Fix dataloaders are not reset when tuning the model

CHANGELOG.md

@@ -83,6 +83,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
+- Fixed dataloaders are not reset when tuning the model ([#7566](https://github.com/PyTorchLightning/pytorch-lightning/pull/7566))
+
 
 - Fixed parsing of multiple training dataloaders ([#7433](https://github.com/PyTorchLightning/pytorch-lightning/pull/7433))
 

pytorch_lightning/tuner/batch_size_scaling.py

@@ -160,7 +160,10 @@ def _run_power_scaling(
             else:
                 raise  # some other error not memory related
 
-        if not changed:
+        if changed:
+            # Force the train dataloader to reset as the batch size has changed
+            trainer.reset_train_dataloader(model)
+        else:
             break
     return new_size
 
@@ -192,7 +195,10 @@ def _run_binsearch_scaling(
             else:
                 new_size, changed = _adjust_batch_size(trainer, batch_arg_name, factor=2.0, desc='succeeded')
 
-            if not changed:
+            if changed:
+                # Force the train dataloader to reset as the batch size has changed
+                trainer.reset_train_dataloader(model)
+            else:
                 break
 
         except RuntimeError as exception:

tests/tuner/test_scale_batch_size.py

@@ -24,14 +24,14 @@
 from pytorch_lightning.utilities import AMPType
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
 from tests.base import EvalModelTemplate
-from tests.helpers import BoringDataModule, BoringModel
+from tests.helpers import BoringDataModule, BoringModel, RandomDataset
 from tests.helpers.datamodules import MNISTDataModule
 from tests.helpers.runif import RunIf
 
 
 class BatchSizeDataModule(BoringDataModule):
 
-    def __init__(self, batch_size=None):
+    def __init__(self, batch_size):
         super().__init__()
         if batch_size is not None:
             self.batch_size = batch_size
@@ -42,21 +42,23 @@ def train_dataloader(self):
 
 class BatchSizeModel(BoringModel):
 
-    def __init__(self, batch_size=None):
+    def __init__(self, batch_size):
         super().__init__()
         if batch_size is not None:
             self.batch_size = batch_size
 
+    def train_dataloader(self):
+        return DataLoader(RandomDataset(32, 64), batch_size=getattr(self, "batch_size", 1))
 
-@pytest.mark.parametrize(
-    "model,datamodule", [
-        (BatchSizeModel(2), None),
-        (BatchSizeModel(2), BatchSizeDataModule(2)),
-        (BatchSizeModel(2), BatchSizeDataModule(None)),
-        (BatchSizeModel(None), BatchSizeDataModule(2)),
-    ]
-)
-def test_scale_batch_size_method_with_model_or_datamodule(tmpdir, model, datamodule):
+
+@pytest.mark.parametrize(["model_bs", "dm_bs"], [
+    (2, -1),
+    (2, 2),
+    (2, None),
+    (None, 2),
+    (16, 16),
+])
+def test_scale_batch_size_method_with_model_or_datamodule(tmpdir, model_bs, dm_bs):
     """ Test the tuner method `Tuner.scale_batch_size` with a datamodule. """
     trainer = Trainer(
         default_root_dir=tmpdir,
@@ -65,14 +67,21 @@ def test_scale_batch_size_method_with_model_or_datamodule(tmpdir, model, datamod
         max_epochs=1,
     )
     tuner = Tuner(trainer)
-    new_batch_size = tuner.scale_batch_size(
-        model=model, mode="binsearch", init_val=4, max_trials=2, datamodule=datamodule
-    )
+
+    model = BatchSizeModel(model_bs)
+    datamodule = BatchSizeDataModule(dm_bs) if dm_bs != -1 else None
+
+    new_batch_size = tuner.scale_batch_size(model, mode="binsearch", init_val=4, max_trials=2, datamodule=datamodule)
     assert new_batch_size == 16
-    if hasattr(model, "batch_size"):
-        assert model.batch_size == 16
-    if datamodule is not None and hasattr(datamodule, "batch_size"):
-        assert datamodule.batch_size == 16
+
+    if model_bs is not None:
+        assert model.batch_size == new_batch_size
+        if dm_bs == -1:
+            # datamodule batch size takes precedence
+            assert trainer.train_dataloader.loaders.batch_size == new_batch_size
+    if dm_bs not in (-1, None):
+        assert datamodule.batch_size == new_batch_size
+        assert trainer.train_dataloader.loaders.batch_size == new_batch_size
 
 
 def test_model_reset_correctly(tmpdir):