Support val_check_interval values higher than number of training batches

CHANGELOG.md

@@ -9,8 +9,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
--
-
+- Added support for setting `val_check_interval` to a value higher than the amount of training batches when `check_val_every_n_epoch=None` ([#11993](https://github.com/PyTorchLightning/pytorch-lightning/pull/11993))
 
 - Include the `pytorch_lightning` version as a header in the CLI config files ([#12532](https://github.com/PyTorchLightning/pytorch-lightning/pull/12532))
 

pytorch_lightning/loops/epoch/training_epoch_loop.py

@@ -501,9 +501,9 @@ def _get_monitor_value(self, key: str) -> Any:
         return self.trainer.callback_metrics.get(key)
 
     def _should_check_val_epoch(self):
-        return (
-            self.trainer.enable_validation
-            and (self.trainer.current_epoch + 1) % self.trainer.check_val_every_n_epoch == 0
+        return self.trainer.enable_validation and (
+            self.trainer.check_val_every_n_epoch is None
+            or (self.trainer.current_epoch + 1) % self.trainer.check_val_every_n_epoch == 0
         )
 
     def _should_check_val_fx(self, batch_idx: int, is_last_batch: bool) -> bool:
@@ -524,7 +524,13 @@ def _should_check_val_fx(self, batch_idx: int, is_last_batch: bool) -> bool:
         if isinstance(self.trainer.limit_train_batches, int) and is_infinite_dataset:
             is_val_check_batch = (batch_idx + 1) % self.trainer.limit_train_batches == 0
         elif self.trainer.val_check_batch != float("inf"):
-            is_val_check_batch = (batch_idx + 1) % self.trainer.val_check_batch == 0
+            # if `check_val_every_n_epoch is `None`, run a validation loop every n training batches
+            # else condition it based on the batch_idx of the current epoch
+            current_iteration = (
+                self._batches_that_stepped if self.trainer.check_val_every_n_epoch is None else batch_idx
+            )
+            is_val_check_batch = (current_iteration + 1) % self.trainer.val_check_batch == 0
+
         return is_val_check_batch
 
     def _save_loggers_on_train_batch_end(self) -> None:

pytorch_lightning/trainer/connectors/data_connector.py

@@ -71,14 +71,21 @@ def _should_reload_val_dl(self) -> bool:
 
     def on_trainer_init(
         self,
-        check_val_every_n_epoch: int,
+        val_check_interval: Union[int, float],
         reload_dataloaders_every_n_epochs: int,
+        check_val_every_n_epoch: Optional[int],
     ) -> None:
         self.trainer.datamodule = None
 
-        if not isinstance(check_val_every_n_epoch, int):
+        if check_val_every_n_epoch is not None and not isinstance(check_val_every_n_epoch, int):
             raise MisconfigurationException(
-                f"check_val_every_n_epoch should be an integer. Found {check_val_every_n_epoch}"
+                f"`check_val_every_n_epoch` should be an integer, found {check_val_every_n_epoch!r}."
+            )
+
+        if check_val_every_n_epoch is None and isinstance(val_check_interval, float):
+            raise MisconfigurationException(
+                "`val_check_interval` should be an integer when `check_val_every_n_epoch=None`,"
+                f" found {val_check_interval!r}."
             )
 
         self.trainer.check_val_every_n_epoch = check_val_every_n_epoch

pytorch_lightning/trainer/trainer.py

@@ -145,7 +145,7 @@ def __init__(
         enable_progress_bar: bool = True,
         overfit_batches: Union[int, float] = 0.0,
         track_grad_norm: Union[int, float, str] = -1,
-        check_val_every_n_epoch: int = 1,
+        check_val_every_n_epoch: Optional[int] = 1,
         fast_dev_run: Union[int, bool] = False,
         accumulate_grad_batches: Optional[Union[int, Dict[int, int]]] = None,
         max_epochs: Optional[int] = None,
@@ -242,10 +242,11 @@ def __init__(
                 :paramref:`~pytorch_lightning.trainer.trainer.Trainer.callbacks`.
                 Default: ``True``.
 
-            check_val_every_n_epoch: Check val every n train epochs.
+            check_val_every_n_epoch: Perform a validation loop every after every `N` training epochs. If ``None``,
+                validation will be done solely based on the number of training batches, requiring ``val_check_interval``
+                to be an integer value.
                 Default: ``1``.
 
-
             default_root_dir: Default path for logs and weights when no logger/ckpt_callback passed.
                 Default: ``os.getcwd()``.
                 Can be remote file paths such as `s3://mybucket/path` or 'hdfs://path/'
@@ -403,7 +404,8 @@ def __init__(
 
             val_check_interval: How often to check the validation set. Pass a ``float`` in the range [0.0, 1.0] to check
                 after a fraction of the training epoch. Pass an ``int`` to check after a fixed number of training
-                batches.
+                batches. An ``int`` value can only be higher than the number of training batches when
+                ``check_val_every_n_epoch=None``.
                 Default: ``1.0``.
 
             enable_model_summary: Whether to enable model summarization by default.
@@ -524,8 +526,9 @@ def __init__(
         # init data flags
         self.check_val_every_n_epoch: int
         self._data_connector.on_trainer_init(
-            check_val_every_n_epoch,
+            val_check_interval,
             reload_dataloaders_every_n_epochs,
+            check_val_every_n_epoch,
         )
 
