Update DataLoader.persistent_workers warnings in ddp_spawn (#6762)

Co-authored-by: Carlos Mocholi <carlossmocholi@gmail.com>

CHANGELOG.md

@@ -116,6 +116,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Changed `PyTorchProfiler` to use `torch.autograd.profiler.record_function` to record functions ([#6349](https://github.com/PyTorchLightning/pytorch-lightning/pull/6349))
 
 
+- Changed warnings and recommendations for dataloaders in `ddp_spawn` ([#6762](https://github.com/PyTorchLightning/pytorch-lightning/pull/6762/))
+
+
 ### Deprecated
 
 - Deprecated `TrainerTrainingTricksMixin` in favor of a separate utilities module for NaN/Inf detection for gradients and parameters ([#6834](https://github.com/PyTorchLightning/pytorch-lightning/pull/6834/))

pytorch_lightning/trainer/data_loading.py

@@ -61,18 +61,36 @@ def _worker_check(self, dataloader: DataLoader, name: str) -> None:
         using_spawn = self.accelerator_connector.distributed_backend == "ddp_spawn"
         if is_dataloader and not on_windows:
             if dataloader.num_workers > 0 and using_spawn:
-                rank_zero_warn(
-                    'Dataloader(num_workers>0) and ddp_spawn do not mix well!'
-                    ' Your performance might suffer dramatically.'
-                    ' Please consider setting accelerator=ddp to use num_workers > 0'
-                    ' (this is a bottleneck of Python .spawn() and PyTorch'
-                )
+                # checks for the attr persistent_workers available in pytorch >= 1.7
+                if hasattr(dataloader, "persistent_workers"):
+                    if not dataloader.persistent_workers:
+                        rank_zero_warn(
+                            'num_workers>0, persistent_workers=False, and accelerator=ddp_spawn'
+                            ' may result in data loading bottlenecks.'
+                            ' Consider setting persistent_workers=True'
+                            ' (this is a limitation of Python .spawn() and PyTorch)'
+                        )
+                else:
+                    rank_zero_warn(
+                        'num_workers>0 and accelerator=ddp_spawn do not mix well'
+                        ' and may result in data loading bottlenecks.'
+                        ' Consider setting accelerator=ddp to use num_workers>0'
+                        ' (this is a limitation of Python .spawn() and PyTorch)'
+                    )
 
             elif dataloader.num_workers == 0 and using_spawn:
-                rank_zero_warn(
-                    'You are using `accelerator=ddp_spawn` with num_workers=0.'
-                    ' For much faster performance, switch to `accelerator=ddp` and set `num_workers>0`'
-                )
+                # checks for the attr persistent_workers available in pytorch >= 1.7
+                if hasattr(dataloader, "persistent_workers"):
+                    if not dataloader.persistent_workers:
+                        rank_zero_warn(
+                            'accelerator=ddp_spawn and num_workers=0 may result in data loading bottlenecks.'
+                            ' Consider setting num_workers>0 and persistent_workers=True'
+                        )
+                else:
+                    rank_zero_warn(
+                        'accelerator=ddp_spawn and num_workers=0 may result in data loading bottlenecks.'
+                        ' Consider setting accelerator=ddp and set num_workers>0'
+                    )
 
             elif dataloader.num_workers <= 2 and multiprocessing.cpu_count() > 2 and not using_spawn:
                 num_cpus = multiprocessing.cpu_count()

tests/trainer/test_data_loading.py

@@ -102,3 +102,25 @@ def check_replace_distrubuted_sampler(tmpdir, save_preds_on_dl_idx, accelerator,
 @pytest.mark.parametrize("mode", [1, 2])
 def test_replace_distrubuted_sampler_custom_dataloader_custom_batch_sampler(tmpdir, mode):
     check_replace_distrubuted_sampler(tmpdir, True, "ddp", 2, 2, mode)
+
+
+@pytest.mark.parametrize("num_workers", [0, 1])
+def test_dataloader_warnings(num_workers):
+
+    class TestModel(BoringModel):
+
+        def on_train_start(self, *_) -> None:
+            raise SystemExit()
+
+    dl = DataLoader(RandomDataset(32, 64), num_workers=num_workers)
+    if hasattr(dl, "persistent_workers"):
+        if num_workers == 0:
+            warn_str = "Consider setting num_workers>0 and persistent_workers=True"
+        else:
+            warn_str = "Consider setting persistent_workers=True"
+    else:
+        warn_str = "Consider setting accelerator=ddp"
+
+    trainer = Trainer(accelerator="ddp_spawn")
+    with pytest.warns(UserWarning, match=warn_str), pytest.raises(SystemExit):
+        trainer.fit(TestModel(), dl)