FSDP use _allgather_base and _reduce_scatter_base (#729)

* Update fully_sharded_data_parallel.py

update fully_sharded_data_parallel to use _allgather_base

* Update reduce_scatter_bucketer.py

Use reduce_scatter_base

* Update fully_sharded_data_parallel.py

nonblocking gradient cpu copy, and nonblocking param rebulds

* Update reduce_scatter_bucketer.py

lints

* Update fully_sharded_data_parallel.py

* Update reduce_scatter_bucketer.py

* Update reduce_scatter_bucketer.py

* lints

* linter, test fix

* linter

* LINTERgit add fairscale/utils/reduce_scatter_bucketer.pygit add fairscale/utils/reduce_scatter_bucketer.py

* LINTERgit add tests/nn/data_parallel/test_fsdp_overlap.pygit add tests/nn/data_parallel/test_fsdp_overlap.py

* Update test_fsdp_overlap.py

* Update fairscale/utils/reduce_scatter_bucketer.py
Co-authored-by: Min Xu <24926999+min-xu-ai@users.noreply.github.com>

* Update fairscale/nn/data_parallel/fully_sharded_data_parallel.py
Co-authored-by: Min Xu <24926999+min-xu-ai@users.noreply.github.com>

* Update reduce_scatter_bucketer.py

* isort
Co-authored-by: Ubuntu <ubuntu@ip-172-31-9-185.ec2.internal>
Co-authored-by: Min Xu <24926999+min-xu-ai@users.noreply.github.com>
Co-authored-by: Ubuntu <ubuntu@ip-172-31-77-164.ec2.internal>

fairscale/nn/data_parallel/fully_sharded_data_parallel.py

@@ -435,7 +435,7 @@ class FullyShardedDataParallel(nn.Module):
 
     @property
     def params_with_grad(self) -> List[Parameter]:
-        """[p for p in self.parameters() if p.grad is not None] """
+        """[p for p in self.parameters() if p.grad is not None]"""
         return [p for p in self.parameters() if p.grad is not None]
 
     @torch.no_grad()
@@ -1315,7 +1315,7 @@ class FullyShardedDataParallel(nn.Module):
         # Optionally move gradients to CPU, typically used if one is running
         # the optimizer on the CPU.
         if self.move_grads_to_cpu:
-            param._cpu_grad.copy_(param.grad.data, non_blocking=False)
+            param._cpu_grad.copy_(param.grad.data, non_blocking=True)
             # Don't let this memory get reused until after the transfer.
             param.grad.data.record_stream(torch.cuda.current_stream())
             param.grad.data = param._cpu_grad
@@ -1448,7 +1448,7 @@ class FullyShardedDataParallel(nn.Module):
                 else:
                     # If self.move_params_to_cpu and force_full_precision, we need to cast
                     # the FP32 CPU param to CUDA for the all-gather.
-                    p_data = p.data.to(p._full_param_padded.device)
+                    p_data = p.data.to(p._full_param_padded.device, non_blocking=True)
 
                     p_size = p._full_param_padded.size()
                     assert p_size.numel() % self.world_size == 0
@@ -1463,6 +1463,10 @@ class FullyShardedDataParallel(nn.Module):
                         output_tensor = p._full_param_padded
 
                     # Fill output_tensor with (p.data for each shard in self.world_size)
+                    if hasattr(dist, "_all_gather_base"):
+                        # New version of PyTorch has all_gather_base, which is faster than chunk and then all_gather.
+                        dist._all_gather_base(output_tensor, p_data, group=self.process_group)  # type: ignore
+                    else:
                         chunks = list(output_tensor.chunk(self.world_size))
                         dist.all_gather(chunks, p_data, group=self.process_group)
 

fairscale/utils/reduce_scatter_bucketer.py

@@ -26,6 +26,11 @@ class Bucket:
             assert len(self.callbacks) == 0
             return
         # reduce-scatter bucket
+        if hasattr(dist, "_reduce_scatter_base"):
+            dist._reduce_scatter_base(  # type: ignore
+                self.output_shard[: self.offset], self.data[:, : self.offset].contiguous(), group=self.group
+            )
+        else:
             dist.reduce_scatter(
                 self.output_shard[: self.offset], list(self.data[:, : self.offset].unbind(0)), group=self.group
             )
@@ -39,7 +44,7 @@ class Bucket:
         self.output_shard = torch.zeros_like(self.data[0])
 
     def setup(self) -> None:
-        """ Setup the buffers if they are not allocated.
+        """Setup the buffers if they are not allocated.
 
         Using ``setup`` and ``teardown``, we can ensure that the bucket
         buffers are only allocated during the backward pass, hence saving more
@@ -122,8 +127,14 @@ class ReduceScatterBucketer:
 
         bucket_shard_size = self._get_shard_size(first_input.element_size(), world_size)
         if first_input_size > bucket_shard_size:
+            # TODO: investigate how to avoid using torch.cat (because it seems to be slow for CPU tensors)
             # input is too big to fit in the bucket, reduce-scatter directly
             output = torch.zeros_like(input_list[0])
+            if hasattr(dist, "_reduce_scatter_base"):
+                input_flattened = torch.cat(input_list)
+                dist._reduce_scatter_base(output, input_flattened, group=group)  # type: ignore
+            else:
+                # fallback
                 dist.reduce_scatter(output, input_list, group=group)
             if callback_fn is not None:
                 callback_fn(output)

