[Feature] Support Pipeline Parallism in torchrun SPMD offline inference for V1 (#17827)

Signed-off-by: Lucia Fang <fanglu@fb.com>

.buildkite/test-pipeline.yaml

@@ -148,6 +148,8 @@ steps:
   # test with tp=2 and external_dp=2
   - VLLM_USE_V1=0 torchrun --nproc-per-node=4 distributed/test_torchrun_example.py
   - torchrun --nproc-per-node=4 distributed/test_torchrun_example.py
+  # test with tp=2 and pp=2
+  - PP_SIZE=2 torchrun --nproc-per-node=4 distributed/test_torchrun_example.py
   # test with internal dp
   - python3 ../examples/offline_inference/data_parallel.py
   - TP_SIZE=2 DP_SIZE=2 pytest -v -s v1/test_async_llm_dp.py

examples/offline_inference/torchrun_example.py

@@ -8,6 +8,8 @@ the argument 2 should match the `tensor_parallel_size` below.
 see `tests/distributed/test_torchrun_example.py` for the unit test.
 """
 
+import torch.distributed as dist
+
 from vllm import LLM, SamplingParams
 
 # Create prompts, the same across all ranks
@@ -27,23 +29,26 @@ sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
 # all ranks have the same random seed, so that sampling can be
 # deterministic across ranks.
 llm = LLM(
-    model="facebook/opt-125m",
+    model="meta-llama/Llama-3.1-8B",
     tensor_parallel_size=2,
+    pipeline_parallel_size=2,
     distributed_executor_backend="external_launcher",
-    seed=0,
+    max_model_len=32768,
+    seed=1,
 )
 
 outputs = llm.generate(prompts, sampling_params)
 
 # all ranks will have the same outputs
-print("-" * 50)
-for output in outputs:
-    prompt = output.prompt
-    generated_text = output.outputs[0].text
-    print(f"Prompt: {prompt!r}\n"
-          f"Generated text: {generated_text!r}")
+if dist.get_rank() == 0:
     print("-" * 50)
-"""
+    for output in outputs:
+        prompt = output.prompt
+        generated_text = output.outputs[0].text
+        print(f"Prompt: {prompt!r}\n"
+              f"Generated text: {generated_text!r}\n")
+        print("-" * 50)
+    """
 Further tips:
 
 1. to communicate control messages across all ranks, use the cpu group,

tests/distributed/test_torchrun_example.py

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 # unit test for `examples/offline_inference/torchrun_example.py`
-
+import os
 import random
 
 import torch.distributed as dist
@@ -25,6 +25,7 @@ sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
 # to test if all ranks agree on the same kv cache configuration.
 llm = LLM(model="facebook/opt-125m",
           tensor_parallel_size=2,
+          pipeline_parallel_size=int(os.getenv("PP_SIZE", 1)),
           distributed_executor_backend="external_launcher",
           gpu_memory_utilization=random.uniform(0.7, 0.9),
           swap_space=random.randint(1, 4),

vllm/config.py

@@ -1695,7 +1695,6 @@ class ParallelConfig:
     """Port of the data parallel master."""
     enable_expert_parallel: bool = False
     """Use expert parallelism instead of tensor parallelism for MoE layers."""
-
     max_parallel_loading_workers: Optional[int] = None
     """Maximum number of parallel loading workers when loading model
     sequentially in multiple batches. To avoid RAM OOM when using tensor

vllm/distributed/device_communicators/custom_all_reduce.py

@@ -265,7 +265,8 @@ class CustomAllreduce:
 
     def close(self):
         if not self.disabled and self._ptr:
-            ops.dispose(self._ptr)
+            if ops is not None:
+                ops.dispose(self._ptr)
             self._ptr = 0
             self.free_shared_buffer(self.meta_ptrs, rank=self.rank)
             self.free_shared_buffer(self.buffer_ptrs, rank=self.rank)
@@ -298,4 +299,5 @@ class CustomAllreduce:
                            rank: Optional[int] = 0) -> None:
         if rank is None:
             rank = dist.get_rank(group=group)
-        ops.free_shared_buffer(pointers[rank])
+        if ops is not None:
+            ops.free_shared_buffer(pointers[rank])

