[Core] separate distributed_init from worker (vllm-project#3904)

vllm/model_executor/parallel_utils/parallel_state.py

@@ -4,6 +4,7 @@
 # Copyright (c) 2022, NVIDIA CORPORATION. All rights reserved.
 """Tensor and pipeline parallel groups."""
 import contextlib
+from typing import Optional
 
 import torch
 
@@ -14,14 +15,59 @@
 # Pipeline model parallel group that the current rank belongs to.
 _PIPELINE_MODEL_PARALLEL_GROUP = None
 
+# when people blindly call `torch.distributed.all_reduce` etc,
+# it will use this group. It is initialized with the `backend`
+# parameter of `init_distributed_environment` below.
+# Essentially, this is `torch.distributed.group.WORLD`.
+# We leave a line here to note that this is device-specific.
+# Note that this variable is not safe to use, because when users
+# call `init_distributed_environment` first, and then destroy
+# the process group themselves, this variable will keep a reference to the
+# destroyed process group, which is not useful.
+_DEVICE_WORLD_GROUP = None
+
+# duing `init_distributed_environment`, we will also initialize a
+# group with `gloo` backend, to allow direct coordination between
+# processes through the CPU.
+_CPU_WORLD_GROUP = None
+
+# In summary, after calling `init_distributed_environment`, we will
+# always have two groups: one for device-specific (and is the default)
+# and one for CPU. All processes will be part of both groups.
+
 # A list of global ranks for each pipeline group to ease calculation of the
 # source rank when broadcasting from the first or last pipeline stage.
 _PIPELINE_GLOBAL_RANKS = None
 
 
+def init_distributed_environment(
+    world_size: int,
+    rank: int,
+    distributed_init_method: Optional[str] = None,
+    local_rank: int = -1,
+    backend: str = "nccl",
+):
+    if not torch.distributed.is_initialized():
+        assert distributed_init_method is not None, (
+            "distributed_init_method must be provided when initializing "
+            "distributed environment")
+        # this backend is used for WORLD
+        torch.distributed.init_process_group(
+            backend=backend,
+            init_method=distributed_init_method,
+            world_size=world_size,
+            rank=rank)
+        global _DEVICE_WORLD_GROUP, _CPU_WORLD_GROUP
+        _DEVICE_WORLD_GROUP = torch.distributed.group.WORLD
+        ranks = list(range(torch.distributed.get_world_size()))
+        _CPU_WORLD_GROUP = torch.distributed.new_group(ranks=ranks,
+                                                       backend="gloo")
+
+
 def initialize_model_parallel(
     tensor_model_parallel_size: int = 1,
     pipeline_model_parallel_size: int = 1,
+    backend: Optional[str] = None,
 ) -> None:
     """
     Initialize model parallel groups.
@@ -48,6 +94,8 @@ def initialize_model_parallel(
     # Get world size and rank. Ensure some consistencies.
     assert torch.distributed.is_initialized()
     world_size: int = torch.distributed.get_world_size()
+    # get the backend of _DEVICE_WORLD_GROUP
+    backend = backend or torch.distributed.get_backend()
 
     if (world_size !=
             tensor_model_parallel_size * pipeline_model_parallel_size):
@@ -69,7 +117,7 @@ def initialize_model_parallel(
     for i in range(num_tensor_model_parallel_groups):
         ranks = range(i * tensor_model_parallel_size,
                       (i + 1) * tensor_model_parallel_size)
-        group = torch.distributed.new_group(ranks)
+        group = torch.distributed.new_group(ranks, backend=backend)
         if rank in ranks:
             _TENSOR_MODEL_PARALLEL_GROUP = group
 
