Add strategy="auto" support on the 1.9.x branch

src/lightning_fabric/CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed an issue causing a wrong environment plugin to be selected when `accelerator=tpu` and `devices > 1` ([#16806](https://github.com/Lightning-AI/lightning/pull/16806))
 - Fixed parsing of defaults for `--accelerator` and `--precision` in Fabric CLI when `accelerator` and `precision` are set to non-default values in the code ([#16818](https://github.com/Lightning-AI/lightning/pull/16818))
+- Fixed `Fabric(strategy="auto")` support ([#16916](https://github.com/Lightning-AI/lightning/pull/16916))
 
 
 ## [1.9.2] - 2023-02-15

src/lightning_fabric/connector.py

@@ -145,7 +145,7 @@ def __init__(
         self.cluster_environment: ClusterEnvironment = self._choose_and_init_cluster_environment()
 
         # 4. Instantiate Strategy - Part 1
-        if self._strategy_flag is None:
+        if self._strategy_flag in (None, "auto"):
             self._strategy_flag = self._choose_strategy()
         # In specific cases, ignore user selection and fall back to a different strategy
         self._check_strategy_and_fallback()
@@ -184,7 +184,11 @@ def _check_config_and_set_final_flags(
         if strategy is not None:
             self._strategy_flag = strategy
 
-        if strategy is not None and strategy not in self._registered_strategies and not isinstance(strategy, Strategy):
+        if (
+            strategy not in (None, "auto")
+            and strategy not in self._registered_strategies
+            and not isinstance(strategy, Strategy)
+        ):
             raise ValueError(
                 f"You selected an invalid strategy name: `strategy={strategy!r}`."
                 " It must be either a string or an instance of `lightning.fabric.strategies.Strategy`."

src/pytorch_lightning/CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 ### Fixed
 
 - Fixed an issue causing a wrong environment plugin to be selected when `accelerator=tpu` and `devices > 1` ([#16806](https://github.com/Lightning-AI/lightning/pull/16806))
+- Fixed `Fabric(strategy="auto")` support. It will choose DDP over DDP-spawn, contrary to `strategy=None` (default) ([#16916](https://github.com/Lightning-AI/lightning/pull/16916))
 
 
 ## [1.9.2] - 2023-02-15

src/pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -207,7 +207,7 @@ def __init__(
         self.cluster_environment: ClusterEnvironment = self._choose_and_init_cluster_environment()
 
         # 4. Instantiate Strategy - Part 1
-        if self._strategy_flag is None:
+        if self._strategy_flag in (None, "auto"):
             self._strategy_flag = self._choose_strategy()
         # In specific cases, ignore user selection and fall back to a different strategy
         self._check_strategy_and_fallback()
@@ -273,7 +273,11 @@ def _check_config_and_set_final_flags(
                     " you can use `Trainer(strategy='ddp_spawn', accelerator='tpu')` instead."
                 )
 
-        if strategy is not None and strategy not in self._registered_strategies and not isinstance(strategy, Strategy):
+        if (
+            strategy not in (None, "auto")
+            and strategy not in self._registered_strategies
+            and not isinstance(strategy, Strategy)
+        ):
             raise ValueError(
                 f"You selected an invalid strategy name: `strategy={strategy!r}`."
                 " It must be either a string or an instance of `pytorch_lightning.strategies.Strategy`."
@@ -639,6 +643,9 @@ def _choose_strategy(self) -> Union[Strategy, str]:
         if len(self._parallel_devices) > 1:
             if _IS_INTERACTIVE:
                 return "ddp_fork"
+            if self._strategy_flag == "auto":
+                # None chooses "ddp_spawn" for backwards compatibility, auto chooses "ddp" for future compatibility
+                return "ddp"
             return "ddp_spawn"
 
         return DDPStrategy.strategy_name

tests/tests_fabric/test_connector.py

@@ -893,3 +893,19 @@ def get_defaults(cls):
     # defaults should match on the intersection of argument names
     for name, connector_default in connector_defaults.items():
         assert connector_default == fabric_defaults[name]
+
+
+@mock.patch("lightning_fabric.accelerators.cuda.num_cuda_devices", return_value=2)
+@mock.patch("lightning_fabric.accelerators.mps.MPSAccelerator.is_available", return_value=False)
+def test_connector_auto_selection(*_):
+    connector = _Connector(accelerator="auto", strategy=None, devices="auto")
+    assert isinstance(connector.accelerator, CUDAAccelerator)
+    assert isinstance(connector.strategy, DDPStrategy)
+    assert isinstance(connector.strategy.launcher, _SubprocessScriptLauncher)
+    assert connector._devices_flag == [0, 1]
+
+    connector = _Connector(accelerator="auto", strategy="auto", devices="auto")
+    assert isinstance(connector.accelerator, CUDAAccelerator)
+    assert isinstance(connector.strategy, DDPStrategy)
+    assert isinstance(connector.strategy.launcher, _SubprocessScriptLauncher)
+    assert connector._devices_flag == [0, 1]

tests/tests_pytorch/trainer/connectors/test_accelerator_connector.py

@@ -892,3 +892,15 @@ def get_defaults(cls):
     for name, connector_default in connector_defaults.items():
         name = lut.get(name, name)
         assert connector_default == trainer_defaults[name]
+
+
+def test_connector_auto_selection(cuda_count_2, mps_count_0):
+    trainer = Trainer(accelerator="auto", strategy=None, devices="auto")
+    assert isinstance(trainer.accelerator, CUDAAccelerator)
+    assert isinstance(trainer.strategy, DDPSpawnStrategy)
+    assert trainer.num_devices == 2
+
+    trainer = Trainer(accelerator="auto", strategy="auto", devices="auto")
+    assert isinstance(trainer.accelerator, CUDAAccelerator)
+    assert isinstance(trainer.strategy, DDPStrategy)
+    assert trainer.num_devices == 2