[Core][Distributed] use cpu/gloo to initialize pynccl (#4248)

tests/distributed/test_pynccl.py

@@ -5,6 +5,7 @@ import torch
 
 from vllm.distributed.device_communicators.pynccl import (NCCLCommunicator,
                                                           ncclGetUniqueId)
+from vllm.distributed.parallel_state import init_distributed_environment
 from vllm.utils import update_environment_variables
 
 
@@ -26,19 +27,23 @@ def distributed_run(fn, world_size):
     for p in processes:
         p.join()
 
+    for p in processes:
+        assert p.exitcode == 0
+
 
-def update_env(fn):
+def worker_fn_wrapper(fn):
     # `multiprocessing.Process` cannot accept environment variables directly
     # so we need to pass the environment variables as arguments
     # and update the environment variables in the function
-    def wrapper(env):
+    def wrapped_fn(env):
         update_environment_variables(env)
+        init_distributed_environment()
         fn()
 
-    return wrapper
+    return wrapped_fn
 
 
-@update_env
+@worker_fn_wrapper
 def worker_fn():
     comm = NCCLCommunicator()
     tensor = torch.ones(16, 1024, 1024, dtype=torch.float32).cuda(comm.rank)
@@ -53,7 +58,7 @@ def test_pynccl():
     distributed_run(worker_fn, 2)
 
 
-@update_env
+@worker_fn_wrapper
 def worker_fn_with_cudagraph():
     with torch.no_grad():
         graph = torch.cuda.CUDAGraph()

vllm/distributed/device_communicators/pynccl.py

@@ -20,14 +20,15 @@
 # variable in the code.
 
 import ctypes
-import datetime
 import platform
+from typing import Optional, Union
 
 # ===================== import region =====================
 import torch
 import torch.distributed as dist
-from torch.distributed import ReduceOp
+from torch.distributed import ProcessGroup, ReduceOp
 
+from vllm.distributed.parallel_state import get_cpu_world_group, get_local_rank
 from vllm.logger import init_logger
 from vllm.utils import find_nccl_library, nccl_integrity_check
 
@@ -59,6 +60,18 @@ except Exception as e:
 
 ncclResult_t = ctypes.c_int
 
+_c_ncclGetErrorString = nccl.ncclGetErrorString
+_c_ncclGetErrorString.restype = ctypes.c_char_p
+_c_ncclGetErrorString.argtypes = [ncclResult_t]
+
+
+def NCCL_CHECK(result: ncclResult_t) -> None:
+    if result != 0:
+        error_str = _c_ncclGetErrorString(result)
+        error_str = error_str.decode("utf-8")
+        raise RuntimeError(f"NCCL error: {error_str}")
+
+
 # equivalent to c declaration:
 # ncclResult_t  ncclGetVersion(int *version);
 _c_ncclGetVersion = nccl.ncclGetVersion
@@ -68,8 +81,7 @@ _c_ncclGetVersion.argtypes = [ctypes.POINTER(ctypes.c_int)]
 
 def ncclGetVersion() -> str:
     version = ctypes.c_int()
-    result = _c_ncclGetVersion(ctypes.byref(version))
-    assert result == 0
+    NCCL_CHECK(_c_ncclGetVersion(ctypes.byref(version)))
     # something like 21903 --> "2.19.3"
     version_str = str(version.value)
     major = version_str[0].lstrip("0")
@@ -91,8 +103,7 @@ _c_ncclGetUniqueId.argtypes = [ctypes.POINTER(NcclUniqueId)]
 
 def ncclGetUniqueId() -> NcclUniqueId:
     unique_id = NcclUniqueId()
-    result = _c_ncclGetUniqueId(ctypes.byref(unique_id))
-    assert result == 0
+    NCCL_CHECK(_c_ncclGetUniqueId(ctypes.byref(unique_id)))
     return unique_id
 
 
@@ -199,66 +210,75 @@ class NCCLCommunicator:
 
     def __init__(
         self,
-        backend=None,
-        init_method=None,
-        timeout=datetime.timedelta(seconds=10),
-        world_size: int = -1,
-        rank: int = -1,
-        store=None,
-        group_name: str = "",
-        pg_options=None,
-        local_rank: int = -1,
+        group: Optional[ProcessGroup] = None,
+        device: Optional[Union[int, str, torch.device]] = None,
     ):
-        if not dist.is_initialized():
-            backend = backend or "nccl"
-            assert backend == 'nccl', (
-                "only use nccl backend for starting the NCCL communicator")
-            dist.init_process_group(backend=backend,
-                                    init_method=init_method,
-                                    timeout=timeout,
-                                    world_size=world_size,
-                                    rank=rank,
-                                    store=store,
-                                    group_name=group_name,
-                                    pg_options=pg_options)
-        self.rank = dist.get_rank()
-        self.world_size = dist.get_world_size()
-        if local_rank == -1:
-            local_rank = self.rank
-        self.local_rank = local_rank
-        # don't use these args, as they can be -1
-        # use `self.rank`, `self.local_rank` and `self.world_size` instead
-        del world_size, rank, local_rank
-        torch.cuda.set_device(self.local_rank)
+        """
+        Args:
+            group: the process group to work on. If None, it will use the
+                default process group.
+            device: the device to bind the NCCLCommunicator to. If None,
+                it will be bind to f"cuda:{local_rank}".
+        It is the caller's responsibility to make sure each communicator
+        is bind to a unique device.
+        """
+        assert dist.is_initialized()
+        group = get_cpu_world_group() if group is None else group
+        assert dist.get_backend(group) != dist.Backend.NCCL, (
+            "NCCLCommunicator should be attached to a non-NCCL group.")
+        self.group = group
+        self.rank = dist.get_rank(group)
+        self.world_size = dist.get_world_size(group)
         if self.rank == 0:
             self.unique_id = ncclGetUniqueId()
         else:
             self.unique_id = NcclUniqueId()
-        tensor = torch.ByteTensor(list(self.unique_id.internal)).cuda(
-            self.local_rank)
-        dist.broadcast(tensor, src=0)
-        byte_list = tensor.cpu().tolist()
+        tensor = torch.ByteTensor(list(self.unique_id.internal))
+        dist.broadcast(tensor, src=0, group=group)
+        byte_list = tensor.tolist()
         for i, byte in enumerate(byte_list):
             self.unique_id.internal[i] = byte
         self.comm = ctypes.c_void_p()
-        result = _c_ncclCommInitRank(ctypes.byref(self.comm), self.world_size,
-                                     self.unique_id, self.rank)
-        assert result == 0
-        self.stream = torch.cuda.Stream(device=f"cuda:{self.local_rank}")
+        if device is None:
+            local_rank = get_local_rank()
+            device = torch.device(f"cuda:{local_rank}")
+        elif isinstance(device, int):
+            device = torch.device(f"cuda:{device}")
+        elif isinstance(device, str):
+            device = torch.device(device)
+        # now `device` is a `torch.device` object
+        assert isinstance(device, torch.device)
+        self.device = device
+        # nccl communicator and stream will use this device
+        current_device = torch.cuda.current_device()
+        try:
+            torch.cuda.set_device(device)
+            NCCL_CHECK(
+                _c_ncclCommInitRank(ctypes.byref(self.comm), self.world_size,
+                                    self.unique_id, self.rank))
+            self.stream = torch.cuda.Stream()
+        finally:
+            torch.cuda.set_device(current_device)
 
