[HotFix] Resolve TPU Training

.gitignore

@@ -155,3 +155,5 @@ cifar-10-batches-py
 # ctags
 tags
 data
+MNIST
+runs

CHANGELOG.md

@@ -288,6 +288,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed missing `process_dataloader` call for `TPUSpawn` when in distributed mode ([#6015](https://github.com/PyTorchLightning/pytorch-lightning/pull/6015))
 
 
+- Fixed synchronization issues with TPU training ([#6027](https://github.com/PyTorchLightning/pytorch-lightning/pull/6027))
+
+
 ## [1.1.8] - 2021-02-08
 
 ### Fixed

pytorch_lightning/accelerators/accelerator.py

@@ -11,10 +11,11 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from typing import Any, Callable, Optional, TYPE_CHECKING, Union
+from typing import Any, Callable, Iterable, Optional, TYPE_CHECKING, Union
 
 import torch
 from torch.optim import Optimizer
+from torch.utils.data import DataLoader
 
 from pytorch_lightning.core import LightningModule
 from pytorch_lightning.plugins.precision import (
@@ -388,3 +389,11 @@ def all_gather(self, tensor: Union[torch.Tensor], group: Optional[Any] = None, s
             A tensor of shape (world_size, batch, ...)
         """
         return all_gather_ddp_if_available(tensor, group=group, sync_grads=sync_grads)
+
+    def process_dataloader(self, dataloader: Union[Iterable, DataLoader]) -> Union[Iterable, DataLoader]:
+        """Wraps the dataloader if necessary
+
+        Args:
+            dataloader: iterable. Ideally of type: :class:`torch.utils.data.DataLoader`
+        """
+        return self.training_type_plugin.process_dataloader(dataloader)

pytorch_lightning/callbacks/model_checkpoint.py

@@ -554,6 +554,14 @@ def _save_top_k_checkpoints(self, trainer, pl_module, metrics):
         epoch = metrics.get("epoch")
         step = metrics.get("step")
 
+        # when `val_loss` is being logged and no ModelCheckpoint is being provided
+        # `val_loss` will be selected for monitor and need to be reduced to
+        # prevent processes divergence
+        # TODO: Move this logic to logger_connector. This also needs to be fixed for any
+        # other monitor logged value which aren't produced from a Metric.
+        if self.monitor == "val_loss":
+            current = trainer.training_type_plugin.reduce(current, reduce_op="mean")
+
         if self.check_monitor_top_k(current):
             self._update_best_and_save(current, epoch, step, trainer, pl_module, metrics)
         elif self.verbose:

pytorch_lightning/plugins/training_type/tpu_spawn.py

@@ -10,7 +10,8 @@
 from pytorch_lightning.plugins.training_type.ddp_spawn import DDPSpawnPlugin
 from pytorch_lightning.plugins.training_type.utils import on_colab_kaggle
 from pytorch_lightning.utilities import _TPU_AVAILABLE, rank_zero_warn
-from pytorch_lightning.utilities.distributed import rank_zero_only
+from pytorch_lightning.utilities.distributed import rank_zero_only, ReduceOp
+from pytorch_lightning.utilities.exceptions import MisconfigurationException
 from pytorch_lightning.utilities.seed import seed_everything
 
 if _TPU_AVAILABLE:
@@ -46,10 +47,6 @@ def create_mp_queue(self):
     def distributed_sampler_kwargs(self) -> dict:
         return dict(num_replicas=xm.xrt_world_size(), rank=xm.get_ordinal())
 
-    @property
-    def should_finalize(self):
-        return self.world_size == 1
-
     @property
     def is_distributed(self):
         return self.world_size != 1
@@ -179,6 +176,29 @@ def reduce_early_stopping_decision(self, should_stop: bool) -> bool:
         should_stop = int(stop.item()) == self.world_size
         return should_stop
 
