Make subprocess launcher the default in Lite (#16388)

Author: awaelchli

src/lightning_fabric/CHANGELOG.md

@@ -35,6 +35,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Merged the implementation of `DDPSpawnStrategy` into `DDPStrategy` and removed `DDPSpawnStrategy` ([#14952](https://github.com/Lightning-AI/lightning/issues/14952))
 - The dataloader wrapper returned from `.setup_dataloaders()` now calls `.set_epoch()` on the distributed sampler if one is used ([#16101](https://github.com/Lightning-AI/lightning/issues/16101))
 - Renamed `Strategy.reduce` to `Strategy.all_reduce` in all strategies ([#16370](https://github.com/Lightning-AI/lightning/issues/16370))
+- When using multiple devices, the strategy now defaults to "ddp" instead of "ddp_spawn" when none is set ([#16388](https://github.com/Lightning-AI/lightning/issues/16388))
 
 ### Removed
 

src/lightning_fabric/connector.py

@@ -383,11 +383,8 @@ def _choose_strategy(self) -> Union[Strategy, str]:
                 device = "cpu"
             # TODO: lazy initialized device, then here could be self._strategy_flag = "single_device"
             return SingleDeviceStrategy(device=device)  # type: ignore
-        if len(self._parallel_devices) > 1:
-            if _IS_INTERACTIVE:
-                return "ddp_fork"
-            return "ddp_spawn"
-
+        if len(self._parallel_devices) > 1 and _IS_INTERACTIVE:
+            return "ddp_fork"
         return "ddp"
 
     def _check_strategy_and_fallback(self) -> None:

tests/tests_fabric/test_connector.py

@@ -46,6 +46,7 @@
     XLAStrategy,
 )
 from lightning_fabric.strategies.ddp import _DDP_FORK_ALIASES
+from lightning_fabric.strategies.launchers.subprocess_script import _SubprocessScriptLauncher
 from lightning_fabric.utilities.exceptions import MisconfigurationException
 
 
@@ -414,6 +415,15 @@ def test_validate_precision_type(precision):
         _Connector(precision=precision)
 
 
+def test_multi_device_default_strategy():
+    """The default strategy when multiple devices are selected is "ddp" with the subprocess launcher."""
+    connector = _Connector(strategy=None, accelerator="cpu", devices=2)
+    assert isinstance(connector.accelerator, CPUAccelerator)
+    assert isinstance(connector.strategy, DDPStrategy)
+    assert connector.strategy._start_method == "popen"
+    assert isinstance(connector.strategy.launcher, _SubprocessScriptLauncher)
+
+
 def test_strategy_choice_ddp_spawn_cpu():
     connector = _Connector(strategy="ddp_spawn", accelerator="cpu", devices=2)
     assert isinstance(connector.accelerator, CPUAccelerator)