Backport ZeRO1: Add bucketting logic to control the size of tensors f…

…or all-gather/reduce-scatter (#6025) (#6806)

Co-authored-by: jeffhataws <jthuynh@amazon.com>
Co-authored-by: Rahul Solanki <rhsoln@amazon.com>
Co-authored-by: guangtai <guangtai@amazon.com>
Co-authored-by: Amithrajith Mamidala <amithrm@amazon.com>

test/test_mp_all_gather.py

@@ -95,6 +95,66 @@ def _mp_fn(index):
             file=sys.stderr)
         print(f'[{index}] {cpu_result}', file=sys.stderr)
         sys.exit(1)
+
+    # Testing with a single replica group and tensor list as input (Bucketized)
+    # TODO: add support for list input with pin_layout=True and output=None
+    result_list = xm.all_gather_bucketized(
+        ordinal_tensors, dim=0, pin_layout=False)
+
+    for i, result in enumerate(result_list):
+      cpu_result = result.cpu()
+      expected = i * 1000 + torch.arange(world_size, dtype=torch.float)
+      if not cpu_result.allclose(expected):
+        print(
+            'xm.all_gather_bucketized() produced wrong reductions for item {i} in result list',
+            file=sys.stderr)
+        print(f'[{index}] {cpu_result}', file=sys.stderr)
+        sys.exit(1)
+
+    # Testing with a single replica group and tensor list as input and output!=None (out-of-place) (Bucketized)
+    # Reuse ordinal_tensors from previous test
+    output_tensors = [
+        torch.zeros([world_size], dtype=torch.float).to(device)
+        for i in range(input_list_size)
+    ]
+    # TODO: add support for list input with pin_layout=True and output!=None
+    result_list = xm.all_gather_bucketized(
+        ordinal_tensors, dim=0, output=output_tensors, pin_layout=False)
+
+    for i, result in enumerate(result_list):
+      cpu_result = result.cpu()
+      expected = i * 1000 + torch.arange(world_size, dtype=torch.float)
+      if not cpu_result.allclose(expected):
+        print(
+            'xm.all_gather() produced wrong reductions for item {i} in result list',
+            file=sys.stderr)
+        print(f'[{index}] {cpu_result}', file=sys.stderr)
+        sys.exit(1)
+
+    # Testing with a single replica group and tensor list as input and output!=None (out-of-place) (Bucketized, zero bucket size)
+    # Reuse ordinal_tensors from previous test
+    output_tensors = [
+        torch.zeros([world_size], dtype=torch.float).to(device)
+        for i in range(input_list_size)
+    ]
+    # TODO: add support for list input with pin_layout=True and output!=None
+    result_list = xm.all_gather_bucketized(
+        ordinal_tensors,
+        dim=0,
+        output=output_tensors,
+        pin_layout=False,
+        bucket_cap_mb=0)
+
+    for i, result in enumerate(result_list):
+      cpu_result = result.cpu()
+      expected = i * 1000 + torch.arange(world_size, dtype=torch.float)
+      if not cpu_result.allclose(expected):
+        print(
+            'xm.all_gather() produced wrong reductions for item {i} in result list',
+            file=sys.stderr)
+        print(f'[{index}] {cpu_result}', file=sys.stderr)
+        sys.exit(1)
+
     # TODO: add test for torch.compile when support for list input is ready
 
   else:

test/test_mp_reduce_scatter.py

@@ -55,6 +55,33 @@ def _mp_fn(index):
 
     xm.rendezvous('test_reduce_scatter_list_input')
 
+    # Testing reduce-scatter with list input bucketized
+    rand_list = [
+        torch.rand((32, shard_size * world_size, 32))
+        for _ in range(input_list_size)
+    ]
+    xrand_list = [rand.to(device) for rand in rand_list]
+
+    # TODO: fix the broken case with pin_layout=True
+    res_list = xm.reduce_scatter_bucketized(
+        xm.REDUCE_SUM,
+        xrand_list,
+        scale,
+        scatter_dim,
+        world_size,
+        pin_layout=False)
+
+    for i, res in enumerate(res_list):
+      expected_world = xm.all_reduce(xm.REDUCE_SUM, xrand_list[i], scale)
+      xm.mark_step()
+
+      slice_idx = torch.tensor(
+          list(range(index * shard_size, (index + 1) * shard_size)))
+      expected = expected_world.cpu().index_select(scatter_dim, slice_idx)
+      assert res.cpu().allclose(expected)
+
+    xm.rendezvous('test_reduce_scatter_list_input_bucketized')
+
     # Testing reduce-scatter with list input and output
     output_list = [
         torch.rand((32, shard_size * world_size, 32))
@@ -83,6 +110,66 @@ def _mp_fn(index):
       assert res.cpu().allclose(expected)
 