tests/nn/data_parallel/test_fsdp_no_sync.py

@@ -150,28 +150,47 @@ class TestNoSyncCommunication(DistributedTest):
         expected_reduce_scatter = num_fsdp
 
         batch = model.module.get_input(torch.device("cuda"))
+
+        # depending on pytorch version the _base methods may not be available
+        method_string_reduce_scatter_base = "torch.distributed._reduce_scatter_base"
+        if hasattr(torch.distributed, "_reduce_scatter_base") is False:
+            # no such method, to make mock_reduce_scatter_base 0 invocation, use an impossible name
+            method_string_reduce_scatter_base = "math.nan"  # just an arbitrary function not going to be called
+
+        method_string_all_gather_base = "torch.distributed._all_gather_base"
+        if hasattr(torch.distributed, "_all_gather_base") is False:
+            # no such method, to make mock_all_gather_base 0 invocation, use an impossible name
+            method_string_all_gather_base = "math.nan"  # just an arbitrary function not going to be called
+
         with patch("torch.distributed.all_gather") as mock_all_gather:
             with patch("torch.distributed.reduce_scatter") as mock_reduce_scatter:
+                with patch(method_string_all_gather_base) as mock_all_gather_base:
+                    with patch(method_string_reduce_scatter_base) as mock_reduce_scatter_base:
                         with model.no_sync():
                             output = model(*batch)
                             loss = model.module.get_loss(batch, output)
                             loss.backward()
 
+                        # the _base methods are activated when they are available.
+                        # the sum of the _base and public methods should stay the same.
+                        assert (
+                            mock_all_gather.call_count + mock_all_gather_base.call_count == expected_all_gather1
+                        ), f"{mock_all_gather.call_count +  mock_all_gather_base.call_count} != {expected_all_gather1}"
                         assert (
-                    mock_all_gather.call_count == expected_all_gather1
-                ), f"{mock_all_gather.call_count} != {expected_all_gather1}"
-                assert mock_reduce_scatter.call_count == 0, f"{mock_reduce_scatter.call_count} != 0"
+                            mock_reduce_scatter.call_count + mock_reduce_scatter_base.call_count == 0
+                        ), f"{mock_reduce_scatter.call_count +  mock_reduce_scatter_base.call_count} != 0"
 
                         output = model(*batch)
                         loss = model.module.get_loss(batch, output)
                         loss.backward()
 
                         assert (
-                    mock_all_gather.call_count == expected_all_gather2
-                ), f"{mock_all_gather.call_count} != {expected_all_gather2}"
+                            mock_all_gather.call_count + mock_all_gather_base.call_count == expected_all_gather2
+                        ), f"{mock_all_gather.call_count + mock_all_gather_base.call_count} != {expected_all_gather2}"
                         assert (
-                    mock_reduce_scatter.call_count == expected_reduce_scatter
-                ), f"{mock_reduce_scatter.call_count} != {expected_reduce_scatter}"
+                            mock_reduce_scatter.call_count + mock_reduce_scatter_base.call_count
+                            == expected_reduce_scatter
+                        ), f"{mock_reduce_scatter.call_count + mock_reduce_scatter_base.call_count} != {expected_reduce_scatter}"
 
 
 if __name__ == "__main__":

tests/nn/data_parallel/test_fsdp_overlap.py

@@ -93,6 +93,9 @@ def _distributed_worker(
     # Save the original torch.distributed.all_gather function since we will
     # patch it to include an artificial delay.
     orig_all_gather = torch.distributed.all_gather
+    orig_all_gather_base = (
+        torch.distributed._all_gather_base if hasattr(torch.distributed, "_all_gather_base") else None
+    )
 
     def run(compute_cycles, all_gather_cycles):
         has_params = all_gather_cycles > 0
@@ -117,6 +120,7 @@ def _distributed_worker(
             cpu_start = time.process_time()
 
             all_gather_called = False
+            all_gather_base_called = False
 
             def _delayed_all_gather(*args, **kwargs):
                 nonlocal all_gather_called
@@ -124,17 +128,30 @@ def _distributed_worker(
                 torch.cuda._sleep(all_gather_cycles)
                 return orig_all_gather(*args, **kwargs)
 
+            def _delayed_all_gather_base(*args, **kwargs):
+                nonlocal all_gather_base_called
+                all_gather_base_called = True
+                torch.cuda._sleep(all_gather_cycles)
+                assert orig_all_gather_base
+                return orig_all_gather_base(*args, **kwargs)
+
+            method_string_all_gather_base = "torch.distributed._all_gather_base"
+            if hasattr(torch.distributed, "_all_gather_base") is False:
+                # no such method, to make mock_all_gather_base 0 invocation, use an impossible name
+                method_string_all_gather_base = "math.nan"
+                pass
             # forward pass
             #
             # Even though both e1 & e2 are on the compute stream, since
             # compute depends on all_gather, e2-e1 includes all_gather time.
             e1.record()
             with patch("torch.distributed.all_gather", _delayed_all_gather):
+                with patch(method_string_all_gather_base, _delayed_all_gather_base):
                     out = model(batch)
                     if has_params and world_size > 1:
-                    assert all_gather_called
+                        assert all_gather_called or all_gather_base_called
                     else:
-                    assert not all_gather_called
+                        assert not all_gather_called and not all_gather_base_called
             e2.record()
 
             # backward pass