Merge branch 't5_pipeline_fixes' into 'main'

pipeline_fixes

See merge request ADLR/megatron-lm!372

megatron/model/language_model.py

@@ -334,10 +334,6 @@ class TransformerLanguageModel(MegatronModule):
         # Decoder (usually set to False, True if part of an encoder-decoder
         # architecture and in decoder-only stage).
         if self.add_decoder:
-            # Temporary assertion until we verify correctness of pipeline parallelism
-            # implementation of T5.
-            assert args.pipeline_model_parallel_size == 1, \
-                'pipeline parallelism is not supported in the presence of decoder'
             self.decoder = ParallelTransformer(
                 self.init_method,
                 output_layer_init_method,

megatron/model/transformer.py

@@ -580,6 +580,7 @@ class ParallelTransformer(MegatronModule):
             assert args.num_layers % args.virtual_pipeline_model_parallel_size == 0, \
                 'num_layers_per_stage must be divisible by ' \
                 'virtual_pipeline_model_parallel_size'
+            assert args.model_type != ModelType.encoder_and_decoder
             # Number of layers in each model chunk is the number of layers in the stage,
             # divided by the number of model chunks in a stage.
             self.num_layers = self.num_layers // args.virtual_pipeline_model_parallel_size
@@ -596,7 +597,16 @@ class ParallelTransformer(MegatronModule):
                 (mpu.get_pipeline_model_parallel_rank() * self.num_layers)
         else:
             # Each stage gets a contiguous set of layers.
-            offset = mpu.get_pipeline_model_parallel_rank() * self.num_layers
+            if args.model_type == ModelType.encoder_and_decoder and \
+                    mpu.get_pipeline_model_parallel_world_size() > 1:
+                pipeline_rank = mpu.get_pipeline_model_parallel_rank()
+                if layer_type == LayerType.encoder:
+                    offset = pipeline_rank * self.num_layers
+                else:
+                    num_ranks_in_enc = args.pipeline_model_parallel_split_rank
+                    offset = (pipeline_rank - num_ranks_in_enc) * self.num_layers
+            else:
+                offset = mpu.get_pipeline_model_parallel_rank() * self.num_layers
 
         self.layers = torch.nn.ModuleList(
             [build_layer(i + 1 + offset) for i in range(self.num_layers)])

megatron/mpu/__init__.py

@@ -25,6 +25,7 @@ from .initialize import get_data_parallel_group
 from .initialize import get_data_parallel_rank
 from .initialize import get_data_parallel_world_size
 from .initialize import get_embedding_group
+from .initialize import get_position_embedding_group
 from .initialize import get_model_parallel_group
 from .initialize import get_tensor_model_parallel_group
 from .initialize import get_pipeline_model_parallel_group
@@ -32,6 +33,7 @@ from .initialize import get_tensor_model_parallel_rank, set_tensor_model_paralle
 from .initialize import get_pipeline_model_parallel_rank, set_pipeline_model_parallel_rank
 from .initialize import is_pipeline_first_stage, is_pipeline_last_stage
 from .initialize import is_rank_in_embedding_group
+from .initialize import is_rank_in_position_embedding_group
 from .initialize import is_pipeline_stage_before_split, is_pipeline_stage_after_split
 from .initialize import is_pipeline_stage_at_split
 from .initialize import get_num_layers

megatron/mpu/initialize.py

@@ -29,6 +29,8 @@ _PIPELINE_MODEL_PARALLEL_GROUP = None
 _MODEL_PARALLEL_GROUP = None
 # Embedding group.
 _EMBEDDING_GROUP = None
+# Position embedding group.
+_POSITION_EMBEDDING_GROUP = None
 # Data parallel group that the current rank belongs to.
 _DATA_PARALLEL_GROUP = None
 
@@ -45,6 +47,9 @@ _MPU_PIPELINE_MODEL_PARALLEL_RANK = None
 # A list of ranks that have a copy of the embedding.
 _EMBEDDING_GLOBAL_RANKS = None
 
