[Feat] Added uneven input support/sync with upstream DDP

Author: rohan-varma

pytorch_lightning/overrides/data_parallel.py

@@ -13,17 +13,20 @@
 # limitations under the License.
 
 import itertools
+import logging
 import threading
 from collections.abc import Iterable, Mapping
 from itertools import chain
 
 import torch
+import torch.distributed as dist
 from torch.cuda._utils import _get_device_index
 from torch.nn import DataParallel
 from torch.nn.parallel import DistributedDataParallel
 from torch.nn.parallel._functions import Gather
 
 from pytorch_lightning.core.step_result import Result
+from pytorch_lightning.utilities import DDP_JOIN_AND_REBUILD_BUCKETS_AVAILABLE
 from pytorch_lightning.utilities.warning_utils import WarningCache
 
 
@@ -161,7 +164,30 @@ def parallel_apply(self, replicas, inputs, kwargs):
         return parallel_apply(replicas, inputs, kwargs, self.device_ids[:len(replicas)])
 
     def forward(self, *inputs, **kwargs):  # pragma: no-cover
-        self._sync_params()
+        # TODO: Update uneven inputs code path when PyTorch 1.8 is released.
+        if DDP_JOIN_AND_REBUILD_BUCKETS_AVAILABLE and self.ddp_join_enabled:
+            ones = torch.ones(
+                1, device=self.device
+            )
+            work = dist.all_reduce(ones, group=self.process_group, async_op=True)
+            self.reducer._set_forward_pass_work_handle(
+                work, self.ddp_join_divide_by_initial_world_size
+            )
+
+        # Calling _rebuild_buckets before forward computation,
+        # It may allocate new buckets before deallocating old buckets
+        # inside _rebuild_buckets. To save peak memory usage,
+        # call _rebuild_buckets before the peak memory usage increases
+        # during forward computation.
+        # This should be called only once during whole training period.
+        if DDP_JOIN_AND_REBUILD_BUCKETS_AVAILABLE and self.reducer._rebuild_buckets():
+            logging.info("Reducer buckets have been rebuilt in this iteration.")
+
+        if self.require_forward_param_sync:
+            self._sync_params()
+        if DDP_JOIN_AND_REBUILD_BUCKETS_AVAILABLE and self.ddp_join_enabled:
+            # Notify joined ranks whether they should sync in backwards pass or not.
+            self._check_global_requires_backward_grad_sync(is_joined_rank=False)
         self.reducer_reset_hooks()
         fx_called: str = ''
 

pytorch_lightning/utilities/__init__.py

@@ -62,6 +62,8 @@ def _module_available(module_path: str) -> bool:
 _FAIRSCALE_PIPE_AVAILABLE = _FAIRSCALE_AVAILABLE and LooseVersion(torch.__version__) >= LooseVersion("1.6.0")
 _BOLTS_AVAILABLE = _module_available('pl_bolts')
 
+DDP_JOIN_AND_REBUILD_BUCKETS_AVAILABLE = LooseVersion(torch.__version__) >= LooseVersion("1.7.0")
+
 FLOAT16_EPSILON = numpy.finfo(numpy.float16).eps
 FLOAT32_EPSILON = numpy.finfo(numpy.float32).eps
 FLOAT64_EPSILON = numpy.finfo(numpy.float64).eps