A graph-based pipeline splitting

.gitignore

@@ -1,6 +1,7 @@
 __pycache__
 build
 pippy.egg-info
+torchpippy.egg-info
 pippy/version.py
 dist
 .idea/

examples/huggingface/pippy_gpt2.py

@@ -21,7 +21,7 @@ def run(args):
     config.n_embd = args.n_embd or config.n_embd
     config.n_layer = args.n_layer or config.n_layer
     config.n_head = args.n_head or config.n_head
-    print("Using device:", args.device)
+    print("[Rank {}] Using device: {}".format(args.rank, args.device))
 
     # Create model
     model_class = GPT2ForSequenceClassification
@@ -38,13 +38,19 @@ def run(args):
     example_inputs = generate_inputs_for_model(
         model_class, gpt2, model_name, args.batch_size, args.device)
 
+    assert not args.autosplit or not args.graphsplit
+
     split_policy = None
     split_spec = None
 
     if args.autosplit:
         # Automatic split
         from pippy import split_into_equal_size
         split_policy = split_into_equal_size(args.world_size)
+    elif args.graphsplit:
+        # Graph-based split
+        from pippy import split_by_graph
+        split_policy = split_by_graph(args.world_size)
     else:
         # Use manual split spec
         decoders_per_rank = (gpt2.config.n_layer + args.world_size - 1) // args.world_size
@@ -106,6 +112,7 @@ def run(args):
     parser.add_argument('--n_layer', type=int, default=None)
     parser.add_argument('--n_head', type=int, default=None)
     parser.add_argument('--autosplit', action="store_true")
+    parser.add_argument('--graphsplit', action="store_true")
 
     args = parser.parse_args()
 

pippy/ModelSplit.py

@@ -5,6 +5,8 @@
 import torch
 import torch.fx as fx
 
+from pippy.graphsplit import split_by_graph_with_num_stages
+
 from ._IR import aten_pipe_split_alias
 
 
@@ -202,3 +204,57 @@ def _split_into_nstages_equal_size(
         return gm
 
     return _split_into_nstages_equal_size
+
+
+"""
+Create a Callable that splits a model into a given number of stages, based on the computation graph, while
+trying to minimize the communication between the stages and to balance the computation
+Input:
+  nstages: the number of stages to split the module into
+Output:
+  a Callable that transforms an input `fx.GraphModule` into an output `fx.GraphModule` that has `pipe_split` inserted
+  between `nstages` stages
+"""
+
+
+def split_by_graph(nstages: int) -> Callable[[fx.GraphModule], fx.GraphModule]:
+    def _split_by_graph(
+        gm: fx.GraphModule,
+    ) -> fx.GraphModule:
+        node_param_sizes = _analyze_node_size(gm)
+        node2stage = split_by_graph_with_num_stages(
+            gm, nstages, node_param_sizes
+        )
+
+        # Remove existing split points
+        for node in gm.graph.nodes:
+            if "pipe_split" in node.name:
+                gm.graph.erase_node(node)
+
+        # Modify the graph by grouping nodes on the same stage and adding
+        # pipe_splits between the stages
+        node_order = [node for node in gm.graph.nodes if node in node2stage]
+        last_node = None
+        for stage_idx in range(nstages):
+            nodes_at_stage = [
+                node
+                for node in node_order
+                if node in node2stage and node2stage[node] == stage_idx
+            ]
+            for idx, node in enumerate(nodes_at_stage):
+                if last_node is not None and last_node.next != node:
+                    last_node.append(node)
+                last_node = node
+            # Insert pipe_split nodes after each stage, except the last one
+            if stage_idx + 1 != nstages and last_node is not None:
+                with gm.graph.inserting_after(last_node):
+                    last_node = gm.graph.call_function(
+                        aten_pipe_split_alias, (), {}
+                    )
+
+        # Since we transformed the graph, recompile the module
+        gm.recompile()
+        gm.graph.lint()
+        return gm
+
+    return _split_by_graph

pippy/__init__.py

@@ -10,7 +10,11 @@
 )
 from ._PipelineStage import PipelineStage
 from .ManualPipelineStage import ManualPipelineStage
-from .ModelSplit import split_into_equal_size, split_on_size_threshold
+from .ModelSplit import (
+    split_by_graph,
+    split_into_equal_size,
+    split_on_size_threshold,
+)
 from .PipelineSchedule import (
     Schedule1F1B,
     ScheduleGPipe,
@@ -27,6 +31,7 @@
     "annotate_split_points",
     "split_into_equal_size",
     "split_on_size_threshold",
+    "split_by_graph",
     "pipeline",
     "Schedule1F1B",
     "ScheduleGPipe",