[Doc] Update Ray Data distributed offline inference example (vllm-pro…

…ject#4871)

examples/offline_inference_distributed.py

@@ -9,19 +9,31 @@
 
 import numpy as np
 import ray
+from packaging.version import Version
+from ray.util.scheduling_strategies import PlacementGroupSchedulingStrategy
 
 from vllm import LLM, SamplingParams
 
+assert Version(ray.__version__) >= Version(
+    "2.22.0"), "Ray version must be at least 2.22.0"
+
 # Create a sampling params object.
 sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
 
+# Set tensor parallelism per instance.
+tensor_parallel_size = 1
+
+# Set number of instances. Each instance will use tensor_parallel_size GPUs.
+num_instances = 1
+
 
 # Create a class to do batch inference.
 class LLMPredictor:
 
     def __init__(self):
         # Create an LLM.
-        self.llm = LLM(model="meta-llama/Llama-2-7b-chat-hf")
+        self.llm = LLM(model="meta-llama/Llama-2-7b-chat-hf",
+                       tensor_parallel_size=tensor_parallel_size)
 
     def __call__(self, batch: Dict[str, np.ndarray]) -> Dict[str, list]:
         # Generate texts from the prompts.
@@ -43,17 +55,41 @@ def __call__(self, batch: Dict[str, np.ndarray]) -> Dict[str, list]:
 # from cloud storage (such as JSONL, Parquet, CSV, binary format).
 ds = ray.data.read_text("s3://anonymous@air-example-data/prompts.txt")
 
+
+# For tensor_parallel_size > 1, we need to create placement groups for vLLM
+# to use. Every actor has to have its own placement group.
+def scheduling_strategy_fn():
+    # One bundle per tensor parallel worker
+    pg = ray.util.placement_group(
+        [{
+            "GPU": 1,
+            "CPU": 1
+        }] * tensor_parallel_size,
+        strategy="STRICT_PACK",
+    )
+    return dict(scheduling_strategy=PlacementGroupSchedulingStrategy(
+        pg, placement_group_capture_child_tasks=True))
+
+
+resources_kwarg = {}
+if tensor_parallel_size == 1:
+    # For tensor_parallel_size == 1, we simply set num_gpus=1.
+    resources_kwarg["num_gpus"] = 1
+else:
+    # Otherwise, we have to set num_gpus=0 and provide
+    # a function that will create a placement group for
+    # each instance.
+    resources_kwarg["num_gpus"] = 0
+    resources_kwarg["ray_remote_args_fn"] = scheduling_strategy_fn
+
 # Apply batch inference for all input data.
 ds = ds.map_batches(
     LLMPredictor,
     # Set the concurrency to the number of LLM instances.
-    concurrency=10,
-    # Specify the number of GPUs required per LLM instance.
-    # NOTE: Do NOT set `num_gpus` when using vLLM with tensor-parallelism
-    # (i.e., `tensor_parallel_size`).
-    num_gpus=1,
+    concurrency=num_instances,
     # Specify the batch size for inference.
     batch_size=32,
+    **resources_kwarg,
 )
 
 # Peek first 10 results.