     xm.rendezvous('test_reduce_scatter_list_input_output')
+
+    # Testing reduce-scatter with list input and output (buckettized)
+    output_list = [
+        torch.rand((32, shard_size * world_size, 32))
+        for _ in range(input_list_size)
+    ]
+    xoutput_list = [output.to(device) for output in output_list]
+
+    # TODO: fix the broken case with pin_layout=True
+    res_list = xm.reduce_scatter_bucketized(
+        xm.REDUCE_SUM,
+        xrand_list,
+        scale,
+        scatter_dim,
+        world_size,
+        output=xoutput_list,
+        pin_layout=False)
+
+    assert (xoutput_list == res_list)
+    for i, res in enumerate(xoutput_list):
+      expected_world = xm.all_reduce(xm.REDUCE_SUM, xrand_list[i], scale)
+      xm.mark_step()
+
+      slice_idx = torch.tensor(
+          list(range(index * shard_size, (index + 1) * shard_size)))
+      expected = expected_world.cpu().index_select(scatter_dim, slice_idx)
+      assert res.cpu().allclose(expected)
+
+    xm.rendezvous('test_reduce_scatter_list_input_output_bucketized')
+
+    # Testing reduce-scatter with list input and output (buckettized, but zero bucket size)
+    output_list = [
+        torch.rand((32, shard_size * world_size, 32))
+        for _ in range(input_list_size)
+    ]
+    xoutput_list = [output.to(device) for output in output_list]
+
+    # TODO: fix the broken case with pin_layout=True
+    res_list = xm.reduce_scatter_bucketized(
+        xm.REDUCE_SUM,
+        xrand_list,
+        scale,
+        scatter_dim,
+        world_size,
+        output=xoutput_list,
+        bucket_cap_mb=0,
+        pin_layout=False)
+
+    assert (xoutput_list == res_list)
+    for i, res in enumerate(xoutput_list):
+      expected_world = xm.all_reduce(xm.REDUCE_SUM, xrand_list[i], scale)
+      xm.mark_step()
+
+      slice_idx = torch.tensor(
+          list(range(index * shard_size, (index + 1) * shard_size)))
+      expected = expected_world.cpu().index_select(scatter_dim, slice_idx)
+      assert res.cpu().allclose(expected)
+
+    xm.rendezvous(
+        'test_reduce_scatter_list_input_output_bucketized, zero bucket size')
   else:
     print(
         'Default device {} is not a TPU device'.format(device), file=sys.stderr)

torch_xla/core/xla_model.py

@@ -634,6 +634,110 @@ def all_gather(value, dim=0, groups=None, output=None, pin_layout=True):
                     f"given {type(value)}.")
 