         # gradient clipping
@@ -1829,11 +1832,12 @@ def reset_train_dataloader(self, model: Optional["pl.LightningModule"] = None) -
 
         if isinstance(self.val_check_interval, int):
             self.val_check_batch = self.val_check_interval
-            if self.val_check_batch > self.num_training_batches:
+            if self.val_check_batch > self.num_training_batches and self.check_val_every_n_epoch is not None:
                 raise ValueError(
                     f"`val_check_interval` ({self.val_check_interval}) must be less than or equal "
                     f"to the number of the training batches ({self.num_training_batches}). "
                     "If you want to disable validation set `limit_val_batches` to 0.0 instead."
+                    "If you want to validate based on the total training batches, set `check_val_every_n_epoch=None`."
                 )
         else:
             if not has_len_all_ranks(self.train_dataloader, self.strategy, module):

tests/trainer/flags/test_check_val_every_n_epoch.py

@@ -12,9 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import pytest
+from torch.utils.data import DataLoader
 
-from pytorch_lightning.trainer import Trainer
-from tests.helpers import BoringModel
+from pytorch_lightning.trainer.trainer import Trainer
+from tests.helpers import BoringModel, RandomDataset
 
 
 @pytest.mark.parametrize(
@@ -46,3 +47,35 @@ def on_validation_epoch_start(self) -> None:
 
     assert model.val_epoch_calls == expected_val_loop_calls
     assert model.val_batches == expected_val_batches
+
+
+def test_check_val_every_n_epoch_with_max_steps(tmpdir):
+    data_samples_train = 2
+    check_val_every_n_epoch = 3
+    max_epochs = 4
+
+    class TestModel(BoringModel):
+        def __init__(self):
+            super().__init__()
+            self.validation_called_at_step = set()
+
+        def validation_step(self, *args):
+            self.validation_called_at_step.add(self.global_step)
+            return super().validation_step(*args)
+
+        def train_dataloader(self):
+            return DataLoader(RandomDataset(32, data_samples_train))
+
+    model = TestModel()
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        max_steps=data_samples_train * max_epochs,
+        check_val_every_n_epoch=check_val_every_n_epoch,
+        num_sanity_val_steps=0,
+    )
+
+    trainer.fit(model)
+
+    assert trainer.current_epoch == max_epochs
+    assert trainer.global_step == max_epochs * data_samples_train
+    assert list(model.validation_called_at_step) == [data_samples_train * check_val_every_n_epoch]

tests/trainer/flags/test_val_check_interval.py

@@ -14,9 +14,12 @@
 import logging
 
 import pytest
+from torch.utils.data import DataLoader
 
-from pytorch_lightning.trainer import Trainer
-from tests.helpers import BoringModel
+from pytorch_lightning.trainer.trainer import Trainer
+from pytorch_lightning.utilities.exceptions import MisconfigurationException
+from tests.helpers import BoringModel, RandomDataset
+from tests.helpers.boring_model import RandomIterableDataset
 
 
 @pytest.mark.parametrize("max_epochs", [1, 2, 3])
@@ -57,3 +60,66 @@ def test_val_check_interval_info_message(caplog, value):
     with caplog.at_level(logging.INFO):
         Trainer()
     assert message not in caplog.text
+
+
+@pytest.mark.parametrize("use_infinite_dataset", [True, False])
+def test_validation_check_interval_exceed_data_length_correct(tmpdir, use_infinite_dataset):
+    data_samples_train = 4
+    max_epochs = 3
+    max_steps = data_samples_train * max_epochs
+
+    class TestModel(BoringModel):
+        def __init__(self):
+            super().__init__()
+            self.validation_called_at_step = set()
+
+        def validation_step(self, *args):
+            self.validation_called_at_step.add(self.global_step)
+            return super().validation_step(*args)
+
+        def train_dataloader(self):
+            train_ds = (
+                RandomIterableDataset(32, count=max_steps + 100)
+                if use_infinite_dataset
+                else RandomDataset(32, length=data_samples_train)
+            )
+            return DataLoader(train_ds)
+
+    model = TestModel()
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        limit_val_batches=1,
+        max_steps=max_steps,
+        val_check_interval=3,
+        check_val_every_n_epoch=None,
+        num_sanity_val_steps=0,
+    )
+
+    trainer.fit(model)
+
+    assert trainer.current_epoch == 1 if use_infinite_dataset else max_epochs
+    assert trainer.global_step == max_steps
+    assert sorted(list(model.validation_called_at_step)) == [3, 6, 9, 12]
+
+
+def test_validation_check_interval_exceed_data_length_wrong():
+    trainer = Trainer(
+        limit_train_batches=10,
+        val_check_interval=100,
+    )
+
+    model = BoringModel()
+    with pytest.raises(ValueError, match="must be less than or equal to the number of the training batches"):
+        trainer.fit(model)
+
+
+def test_val_check_interval_float_with_none_check_val_every_n_epoch():
+    """Test that an exception is raised when `val_check_interval` is set to float with
+    `check_val_every_n_epoch=None`"""
+    with pytest.raises(
+        MisconfigurationException, match="`val_check_interval` should be an integer when `check_val_every_n_epoch=None`"
+    ):
+        Trainer(
+            val_check_interval=0.5,
+            check_val_every_n_epoch=None,
+        )