[Core][Doc] Default to multiprocessing for single-node distributed case

docs/source/serving/distributed_serving.rst

@@ -3,7 +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.
+
+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.
 
 .. code-block:: console
 
@@ -25,10 +27,12 @@ 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
 

vllm/config.py

@@ -600,9 +600,24 @@ def __init__(
                                  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"
+            logger.info("Defaulting to use %s for distributed inference",
+                        backend)
 
         self._verify_args()