Fix DistribType for ddp_cpu (spawn) (#7492)

Author: alanhdu

CHANGELOG.md

@@ -90,6 +90,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed recursive passing of `wrong_type` keyword argument in `pytorch_lightning.utilities.apply_to_collection` ([#7433](https://github.com/PyTorchLightning/pytorch-lightning/pull/7433))
 
 
+- Fixed setting correct `DistribType` for `ddp_cpu` (spawn) backend ([#7492](https://github.com/PyTorchLightning/pytorch-lightning/pull/7492))
+
 
 ## [1.3.1] - 2021-05-11
 

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -519,7 +519,7 @@ def set_distributed_mode(self, distributed_backend: Optional[str] = None):
 
         # special case with DDP on CPUs
         if self.distributed_backend == "ddp_cpu":
-            self._distrib_type = DistributedType.DDP
+            self._distrib_type = DistributedType.DDP_SPAWN
             if self.num_gpus > 0:
                 rank_zero_warn(
                     'You requested one or more GPUs, but set the backend to `ddp_cpu`. Training will not use GPUs.'

tests/accelerators/test_accelerator_connector.py

@@ -430,13 +430,15 @@ def test_ipython_incompatible_backend_error(*_):
     with pytest.raises(MisconfigurationException, match="backend ddp is not compatible"):
         Trainer(accelerator="ddp", gpus=2)
 
-    with pytest.raises(MisconfigurationException, match="backend ddp is not compatible"):
-        Trainer(accelerator="ddp_cpu", num_processes=2)
-
     with pytest.raises(MisconfigurationException, match="backend ddp2 is not compatible"):
         Trainer(accelerator="ddp2", gpus=2)
 
 
+@mock.patch("pytorch_lightning.utilities._IS_INTERACTIVE", return_value=True)
+def test_ipython_compatible_backend(*_):
+    Trainer(accelerator="ddp_cpu", num_processes=2)
+
+
 @pytest.mark.parametrize(
     ["accelerator", "plugin"],
     [('ddp_spawn', 'ddp_sharded'), (None, 'ddp_sharded')],

tests/trainer/test_trainer.py

@@ -1132,7 +1132,7 @@ def test_num_sanity_val_steps_neg_one(tmpdir, limit_val_batches):
         ),
         (
             dict(accelerator="ddp_cpu", num_processes=2, gpus=None),
-            dict(_distrib_type=DistributedType.DDP, _device_type=DeviceType.CPU, num_gpus=0, num_processes=2),
+            dict(_distrib_type=DistributedType.DDP_SPAWN, _device_type=DeviceType.CPU, num_gpus=0, num_processes=2),
         ),
         (
             dict(accelerator="ddp2", gpus=None),
@@ -1152,7 +1152,7 @@ def test_num_sanity_val_steps_neg_one(tmpdir, limit_val_batches):
         ),
         (
             dict(accelerator="ddp_cpu", num_processes=2, gpus=1),
-            dict(_distrib_type=DistributedType.DDP, _device_type=DeviceType.CPU, num_gpus=0, num_processes=2),
+            dict(_distrib_type=DistributedType.DDP_SPAWN, _device_type=DeviceType.CPU, num_gpus=0, num_processes=2),
         ),
         (
             dict(accelerator="ddp2", gpus=1),