+    def reduce(self, output, group: Optional[Any] = None, reduce_op: Optional[Union[ReduceOp, str]] = None):
+        if not isinstance(output, torch.Tensor):
+            output = torch.tensor(output, device=self.device)
+
+        if (isinstance(reduce_op, ReduceOp) and ReduceOp != ReduceOp.SUM) \
+           or isinstance(reduce_op, str) and reduce_op.lower() not in ("sum", "mean", "avg"):
+            raise MisconfigurationException(
+                "Currently, TPUSpawn TrainingTypePlugin only support `sum`, `mean`, `avg` reduce operation."
+            )
+
+        divide_by_world_size = False
+
+        output = xm.mesh_reduce('reduce', output, sum)
+
+        if isinstance(reduce_op, str) and reduce_op.lower() in ("avg", "mean"):
+            divide_by_world_size = True
+        # sync all processes before reduction
+
+        if divide_by_world_size:
+            output = output / self.world_size
+
+        return output
+
     def post_dispatch(self) -> None:
         # TODO: Check if trainer references can be resolved otherwise
         model = self.lightning_module
@@ -213,6 +233,10 @@ def __load_weights_on_main_process(self) -> None:
 
         self._model = model
 
+    def _close_logger(self, trainer) -> None:
+        if hasattr(trainer, "logger"):
+            trainer.logger.finalize("success")
+
     @property
     def xmp_spawn_kwargs(self):
         return {
@@ -225,9 +249,11 @@ def start_training(self, trainer) -> None:
         # todo: precision pluging is call in accelerator setup and should be moved
         if 'XLA_USE_BF16' in os.environ:
             del os.environ["XLA_USE_BF16"]
+        self._close_logger(trainer)
         xmp.spawn(self.new_process, **self.xmp_spawn_kwargs)
 
     def start_testing(self, trainer) -> None:
+        self._close_logger(trainer)
         xmp.spawn(self.new_process, **self.xmp_spawn_kwargs)
 
     def start_predicting(self, trainer) -> None:

pytorch_lightning/plugins/training_type/training_type_plugin.py

@@ -35,10 +35,6 @@ def __init__(self) -> None:
         self._results = None
         self.global_rank = 0
 
-    @property
-    def should_finalize(self):
-        return True
-
     @property
     @abstractmethod
     def on_gpu(self) -> bool:

pytorch_lightning/trainer/trainer.py

@@ -711,7 +711,7 @@ def run_evaluation(self, max_batches=None, on_epoch=False):
         for dataloader_idx, dataloader in enumerate(dataloaders):
             # bookkeeping
             dl_outputs = []
-            dataloader = self.training_type_plugin.process_dataloader(dataloader)
+            dataloader = self.accelerator.process_dataloader(dataloader)
             dl_max_batches = self.evaluation_loop.max_batches[dataloader_idx]
 
             for batch_idx, batch in enumerate(dataloader):
@@ -823,7 +823,7 @@ def run_predict(self):
 
         # run validation/testing
         for dataloader_idx, dataloader in enumerate(dataloaders):
-            dataloader = self.training_type_plugin.process_dataloader(dataloader)
+            dataloader = self.accelerator.process_dataloader(dataloader)
             dl_max_batches = self.predict_loop.max_batches[dataloader_idx]
 
             for batch_idx, batch in enumerate(dataloader):

pytorch_lightning/trainer/training_loop.py

@@ -140,7 +140,7 @@ def on_train_end(self):
         # todo: TPU 8 cores hangs in flush with TensorBoard. Might do for all loggers.
         # It might be related to xla tensors blocked when moving the cpu
         # kill loggers
-        if self.trainer.logger is not None and self.trainer.training_type_plugin.should_finalize:
+        if self.trainer.logger is not None:
             self.trainer.logger.finalize("success")
 
         # summarize profile results
@@ -502,7 +502,7 @@ def tbptt_split_batch(self, batch):
 
     def run_training_epoch(self):
         # modify dataloader if needed (ddp, etc...)
-        train_dataloader = self.trainer.training_type_plugin.process_dataloader(self.trainer.train_dataloader)
+        train_dataloader = self.trainer.accelerator.process_dataloader(self.trainer.train_dataloader)
 
         # track epoch output
         epoch_output = [[] for _ in range(self.num_optimizers)]

tests/models/test_tpu.py

@@ -264,9 +264,6 @@ def test_distributed_backend_set_when_using_tpu(tmpdir, tpu_cores):
 
 
 @pytest.mark.skipif(not _TPU_AVAILABLE, reason="test requires TPU machine")
-@pytest.mark.skipif(
-    not os.getenv("PL_RUNNING_SPECIAL_TESTS", '0') == '1', reason="test should be run outside of pytest"
-)
 @pl_multi_process_test
 def test_broadcast_on_tpu():
     """ Checks if an object from the master process is broadcasted to other processes correctly"""