[Core][Doc] Default to multiprocessing for single-node distributed case (#5230)

Co-authored-by: Antoni Baum <antoni.baum@protonmail.com>

docs/source/serving/distributed_serving.rst

@@ -3,11 +3,9 @@
 Distributed Inference and Serving
 =================================
 
-vLLM supports distributed tensor-parallel inference and serving. Currently, we support `Megatron-LM's tensor parallel algorithm <https://arxiv.org/pdf/1909.08053.pdf>`_. We manage the distributed runtime with `Ray <https://github.com/ray-project/ray>`_. To run distributed inference, install Ray with:
+vLLM supports distributed tensor-parallel inference and serving. Currently, we support `Megatron-LM's tensor parallel algorithm <https://arxiv.org/pdf/1909.08053.pdf>`_. We manage the distributed runtime with either `Ray <https://github.com/ray-project/ray>`_ or python native multiprocessing. Multiprocessing can be used when deploying on a single node, multi-node inferencing currently requires Ray.
 
-.. code-block:: console
+Multiprocessing will be used by default when not running in a Ray placement group and if there are sufficient GPUs available on the same node for the configured :code:`tensor_parallel_size`, otherwise Ray will be used. This default can be overridden via the :code:`LLM` class :code:`distributed-executor-backend` argument or :code:`--distributed-executor-backend` API server argument. Set it to :code:`mp` for multiprocessing or :code:`ray` for Ray. It's not required for Ray to be installed for the multiprocessing case.
-
-    $ pip install ray
 
 To run multi-GPU inference with the :code:`LLM` class, set the :code:`tensor_parallel_size` argument to the number of GPUs you want to use. For example, to run inference on 4 GPUs:
 
@@ -25,10 +23,12 @@ To run multi-GPU serving, pass in the :code:`--tensor-parallel-size` argument wh
     $     --model facebook/opt-13b \
     $     --tensor-parallel-size 4
 
-To scale vLLM beyond a single machine, start a `Ray runtime <https://docs.ray.io/en/latest/ray-core/starting-ray.html>`_ via CLI before running vLLM:
+To scale vLLM beyond a single machine, install and start a `Ray runtime <https://docs.ray.io/en/latest/ray-core/starting-ray.html>`_ via CLI before running vLLM:
 
 .. code-block:: console
 
+    $ pip install ray
+
     $ # On head node
     $ ray start --head
 

tests/spec_decode/e2e/conftest.py

@@ -77,7 +77,11 @@ class AsyncLLM:
             swap_space=swap_space,
             enforce_eager=enforce_eager,
             max_seq_len_to_capture=max_seq_len_to_capture,
+            # For now use ray for the distributed back-end, since
+            # we rely on the use of engine_use_ray=True to avoid
+            # reinitializing CUDA in the same process (driver worker)
             engine_use_ray=True,
+            distributed_executor_backend="ray",
             disable_custom_all_reduce=disable_custom_all_reduce,
             **kwargs,
         )

vllm/config.py

@@ -603,9 +603,25 @@ class ParallelConfig:
                                  f"'{self.distributed_executor_backend}'.")
 
         if self.distributed_executor_backend is None and self.world_size > 1:
+            # We use multiprocessing by default if world_size fits on the
+            # current node and we aren't in a ray placement group.
+            from torch.cuda import device_count
+
             from vllm.executor import ray_utils
+            backend = "mp"
             ray_found = ray_utils.ray is not None
-            self.distributed_executor_backend = "ray" if ray_found else "mp"
+            if device_count() < self.world_size:
+                if not ray_found:
+                    raise ValueError("Unable to load Ray which is "
+                                     "required for multi-node inference")
+                backend = "ray"
+            elif ray_found:
+                from ray.util import get_current_placement_group
+                if self.placement_group or get_current_placement_group():
+                    backend = "ray"
+            self.distributed_executor_backend = backend
+            logger.info("Defaulting to use %s for distributed inference",
+                        backend)
 
         self._verify_args()
 

vllm/executor/multiproc_gpu_executor.py

@@ -19,10 +19,6 @@ class MultiprocessingGPUExecutor(DistributedGPUExecutor):
     """Python multiprocessing-based multi-GPU executor"""
 
     def _init_executor(self) -> None:
-        assert (
-            not self.speculative_config
-        ), "Speculative decoding not yet supported for MultiProcGPU backend."
-
         # Create the parallel GPU workers.
         world_size = self.parallel_config.tensor_parallel_size
 

vllm/executor/multiproc_worker_utils.py

@@ -65,10 +65,11 @@ def _set_future_result(future: Union[ResultFuture, asyncio.Future],
         future.set_result(result)
         return
     loop = future.get_loop()
-    if result.exception is not None:
+    if not loop.is_closed():
-        loop.call_soon_threadsafe(future.set_exception, result.exception)
+        if result.exception is not None:
-    else:
+            loop.call_soon_threadsafe(future.set_exception, result.exception)
-        loop.call_soon_threadsafe(future.set_result, result.value)
+        else:
+            loop.call_soon_threadsafe(future.set_result, result.value)
 
 
 class ResultHandler(threading.Thread):