@@ -80,7 +128,7 @@ def initialize_model_parallel(
         "pipeline model parallel group is already initialized")
     for i in range(num_pipeline_model_parallel_groups):
         ranks = range(i, world_size, num_pipeline_model_parallel_groups)
-        group = torch.distributed.new_group(ranks)
+        group = torch.distributed.new_group(ranks, backend=backend)
         if rank in ranks:
             _PIPELINE_MODEL_PARALLEL_GROUP = group
             _PIPELINE_GLOBAL_RANKS = ranks
@@ -89,14 +137,17 @@ def initialize_model_parallel(
 def ensure_model_parallel_initialized(
     tensor_model_parallel_size: int,
     pipeline_model_parallel_size: int,
+    backend: Optional[str] = None,
 ) -> None:
     """Helper to initialize model parallel groups if they are not initialized,
     or ensure tensor-parallel and pipeline-parallel sizes are equal to expected
     values if the model parallel groups are initialized.
     """
+    # get the backend of _DEVICE_WORLD_GROUP
+    backend = backend or torch.distributed.get_backend()
     if not model_parallel_is_initialized():
         initialize_model_parallel(tensor_model_parallel_size,
-                                  pipeline_model_parallel_size)
+                                  pipeline_model_parallel_size, backend)
         return
 
     assert (
@@ -117,6 +168,12 @@ def model_parallel_is_initialized():
             and _PIPELINE_MODEL_PARALLEL_GROUP is not None)
 
 
+def get_cpu_world_group():
+    """Get the CPU world group."""
+    assert _CPU_WORLD_GROUP is not None, ("CPU world group is not initialized")
+    return _CPU_WORLD_GROUP
+
+
 def get_tensor_model_parallel_group():
     """Get the tensor model parallel group the caller rank belongs to."""
     assert _TENSOR_MODEL_PARALLEL_GROUP is not None, (

vllm/test_utils.py

@@ -1,8 +1,8 @@
 import ray
 
-from vllm.config import ParallelConfig
+from vllm.model_executor.parallel_utils.parallel_state import (
+    ensure_model_parallel_initialized, init_distributed_environment)
 from vllm.utils import get_open_port
-from vllm.worker.worker import init_distributed_environment
 
 
 def init_test_distributed_environment(
@@ -12,15 +12,14 @@ def init_test_distributed_environment(
     distributed_init_port: str,
     local_rank: int = -1,
 ) -> None:
-    parallel_config = ParallelConfig(pipeline_parallel_size,
-                                     tensor_parallel_size,
-                                     worker_use_ray=True)
     distributed_init_method = f"tcp://localhost:{distributed_init_port}"
     init_distributed_environment(
-        parallel_config,
-        rank,
+        world_size=pipeline_parallel_size * tensor_parallel_size,
+        rank=rank,
         distributed_init_method=distributed_init_method,
         local_rank=local_rank)
+    ensure_model_parallel_initialized(tensor_parallel_size,
+                                      pipeline_parallel_size)
 
 
 def multi_process_tensor_parallel(

vllm/worker/cpu_worker.py

@@ -13,7 +13,7 @@
 from vllm.model_executor.parallel_utils.communication_op import (
     broadcast_tensor_dict)
 from vllm.model_executor.parallel_utils.parallel_state import (
-    ensure_model_parallel_initialized)
+    ensure_model_parallel_initialized, init_distributed_environment)
 from vllm.sequence import SamplerOutput, SequenceGroupMetadata
 from vllm.utils import STR_DTYPE_TO_TORCH_DTYPE
 from vllm.worker.model_runner import ModelRunner
@@ -251,26 +251,12 @@ def init_distributed_environment(self) -> None:
         parallel_config = self.parallel_config
         rank = self.rank
         distributed_init_method = self.distributed_init_method
-
-        if torch.distributed.is_initialized():
-            torch_world_size = torch.distributed.get_world_size()
-            if torch_world_size != parallel_config.world_size:
-                raise RuntimeError(
-                    "torch.distributed is already initialized but the torch "
-                    "world size does not match parallel_config.world_size "
-                    f"({torch_world_size} vs. {parallel_config.world_size}).")
-        elif not distributed_init_method:
-            raise ValueError(
-                "distributed_init_method must be set if torch.distributed "
-                "is not already initialized")
-        else:
-            backend = "gloo"
-            torch.distributed.init_process_group(
-                backend=backend,
-                world_size=parallel_config.world_size,
-                rank=rank,
-                init_method=distributed_init_method,
-            )
+        init_distributed_environment(
+            world_size=parallel_config.world_size,
+            rank=rank,
+            distributed_init_method=distributed_init_method,
+            backend="gloo",
+        )
 
