Disable spec-decode + chunked-prefill for draft models with tensor parallelism > 1 (#10136)

Signed-off-by: Sourashis Roy <sroy@roblox.com>

tests/spec_decode/e2e/test_compatibility.py

@@ -50,3 +50,49 @@ def test_spec_decode_xfail_spec_max_model_len(test_llm_generator):
     with pytest.raises(ValueError, match="cannot be larger than"):
         get_output_from_llm_generator(test_llm_generator, prompts,
                                       sampling_params)
+
+
+@pytest.mark.parametrize("common_llm_kwargs",
+                         [{
+                             "model": "meta-llama/Llama-2-7b-chat-hf",
+                             "speculative_model": "JackFram/llama-68m",
+                             "num_speculative_tokens": 5,
+                             "enable_chunked_prefill": "True",
+                         }])
+@pytest.mark.parametrize("per_test_common_llm_kwargs", [
+    {
+        "tensor_parallel_size": 2,
+        "speculative_draft_tensor_parallel_size": 2,
+    },
+    {
+        "tensor_parallel_size": 4,
+        "speculative_draft_tensor_parallel_size": 4,
+    },
+    {
+        "tensor_parallel_size": 8,
+        "speculative_draft_tensor_parallel_size": 8,
+    },
+])
+@pytest.mark.parametrize("test_llm_kwargs", [{}])
+@pytest.mark.parametrize("seed", [1])
+def test_spec_decode_xfail_chunked_prefill_draft_model_tp_not_one(
+        test_llm_generator):
+    """Verify that speculative decoding fails if chunked prefill is enabled for 
+    draft model with tensor parallelism of more than 1.
+    """
+    output_len = 128
+    temperature = 0.0
+
+    prompts = [
+        "Hello, my name is",
+    ]
+
+    sampling_params = SamplingParams(
+        max_tokens=output_len,
+        ignore_eos=True,
+        temperature=temperature,
+    )
+
+    with pytest.raises(ValueError, match="with tensor parallel size 1"):
+        get_output_from_llm_generator(test_llm_generator, prompts,
+                                      sampling_params)

vllm/config.py

@@ -1388,6 +1388,23 @@ class SpeculativeConfig:
                     "Chunked prefill and hidden-state based draft models are "
                     "not compatible.")
 
+            speculative_draft_tensor_parallel_size = \
+                SpeculativeConfig._verify_and_get_draft_model_tensor_parallel_size(
+                    target_parallel_config,
+                    speculative_draft_tensor_parallel_size,
+                    draft_hf_config
+            )
+
+            if (enable_chunked_prefill and \
+                 speculative_draft_tensor_parallel_size != 1):
+                # TODO - Investigate why the error reported in
+                # https://github.com/vllm-project/vllm/pull/9291#issuecomment-2463266258
+                # is happening and re-enable it.
+                raise ValueError(
+                    "Chunked prefill and speculative decoding can be enabled "
+                    "simultaneously only for draft models with tensor "
+                    "parallel size 1.")
+
             draft_model_config.max_model_len = (
                 SpeculativeConfig._maybe_override_draft_max_model_len(
                     speculative_max_model_len,
@@ -1466,15 +1483,16 @@ class SpeculativeConfig:
         )
 
     @staticmethod
-    def create_draft_parallel_config(
+    def _verify_and_get_draft_model_tensor_parallel_size(
             target_parallel_config: ParallelConfig,
             speculative_draft_tensor_parallel_size: Optional[int],
-        draft_hf_config: PretrainedConfig,
-    ) -> ParallelConfig:
-        """Create a parallel config for use by the draft worker.
-
-        This is mostly a copy of the target parallel config, except the tp_size.
+            draft_hf_config: PretrainedConfig) -> int:
+        """
+        Verifies and adjusts the tensor parallel size for a draft model
+        specified using speculative_draft_tensor_parallel_size.
         """
+        # If speculative_draft_tensor_parallel_size is unset then set it
+        # appropriately else verify that it is set correctly.
         if speculative_draft_tensor_parallel_size is None:
             if draft_hf_config.model_type == "mlp_speculator":
                 speculative_draft_tensor_parallel_size = 1
@@ -1490,7 +1508,18 @@ class SpeculativeConfig:
             raise ValueError(
                 f"{speculative_draft_tensor_parallel_size=} cannot be "
                 f"other value than 1 or target model tensor_parallel_size")
+        return speculative_draft_tensor_parallel_size
 
+    @staticmethod
+    def create_draft_parallel_config(
+        target_parallel_config: ParallelConfig,
+        speculative_draft_tensor_parallel_size: int,
+        draft_hf_config: PretrainedConfig,
+    ) -> ParallelConfig:
+        """Create a parallel config for use by the draft worker.
+
+        This is mostly a copy of the target parallel config, except the tp_size.
+        """
         draft_parallel_config = ParallelConfig(
             pipeline_parallel_size=target_parallel_config.
             pipeline_parallel_size,