Set default strategy to ddp_fork in interactive environments (#13746)

src/pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -87,6 +87,7 @@ from pytorch_lightning.utilities.imports import (
     _HOROVOD_AVAILABLE,
     _HPU_AVAILABLE,
     _IPU_AVAILABLE,
+    _IS_INTERACTIVE,
     _TORCH_GREATER_EQUAL_1_11,
     _TPU_AVAILABLE,
 )
@@ -588,7 +589,9 @@ class AcceleratorConnector:
             # 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:
-            return DDPSpawnStrategy.strategy_name
+            if _IS_INTERACTIVE:
+                return "ddp_fork"
+            return "ddp_spawn"
 
         return DDPStrategy.strategy_name
 

tests/tests_pytorch/accelerators/test_accelerator_connector.py

@@ -424,6 +424,18 @@ def test_strategy_choice_ddp_spawn_cpu():
     assert trainer.strategy.launcher._start_method == "spawn"
 
 
+@RunIf(skip_windows=True)
+@mock.patch("pytorch_lightning.trainer.connectors.accelerator_connector._IS_INTERACTIVE", True)
+def test_strategy_choice_ddp_fork_in_interactive():
+    """Test that when accelerator and strategy are unspecified, the connector chooses DDP Fork in interactive
+    environments by default."""
+    trainer = Trainer(devices=2)
+    assert isinstance(trainer.accelerator, CPUAccelerator)
+    assert isinstance(trainer.strategy, DDPSpawnStrategy)
+    assert isinstance(trainer.strategy.cluster_environment, LightningEnvironment)
+    assert trainer.strategy.launcher._start_method == "fork"
+
+
 @RunIf(skip_windows=True)
 def test_strategy_choice_ddp_fork_cpu():
     trainer = Trainer(strategy="ddp_fork", accelerator="cpu", devices=2)