         # A small all_reduce for warmup.
         torch.distributed.all_reduce(torch.zeros(1).cpu())

vllm/worker/worker.py

@@ -15,7 +15,7 @@
     broadcast_tensor_dict)
 from vllm.model_executor.parallel_utils.custom_all_reduce import init_custom_ar
 from vllm.model_executor.parallel_utils.parallel_state import (
-    ensure_model_parallel_initialized)
+    ensure_model_parallel_initialized, init_distributed_environment)
 from vllm.sequence import SamplerOutput, SequenceGroupMetadata
 from vllm.worker.cache_engine import CacheEngine
 from vllm.worker.model_runner import ModelRunner
@@ -97,9 +97,9 @@ def init_device(self) -> None:
             raise RuntimeError(
                 f"Not support device type: {self.device_config.device}")
         # Initialize the distributed environment.
-        init_distributed_environment(self.parallel_config, self.rank,
-                                     self.distributed_init_method,
-                                     self.local_rank)
+        init_worker_distributed_environment(self.parallel_config, self.rank,
+                                            self.distributed_init_method,
+                                            self.local_rank)
         # Set random seed.
         set_random_seed(self.model_config.seed)
 
@@ -248,31 +248,15 @@ def get_cache_block_size_bytes(self, block_size: int,
                                                 self.parallel_config)
 
 
-def init_distributed_environment(
+def init_worker_distributed_environment(
     parallel_config: ParallelConfig,
     rank: int,
     distributed_init_method: Optional[str] = None,
     local_rank: int = -1,
 ) -> None:
     """Initialize the distributed environment."""
-    if torch.distributed.is_initialized():
-        torch_world_size = torch.distributed.get_world_size()
-        if torch_world_size != parallel_config.world_size:
-            raise RuntimeError(
-                "torch.distributed is already initialized but the torch world "
-                "size does not match parallel_config.world_size "
-                f"({torch_world_size} vs. {parallel_config.world_size}).")
-    elif not distributed_init_method:
-        raise ValueError(
-            "distributed_init_method must be set if torch.distributed "
-            "is not already initialized")
-    else:
-        torch.distributed.init_process_group(
-            backend="nccl",
-            world_size=parallel_config.world_size,
-            rank=rank,
-            init_method=distributed_init_method,
-        )
+    init_distributed_environment(parallel_config.world_size, rank,
+                                 distributed_init_method, local_rank)
 
     if pynccl_utils.is_initialized():
         pynccl_world_size = pynccl_utils.get_world_size()
@@ -291,17 +275,18 @@ def init_distributed_environment(
             init_method=distributed_init_method,
         )
 
-    # A small all_reduce for warmup.
-    torch.distributed.all_reduce(torch.zeros(1).cuda())
-    if pynccl_utils.is_initialized():
-        pynccl_utils.all_reduce(torch.zeros(1).cuda())
     ensure_model_parallel_initialized(parallel_config.tensor_parallel_size,
                                       parallel_config.pipeline_parallel_size)
 
     # Initialize a custom fast all-reduce implementation.
     if not parallel_config.disable_custom_all_reduce:
         init_custom_ar()
 
+    # A small all_reduce for warmup.
+    torch.distributed.all_reduce(torch.zeros(1).cuda())
+    if pynccl_utils.is_initialized():
+        pynccl_utils.all_reduce(torch.zeros(1).cuda())
+
 
 def _check_if_gpu_supports_dtype(torch_dtype: torch.dtype):
     # Check if the GPU supports the dtype.