[Frontend] Enable support for CPU backend in AsyncLLMEngine. (#3993)

Signed-off-by: Tao He <sighingnow@gmail.com>

vllm/engine/async_llm_engine.py

@@ -343,6 +343,11 @@ class AsyncLLMEngine:
         if engine_config.device_config.device_type == "neuron":
             from vllm.executor.neuron_executor import NeuronExecutorAsync
             executor_class = NeuronExecutorAsync
+        elif engine_config.device_config.device_type == "cpu":
+            assert not engine_config.parallel_config.worker_use_ray, (
+                "Ray is not supported with the CPU backend.")
+            from vllm.executor.cpu_executor import CPUExecutorAsync
+            executor_class = CPUExecutorAsync
         elif engine_config.parallel_config.worker_use_ray:
             initialize_ray_cluster(engine_config.parallel_config)
             from vllm.executor.ray_gpu_executor import RayGPUExecutorAsync

vllm/executor/cpu_executor.py

@@ -4,11 +4,12 @@ from typing import Dict, List, Set, Tuple
 import torch
 
 from vllm.config import CacheConfig, ModelConfig, SchedulerConfig
-from vllm.executor.executor_base import ExecutorBase
+from vllm.executor.executor_base import ExecutorAsyncBase, ExecutorBase
 from vllm.logger import init_logger
 from vllm.lora.request import LoRARequest
 from vllm.sequence import SamplerOutput, SequenceGroupMetadata
-from vllm.utils import get_distributed_init_method, get_ip, get_open_port
+from vllm.utils import (get_distributed_init_method, get_ip, get_open_port,
+                        make_async)
 
 logger = init_logger(__name__)
 
@@ -100,6 +101,28 @@ class CPUExecutor(ExecutorBase):
         return
 
 
+class CPUExecutorAsync(CPUExecutor, ExecutorAsyncBase):
+
+    async def execute_model_async(
+        self,
+        seq_group_metadata_list: List[SequenceGroupMetadata],
+        blocks_to_swap_in: Dict[int, int],
+        blocks_to_swap_out: Dict[int, int],
+        blocks_to_copy: Dict[int, List[int]],
+    ) -> SamplerOutput:
+        output = await make_async(self.driver_worker.execute_model)(
+            seq_group_metadata_list=seq_group_metadata_list,
+            blocks_to_swap_in=blocks_to_swap_in,
+            blocks_to_swap_out=blocks_to_swap_out,
+            blocks_to_copy=blocks_to_copy)
+        return output
+
+    async def check_health_async(self) -> None:
+        # CPUExecutor will always be healthy as long as
+        # it's running.
+        return
+
+
 def _verify_and_get_model_config(config: ModelConfig) -> ModelConfig:
     if config.dtype == torch.float16:
         logger.warning("float16 is not supported on CPU, casting to bfloat16.")