Add max_acc_splits (facebookincubator#1017)

Summary:
Pull Request resolved: facebookincubator#1017

X-link: pytorch/pytorch#133041

Model owners can set the lower_settings with max_acc_splits=2, and lowering will fail during model iteration, to alert them of possible performance degradation from increased fragmentation.

Differential Revision: D60133589

fx2ait/fx2ait/ait_splitter.py

@@ -115,12 +115,14 @@ def __init__(
         min_acc_module_size=DEFAULT_MIN_ACC_MODULE_SIZE,
         allow_int_inputs=False,
         debug_operator_range=None,
+        max_acc_splits=-1,
     ):
         super().__init__()
         self.min_acc_module_size = min_acc_module_size
         self.exclude_support_node_name: set = set()
         self.allow_int_inputs: bool = allow_int_inputs
         self.debug_operator_range = debug_operator_range
+        self.max_acc_splits = max_acc_splits
 
 
 class SelectedOperatorSupport(ops.OperatorSupportBase):

fx2ait/fx2ait/lower/lower.py

@@ -100,6 +100,7 @@ def default_split_function(
     settings = AITSplitterSettings(
         min_acc_module_size=lower_settings.min_acc_module_size,
         allow_int_inputs=lower_settings.allow_int_inputs,
+        max_acc_splits=lower_settings.max_acc_splits,
     )
     splitter = AITSplitter(model, inputs, settings=settings)
     splitter.node_support_preview()

fx2ait/fx2ait/lower/lower_settings.py

@@ -61,6 +61,9 @@ class LowerSettings:
 
     max_batch_size: int = 2048
     min_acc_module_size: int = 10
+    # Maximum number of splits for lowered module
+    # (eg. if lowered module is split into _run_on_gpu_0(unlowered submodule) and _run_on_acc_1(lowered submodule) it has 2 splits)
+    max_acc_splits: int = -1
     workdir: str = ""
     name: str = ""
     dll_name: str = "ait_engine.so"

fx2ait/fx2ait/test/test_ait_splitter.py

@@ -250,3 +250,58 @@ def is_node_supported(self, submodules, node: torch.fx.Node) -> bool:
             dict(split_results_relu_allowed.split_module.named_children()).keys(),
             {"_run_on_acc_0"},
         )
+
+    def test_fail_if_exceed_max_acc_split_limit(self):
+        class TestModule(torch.nn.Module):
+            def forward(self, a):
+                b = torch.sin(a)
+                c = torch.relu(b)
+                d = torch.cos(c)
+                e = torch.sigmoid(d)
+                f = torch.tanh(e)
+                return f
+
+        # Support all ops
+        _support_dict = {
+            "acc_ops.sin": None,
+            "acc_ops.cos": None,
+            "acc_ops.relu": None,
+            "acc_ops.sigmoid": None,
+            "acc_ops.tanh": None,
+        }
+        custom_op_support = op_support.OperatorSupport(_support_dict)
+
+        # With no ops excluded, the entire module should be lowered
+        # into one acc graph
+        mod = acc_tracer.trace(TestModule(), [torch.randn(2, 3)])
+        settings = AITSplitterSettings(min_acc_module_size=0, max_acc_splits=1)
+        splitter = AITSplitter(
+            mod,
+            (torch.randn(2, 3),),
+            custom_op_support,
+            settings,
+        )
+
+        res_all_nodes_supported = splitter.generate_split_results()
+        split_named_mods = dict(res_all_nodes_supported.split_module.named_children())
+        self.assertEqual(len(split_named_mods), 1)
+        self.assertIn("_run_on_acc_0", split_named_mods)
+
+        # Add "relu" to exclude_support_node_name
+        # The graph should be split into 3 parts now(_run_on_acc_0, _run_on_gpu_1, _run_on_acc_2)
+        mod = acc_tracer.trace(TestModule(), [torch.randn(2, 3)])
+        for node in mod.graph.nodes:
+            if node.target == acc_ops.relu:
+                settings.exclude_support_node_name.add(node.name)
+        splitter = AITSplitter(
+            mod,
+            (torch.randn(2, 3),),
+            custom_op_support,
+            settings,
+        )
+        # Split should fail now
+        with self.assertRaisesRegex(
+            ValueError,
+            "Cannot fulfill max_acc_splits limit. This may cause split fragmentation and result in performance issues.",
+        ):
+            splitter.generate_split_results()