vllm/engine/arg_utils.py

@@ -1383,9 +1383,10 @@ class EngineArgs:
             return False
 
         if (self.pipeline_parallel_size > 1
-                and self.distributed_executor_backend not in ["ray", "mp"]):
+                and self.distributed_executor_backend
+                not in ("ray", "mp", "external_launcher")):
             name = "Pipeline Parallelism without Ray distributed executor " \
-                    "or multiprocessing executor"
+                    "or multiprocessing executor or external launcher"
             _raise_or_fallback(feature_name=name, recommend_to_remove=False)
             return False
 

vllm/executor/uniproc_executor.py

@@ -86,9 +86,6 @@ class ExecutorWithExternalLauncher(UniProcExecutor):
     def _init_executor(self) -> None:
         """Initialize the worker and load the model.
         """
-        assert self.vllm_config.parallel_config.pipeline_parallel_size == 1, \
-            ("ExecutorWithExternalLauncher does not "
-            "support pipeline parallelism.")
         assert self.vllm_config.scheduler_config.delay_factor == 0.0, \
             ("ExecutorWithExternalLauncher needs deterministic "
             "execution, so it"

vllm/v1/worker/gpu_model_runner.py

@@ -22,7 +22,8 @@ from vllm.distributed.kv_transfer import (get_kv_transfer_group,
                                           has_kv_transfer_group)
 from vllm.distributed.kv_transfer.kv_connector.v1 import KVConnectorBase_V1
 from vllm.distributed.parallel_state import (
-    get_pp_group, graph_capture, prepare_communication_buffer_for_model)
+    get_pp_group, get_tp_group, graph_capture,
+    prepare_communication_buffer_for_model)
 from vllm.forward_context import get_forward_context, set_forward_context
 from vllm.logger import init_logger
 from vllm.model_executor.layers.rotary_embedding import MRotaryEmbedding
@@ -1162,13 +1163,32 @@ class GPUModelRunner(LoRAModelRunnerMixin):
             hidden_states, aux_hidden_states = model_output
         else:
             hidden_states = model_output
-
+        # Broadcast PP output for external_launcher (torchrun)
+        # to make sure we are synced across pp ranks
+        # TODO: Support overlapping mirco-batches
+        # https://github.com/vllm-project/vllm/issues/18019
+        broadcast_pp_output = \
+            self.parallel_config.distributed_executor_backend \
+            == "external_launcher" and len(get_pp_group().ranks) > 0
         if not get_pp_group().is_last_rank:
             # For mid-pipeline stages, return the hidden states.
-            return hidden_states
-
-        sample_hidden_states = hidden_states[logits_indices]
-        logits = self.model.compute_logits(sample_hidden_states, None)
+            if not broadcast_pp_output:
+                return hidden_states
+            assert isinstance(hidden_states, IntermediateTensors)
+            get_pp_group().send_tensor_dict(hidden_states.tensors,
+                                            all_gather_group=get_tp_group())
+            logits = None
+        else:
+            sample_hidden_states = hidden_states[logits_indices]
+            logits = self.model.compute_logits(sample_hidden_states, None)
+        if broadcast_pp_output:
+            model_output_broadcast_data = {
+                "logits": logits.contiguous(),
+            } if logits is not None else {}
+            model_output_broadcast_data = get_pp_group().broadcast_tensor_dict(
+                model_output_broadcast_data, src=len(get_pp_group().ranks) - 1)
+            assert model_output_broadcast_data is not None
+            logits = model_output_broadcast_data["logits"]
 
         # Apply structured output bitmasks if present
         if scheduler_output.grammar_bitmask is not None:
@@ -1186,6 +1206,7 @@ class GPUModelRunner(LoRAModelRunnerMixin):
             # creates a new tensor with separate storage from the original
             # logits tensor. This means any in-place operations on bonus_logits
             # won't affect the original logits tensor.
+            assert logits is not None
             bonus_logits = logits[spec_decode_metadata.bonus_logits_indices]
             sampler_output = self.sampler(
                 logits=bonus_logits,

vllm/v1/worker/gpu_worker.py

@@ -275,13 +275,13 @@ class Worker(WorkerBase):
 
         output = self.model_runner.execute_model(scheduler_output,
                                                  intermediate_tensors)
-
-        if not get_pp_group().is_last_rank:
+        parallel_config = self.vllm_config.parallel_config
+        if parallel_config.distributed_executor_backend != "external_launcher" \
+            and not get_pp_group().is_last_rank:
             assert isinstance(output, IntermediateTensors)
             get_pp_group().send_tensor_dict(output.tensors,
                                             all_gather_group=get_tp_group())
             return None
-
         assert isinstance(output, ModelRunnerOutput)
         return output if self.is_driver_worker else None