+# A list of ranks that have a copy of the position embedding.
+_POSITION_EMBEDDING_GLOBAL_RANKS = None
+
 # A list of global ranks for each pipeline group to ease calculation of the source
 # rank when broadcasting from the first or last pipeline stage.
 _PIPELINE_GLOBAL_RANKS = None
@@ -165,6 +170,10 @@ def initialize_model_parallel(tensor_model_parallel_size_=1,
     global _EMBEDDING_GLOBAL_RANKS
     assert _EMBEDDING_GROUP is None, \
         'embedding group is already initialized'
+    global _POSITION_EMBEDDING_GROUP
+    global _POSITION_EMBEDDING_GLOBAL_RANKS
+    assert _POSITION_EMBEDDING_GROUP is None, \
+        'position embedding group is already initialized'
     for i in range(num_pipeline_model_parallel_groups):
         ranks = range(i, world_size,
                       num_pipeline_model_parallel_groups)
@@ -176,19 +185,31 @@ def initialize_model_parallel(tensor_model_parallel_size_=1,
         # first and last stages).
         if len(ranks) > 1:
             embedding_ranks = [ranks[0], ranks[-1]]
-            if pipeline_model_parallel_split_rank_ is not None and \
-                    ranks[pipeline_model_parallel_split_rank_] not in embedding_ranks:
-                embedding_ranks = [ranks[0],
-                                   ranks[pipeline_model_parallel_split_rank_],
-                                   ranks[-1]]
+            position_embedding_ranks = [ranks[0]]
+            if pipeline_model_parallel_split_rank_ is not None:
+                if ranks[pipeline_model_parallel_split_rank_] not in embedding_ranks:
+                    embedding_ranks = [ranks[0],
+                                       ranks[pipeline_model_parallel_split_rank_],
+                                       ranks[-1]]
+                if ranks[pipeline_model_parallel_split_rank_] not in position_embedding_ranks:
+                    position_embedding_ranks = [ranks[0],
+                                       ranks[pipeline_model_parallel_split_rank_]]
         else:
             embedding_ranks = ranks
+            position_embedding_ranks = ranks
+
         group = torch.distributed.new_group(embedding_ranks)
         if rank in embedding_ranks:
             _EMBEDDING_GROUP = group
         if rank in ranks:
             _EMBEDDING_GLOBAL_RANKS = embedding_ranks
 
+        group = torch.distributed.new_group(position_embedding_ranks)
+        if rank in position_embedding_ranks:
+            _POSITION_EMBEDDING_GROUP = group
+        if rank in ranks:
+            _POSITION_EMBEDDING_GLOBAL_RANKS = position_embedding_ranks
+
 
 def model_parallel_is_initialized():
     """Check if model and data parallel groups are initialized."""
@@ -234,6 +255,13 @@ def get_embedding_group():
     return _EMBEDDING_GROUP
 
 
+def get_position_embedding_group():
+    """Get the position embedding group the caller rank belongs to."""
+    assert _POSITION_EMBEDDING_GROUP is not None, \
+        'position embedding group is not initialized'
+    return _POSITION_EMBEDDING_GROUP
+
+
 def set_tensor_model_parallel_world_size(world_size):
     """Set the tensor model parallel size"""
     global _MPU_TENSOR_MODEL_PARALLEL_WORLD_SIZE
@@ -352,6 +380,13 @@ def is_rank_in_embedding_group(ignore_virtual=False):
     return False
 
 
+def is_rank_in_position_embedding_group():
+    """Return true if current rank is in position embedding group, False otherwise."""
+    rank = torch.distributed.get_rank()
+    global _POSITION_EMBEDDING_GLOBAL_RANKS
+    return rank in _POSITION_EMBEDDING_GLOBAL_RANKS
+
+
 def is_pipeline_stage_before_split(rank=None):
     """Return True if pipeline stage executes encoder block for a model
     with both encoder and decoder."""
@@ -467,3 +502,5 @@ def destroy_model_parallel():
     _DATA_PARALLEL_GROUP = None
     global _EMBEDDING_GROUP
     _EMBEDDING_GROUP = None
+    global _POSITION_EMBEDDING_GROUP
+    _POSITION_EMBEDDING_GROUP = None

megatron/training.py

@@ -427,6 +427,20 @@ def train_step(forward_step_func, data_iterator,
             else:
                 grad = word_embeddings_weight.grad
             torch.distributed.all_reduce(grad, group=mpu.get_embedding_group())
+
+    # All-reduce position_embeddings grad across first (encoder) and split (decoder) 
+    # stages to ensure that position embeddings parameters stay in sync.
+    # This should only run for T5 models with pipeline parallelism
+    if mpu.is_rank_in_position_embedding_group() and \
+            mpu.get_pipeline_model_parallel_world_size() > 1 and \
+            args.pipeline_model_parallel_split_rank is not None:
+        unwrapped_model = model[0]
+        unwrapped_model = unwrap_model(
+            unwrapped_model, (torchDDP, LocalDDP, Float16Module))
+        assert args.DDP_impl == 'local', \
+            'T5 model is only supported with local DDP mode'
+        grad = unwrapped_model.language_model.embedding.position_embeddings.weight.main_grad
+        torch.distributed.all_reduce(grad, group=mpu.get_position_embedding_group())
     timers('backward-embedding-all-reduce').stop()
 
     # Update parameters.