[Misc] Better RayExecutor and multiprocessing compatibility (#14705)

Signed-off-by: Cody Yu <hao.yu.cody@gmail.com>

vllm/engine/arg_utils.py

@@ -26,7 +26,7 @@ from vllm.plugins import load_general_plugins
 from vllm.test_utils import MODEL_WEIGHTS_S3_BUCKET, MODELS_ON_S3
 from vllm.transformers_utils.utils import check_gguf_file
 from vllm.usage.usage_lib import UsageContext
-from vllm.utils import FlexibleArgumentParser, StoreBoolean
+from vllm.utils import FlexibleArgumentParser, StoreBoolean, is_in_ray_actor
 
 if TYPE_CHECKING:
     from vllm.transformers_utils.tokenizer_group import BaseTokenizerGroup
@@ -1245,6 +1245,18 @@ class EngineArgs:
             cpu_offload_gb=self.cpu_offload_gb,
             calculate_kv_scales=self.calculate_kv_scales,
         )
+
+        # Get the current placement group if Ray is initialized and
+        # we are in a Ray actor. If so, then the placement group will be
+        # passed to spawned processes.
+        placement_group = None
+        if is_in_ray_actor():
+            import ray
+
+            # This call initializes Ray automatically if it is not initialized,
+            # but we should not do this here.
+            placement_group = ray.util.get_current_placement_group()
+
         parallel_config = ParallelConfig(
             pipeline_parallel_size=self.pipeline_parallel_size,
             tensor_parallel_size=self.tensor_parallel_size,
@@ -1257,6 +1269,7 @@ class EngineArgs:
                 self.tokenizer_pool_extra_config,
             ),
             ray_workers_use_nsight=self.ray_workers_use_nsight,
+            placement_group=placement_group,
             distributed_executor_backend=self.distributed_executor_backend,
             worker_cls=self.worker_cls,
             worker_extension_cls=self.worker_extension_cls,

vllm/executor/multiproc_worker_utils.py

@@ -16,7 +16,7 @@ import torch
 
 from vllm.config import VllmConfig
 from vllm.logger import init_logger
-from vllm.utils import _check_multiproc_method, get_mp_context, run_method
+from vllm.utils import _maybe_force_spawn, get_mp_context, run_method
 
 logger = init_logger(__name__)
 
@@ -291,7 +291,7 @@ def set_multiprocessing_worker_envs(parallel_config):
     in a multiprocessing environment. This should be called by the parent 
     process before worker processes are created"""
 
-    _check_multiproc_method()
+    _maybe_force_spawn()
 
     # Configure thread parallelism if OMP_NUM_THREADS isn't set
     #

vllm/executor/ray_utils.py

@@ -284,8 +284,9 @@ def initialize_ray_cluster(
     assert_ray_available()
     from vllm.platforms import current_platform
 
-    # Connect to a ray cluster.
-    if current_platform.is_rocm() or current_platform.is_xpu():
+    if ray.is_initialized():
+        logger.info("Ray is already initialized. Skipping Ray initialization.")
+    elif current_platform.is_rocm() or current_platform.is_xpu():
         # Try to connect existing ray instance and create a new one if not found
         try:
             ray.init("auto", ignore_reinit_error=True)
@@ -299,19 +300,21 @@ def initialize_ray_cluster(
     else:
         ray.init(address=ray_address, ignore_reinit_error=True)
 
-    if parallel_config.placement_group:
-        # Placement group is already set.
-        return
-
     device_str = current_platform.ray_device_key
     if not device_str:
         raise ValueError(
             f"current platform {current_platform.device_name} does not "
             "support ray.")
 
-    # Create placement group for worker processes
-    current_placement_group = ray.util.get_current_placement_group()
+    # Create or get the placement group for worker processes
+    if parallel_config.placement_group:
+        current_placement_group = parallel_config.placement_group
+    else:
+        current_placement_group = ray.util.get_current_placement_group()
+
     if current_placement_group:
+        logger.info("Using the existing placement group")
+
         # We are in a placement group
         bundles = current_placement_group.bundle_specs
         # Verify that we can use the placement group.
@@ -331,6 +334,8 @@ def initialize_ray_cluster(
                 f"Required number of devices: {parallel_config.world_size}. "
                 f"Total number of devices: {device_bundles}.")
     else:
+        logger.info("No current placement group found. "
+                    "Creating a new placement group.")
         num_devices_in_cluster = ray.cluster_resources().get(device_str, 0)
         # Log a warning message and delay resource allocation failure response.
         # Avoid immediate rejection to allow user-initiated placement group

vllm/utils.py

@@ -2147,20 +2147,48 @@ def zmq_socket_ctx(path: str, socket_type: Any) -> Iterator[zmq.Socket]:
         ctx.destroy(linger=0)
 
 
-def _check_multiproc_method():
-    if (cuda_is_initialized()
-            and os.environ.get("VLLM_WORKER_MULTIPROC_METHOD") != "spawn"):
-        logger.warning("CUDA was previously initialized. We must use "
-                       "the `spawn` multiprocessing start method. Setting "
-                       "VLLM_WORKER_MULTIPROC_METHOD to 'spawn'. "
-                       "See https://docs.vllm.ai/en/latest/getting_started/"
-                       "troubleshooting.html#python-multiprocessing "
-                       "for more information.")
+def is_in_ray_actor():
+    """Check if we are in a Ray actor."""
+
+    try:
+        import ray
+        return (ray.is_initialized()
+                and ray.get_runtime_context().get_actor_id() is not None)
+    except ImportError:
+        return False
+
+
+def _maybe_force_spawn():
+    """Check if we need to force the use of the `spawn` multiprocessing start
+    method.
+    """
+    if os.environ.get("VLLM_WORKER_MULTIPROC_METHOD") == "spawn":
+        return
+
+    reason = None
+    if cuda_is_initialized():
+        reason = "CUDA is initialized"
+    elif is_in_ray_actor():
+        reason = "In a Ray actor and can only be spawned"
+
+    if reason is not None:
+        logger.warning(
+            "We must use the `spawn` multiprocessing start method. "
+            "Overriding VLLM_WORKER_MULTIPROC_METHOD to 'spawn'. "
+            "See https://docs.vllm.ai/en/latest/getting_started/"
+            "troubleshooting.html#python-multiprocessing "
+            "for more information. Reason: %s", reason)
         os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = "spawn"
 
 
 def get_mp_context():
-    _check_multiproc_method()
+    """Get a multiprocessing context with a particular method (spawn or fork).
+    By default we follow the value of the VLLM_WORKER_MULTIPROC_METHOD to
+    determine the multiprocessing method (default is fork). However, under
+    certain conditions, we may enforce spawn and override the value of
+    VLLM_WORKER_MULTIPROC_METHOD.
+    """
+    _maybe_force_spawn()
     mp_method = envs.VLLM_WORKER_MULTIPROC_METHOD
     return multiprocessing.get_context(mp_method)