides good performance for most workloads. You can adjust this value based on your specific needs:
+The default value is 10, which is a conservative value. You may get improved performance by tuning
+the concurrency limit. You can adjust this value based on your specific needs:
 
 ```python
 import zarr

docs/user-guide/performance.md

@@ -97,7 +97,7 @@ def enable_gpu(self) -> ConfigSet:
                 "order": "C",
                 "write_empty_chunks": False,
             },
-            "async": {"concurrency": 64, "timeout": None},
+            "async": {"concurrency": 10, "timeout": None},
             "threading": {"max_workers": None},
             "json_indent": 2,
             "codec_pipeline": {

src/zarr/core/config.py

@@ -54,7 +54,7 @@ def test_config_defaults_set() -> None:
                     "order": "C",
                     "write_empty_chunks": False,
                 },
-                "async": {"concurrency": 64, "timeout": None},
+                "async": {"concurrency": 10, "timeout": None},
                 "threading": {"max_workers": None},
                 "json_indent": 2,
                 "codec_pipeline": {
@@ -100,15 +100,15 @@ def test_config_defaults_set() -> None:
         ]
     )
     assert config.get("array.order") == "C"
-    assert config.get("async.concurrency") == 64
+    assert config.get("async.concurrency") == 10
     assert config.get("async.timeout") is None
     assert config.get("codec_pipeline.batch_size") == 1
     assert config.get("json_indent") == 2
 
 
 @pytest.mark.parametrize(
     ("key", "old_val", "new_val"),
-    [("array.order", "C", "F"), ("async.concurrency", 64, 128), ("json_indent", 2, 0)],
+    [("array.order", "C", "F"), ("async.concurrency", 10, 128), ("json_indent", 2, 0)],
 )
 def test_config_defaults_can_be_overridden(key: str, old_val: Any, new_val: Any) -> None:
     assert config.get(key) == old_val