     def all_reduce(self,
                    tensor: torch.Tensor,
                    op: ReduceOp = ReduceOp.SUM,
                    stream=None):
+        # nccl communicator created on a specific device
+        # will only work on tensors on the same device
+        # otherwise it will cause "illegal memory access"
+        assert tensor.device == self.device, (
+            f"this nccl communicator is created to work on {self.device}, "
+            f"but the input tensor is on {tensor.device}")
         if stream is None:
             stream = self.stream
-        result = _c_ncclAllReduce(ctypes.c_void_p(tensor.data_ptr()),
-                                  ctypes.c_void_p(tensor.data_ptr()),
-                                  tensor.numel(),
-                                  ncclDataTypeEnum.from_torch(tensor.dtype),
-                                  ncclRedOpTypeEnum.from_torch(op), self.comm,
-                                  ctypes.c_void_p(stream.cuda_stream))
-        assert result == 0
+        NCCL_CHECK(
+            _c_ncclAllReduce(ctypes.c_void_p(tensor.data_ptr()),
+                             ctypes.c_void_p(tensor.data_ptr()),
+                             tensor.numel(),
+                             ncclDataTypeEnum.from_torch(tensor.dtype),
+                             ncclRedOpTypeEnum.from_torch(op), self.comm,
+                             ctypes.c_void_p(stream.cuda_stream)))
 
     def __del__(self):
         # `dist` module might have been already destroyed

vllm/distributed/device_communicators/pynccl_utils.py

@@ -2,7 +2,7 @@ import contextlib
 from typing import Optional
 
 import torch
-from torch.distributed import ReduceOp
+from torch.distributed import ProcessGroup, ReduceOp
 
 from vllm.logger import init_logger
 
@@ -37,17 +37,11 @@ def set_pynccl_stream(stream: torch.cuda.Stream):
         pass
 
 
-def init_process_group(world_size: int,
-                       rank: int,
-                       init_method: str,
-                       local_rank: int = -1) -> None:
+def init_process_group(group: Optional[ProcessGroup] = None) -> None:
     assert not is_initialized()
     global comm
     logger.info(f"vLLM is using nccl=={ncclGetVersion()}")
-    comm = NCCLCommunicator(init_method=init_method,
-                            world_size=world_size,
-                            local_rank=local_rank,
-                            rank=rank)
+    comm = NCCLCommunicator(group=group)
 
 
 def all_reduce(input_: torch.Tensor, op=ReduceOp.SUM) -> None:

vllm/distributed/parallel_state.py

@@ -4,6 +4,7 @@
 # Copyright (c) 2022, NVIDIA CORPORATION. All rights reserved.
 """Tensor and pipeline parallel groups."""
 import contextlib
+import os
 from typing import Optional
 
 import torch
@@ -73,6 +74,11 @@ def init_distributed_environment(
         ranks = list(range(torch.distributed.get_world_size()))
         _CPU_WORLD_GROUP = torch.distributed.new_group(ranks=ranks,
                                                        backend="gloo")
+        # set the local rank
+        # local_rank is not available in torch ProcessGroup,
+        # see https://github.com/pytorch/pytorch/issues/122816
+        if local_rank == -1 and distributed_init_method == "env://":
+            local_rank = int(os.environ['LOCAL_RANK'])
         global _LOCAL_RANK
         _LOCAL_RANK = local_rank
 

vllm/worker/worker.py

@@ -298,12 +298,9 @@ def init_worker_distributed_environment(
     elif parallel_config.world_size > 1:
         # NOTE(woosuk): We don't initialize pynccl process group when world size
         # is 1.
-        pynccl_utils.init_process_group(
-            world_size=parallel_config.world_size,
-            local_rank=local_rank,
-            rank=rank,
-            init_method=distributed_init_method,
-        )
+        # NOTE(kaichao): By default, pynccl will use information inside
+        # `parallel_state` for initialization.
+        pynccl_utils.init_process_group()
 
     ensure_model_parallel_initialized(parallel_config.tensor_parallel_size,
                                       parallel_config.pipeline_parallel_size)