 
+class CoalescingBuckets(object):
+
+  def __init__(self, func, input_list, output_list=None, bucket_cap_mb=160):
+    if not isinstance(input_list, list) or any(
+        not isinstance(v, torch.Tensor) for v in input_list):
+      raise TypeError(
+          f"`input_list` needs to be a list of Tensors, but given {type(input_list)}."
+      )
+    if output_list != None:
+      if not isinstance(output_list, list) or any(
+          not isinstance(v, torch.Tensor) for v in output_list):
+        raise TypeError(
+            f"`output_list` needs to be a list of Tensors, but given {type(output_list)}."
+        )
+      if len(output_list) != len(input_list):
+        raise ValueError(
+            "`output_list` length doesn't match `input_list` length: "
+            f"{len(output_list)} vs {len(input_list)}.")
+    self._func = func
+    self._input_list = input_list
+    self._output_list = output_list
+    self._total = 0
+    self._tensor_bucket = []
+    self._output_bucket = [] if output_list else None
+    self._bucket_cap = bucket_cap_mb * 1024 * 1024
+    self._out_tensors = []
+
+  def flush(self):
+    if len(self._tensor_bucket) == 1:
+      # Use non-coalesced CCOp if its just one tensor
+      output = self._output_bucket[0] if self._output_bucket else None
+      self._out_tensors.append(self._func(self._tensor_bucket[0], output))
+    elif len(self._tensor_bucket):
+      self._out_tensors.extend(
+          self._func(self._tensor_bucket, self._output_bucket))
+    self._total = 0
+    self._tensor_bucket = []
+    self._output_bucket = [] if self._output_list else None
+
+  def add(self, tensor, idx):
+    self._total += tensor.numel() * tensor.element_size()
+    self._tensor_bucket.append(tensor)
+    if self._output_list != None:
+      self._output_bucket.append(self._output_list[idx])
+
+  def __call__(self):
+    for idx, tensor in enumerate(self._input_list):
+      tensor_bytes = tensor.numel() * tensor.element_size()
+
+      # Aim for target bucket_cap_mb: flush new tensor with bucket if bucket content
+      # is small (1/2 cap) but don't combine if combined total is over 2x cap
+      total_new = self._total + tensor_bytes
+      if tensor_bytes > self._bucket_cap and self._total < 0.5 * self._bucket_cap and total_new <= 2 * self._bucket_cap:
+        self.add(tensor, idx)
+        self.flush()
+      else:
+        # Bucketize till the total spills over
+        if total_new > self._bucket_cap:
+          self.flush()
+        self.add(tensor, idx)
+
+    # Flush the last remaining bucket
+    self.flush()
+
+    assert len(self._out_tensors) == len(self._input_list)
+
+    return self._out_tensors
+
+
+def all_gather_bucketized(input_list,
+                          dim=0,
+                          groups=None,
+                          output=None,
+                          pin_layout=False,
+                          bucket_cap_mb=160):
+  """Performs an all-gather operation along a given dimension, with bucketization.
+
+  Args:
+    See all_gather for the args: dim, groups, output, pin_layout
+    input_list: List of input tensors
+    bucket_cap_mb: Number of MegaBytes of the tensor bucket to fill before doing all-gather.
+
+  Returns:
+    A list of tensors each of which has, in the ``dim`` dimension, all the values from the
+    participating replicas.
+  """
+  # sanity checks
+  if pin_layout:
+    raise RuntimeError(
+        "For xm.all_gather_bucketized, pin_layout=True is not yet supported.")
+
+  def _all_gather_coalesced(_input_list, _output_list=None):
+    return all_gather(
+        value=_input_list,
+        dim=dim,
+        groups=groups,
+        output=_output_list,
+        pin_layout=pin_layout)
+
+  buckets = CoalescingBuckets(
+      _all_gather_coalesced, input_list, output, bucket_cap_mb=bucket_cap_mb)
+  return buckets()
+
+
 def all_to_all(value,
                split_dimension,
                concat_dimension,
@@ -847,6 +951,50 @@ def reduce_scatter(reduce_type,
                     f"given {type(input)}.")
 
 
+def reduce_scatter_bucketized(reduce_type,
+                              input_list,
+                              scale,
+                              scatter_dim,
+                              shard_count,
+                              groups=None,
+                              output=None,
+                              pin_layout=False,
+                              bucket_cap_mb=160):
+  """Performs a XLA `ReduceScatter()` operation on a list of tensors (bucketized).
+
+  See: https://www.tensorflow.org/xla/operation_semantics#reducescatter
+
+  Args:
+    see reduce_scatter for reduce_type, scale, scatter_dim, shard_count, groups, pin_layout
+    input_list: List of input tensors
+    output: Optional list of output torch.Tensor
+    bucket_cap_mb: Number of MegaBytes of the tensor bucket to fill before doing all-gather.
+
+  Returns:
+    A list of `torch.Tensors` with all the values reduced across replicas. Each process
+    gets a shard split along the `scatter_dim`. All other dimensions are
+    the same as the input.
+  """
+
+  def _reduce_scatter_coalesced(_input_list, _output_list=None):
+    return reduce_scatter(
+        reduce_type=reduce_type,
+        input=_input_list,
+        scale=scale,
+        scatter_dim=scatter_dim,
+        shard_count=shard_count,
+        groups=groups,
+        output=_output_list,
+        pin_layout=pin_layout)
+
+  buckets = CoalescingBuckets(
+      _reduce_scatter_coalesced,
+      input_list,
+      output,
+      bucket_cap_mb=bucket_cap_mb)
+  return buckets()
+
+
 def add_step_closure(closure, args=(), run_async=False):
   """Adds a closure to the list of the ones to be run at the end of the step.