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luopingyi
2026-01-09 13:34:11 +08:00
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vllm/distributed/device_communicators/pynccl.py Normal file
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# This file is a pure Python wrapper for the NCCL library.
# The main purpose is to use NCCL combined with CUDA graph.
# Before writing this script, we tried the following approach:
# 1. We tried to use `cupy`, it calls NCCL correctly, but `cupy` itself
# often gets stuck when initializing the NCCL communicator.
# 2. We tried to use `torch.distributed`, but `torch.distributed.all_reduce`
# contains many other potential cuda APIs, that are not allowed during
# capturing the CUDA graph. For further details, please check
# https://discuss.pytorch.org/t/pytorch-cudagraph-with-nccl-operation-failed/ .
#
# Another rejected idea is to write a C/C++ binding for NCCL. It is usually
# doable, but we often encounter issues related with nccl versions, and need
# to switch between different versions of NCCL. See
# https://github.com/NVIDIA/nccl/issues/1234 for more details.
# A C/C++ binding is not flexible enough to handle this. It requires
# recompilation of the code every time we want to switch between different
# versions. This current implementation, with a **pure** Python wrapper, is
# more flexible. We can easily switch between different versions of NCCL by
# changing the environment variable `VLLM_NCCL_SO_PATH`, or the `so_file`
# variable in the code.
import ctypes
import platform
from typing import Optional, Union
# ===================== import region =====================
import torch
import torch.distributed as dist
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
logger = init_logger(__name__)
so_file = find_nccl_library()
try:
# load the library in another process.
# if it core dumps, it will not crash the current process
nccl_integrity_check(so_file)
nccl = ctypes.CDLL(so_file)
except Exception as e:
logger.error(
"Failed to load NCCL library from %s ."
"It is expected if you are not running on NVIDIA/AMD GPUs."
"Otherwise, the nccl library might not exist, be corrupted "
"or it does not support the current platform %s."
"One solution is to download libnccl2 version 2.18 from "
"https://developer.download.nvidia.com/compute/cuda/repos/ "
"and extract the libnccl.so.2 file. If you already have the "
"library, please set the environment variable VLLM_NCCL_SO_PATH"
" to point to the correct nccl library path.", so_file,
platform.platform())
raise e
# === export types and functions from nccl to Python ===
# for the original nccl definition, please check
# https://github.com/NVIDIA/nccl/blob/master/src/nccl.h.in
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
_c_ncclGetVersion.restype = ctypes.c_int
_c_ncclGetVersion.argtypes = [ctypes.POINTER(ctypes.c_int)]
def ncclGetVersion() -> str:
version = ctypes.c_int()
NCCL_CHECK(_c_ncclGetVersion(ctypes.byref(version)))
# something like 21903 --> "2.19.3"
version_str = str(version.value)
major = version_str[0].lstrip("0")
minor = version_str[1:3].lstrip("0")
patch = version_str[3:].lstrip("0")
return f"{major}.{minor}.{patch}"
class NcclUniqueId(ctypes.Structure):
_fields_ = [("internal", ctypes.c_byte * 128)]
# equivalent to c declaration:
# ncclResult_t ncclGetUniqueId(ncclUniqueId* uniqueId);
_c_ncclGetUniqueId = nccl.ncclGetUniqueId
_c_ncclGetUniqueId.restype = ctypes.c_int
_c_ncclGetUniqueId.argtypes = [ctypes.POINTER(NcclUniqueId)]
def ncclGetUniqueId() -> NcclUniqueId:
unique_id = NcclUniqueId()
NCCL_CHECK(_c_ncclGetUniqueId(ctypes.byref(unique_id)))
return unique_id
# equivalent to c declaration:
# ncclResult_t ncclCommInitRank(
# ncclComm_t* comm, int nranks, ncclUniqueId commId, int rank);
# note that ncclComm_t is a pointer type, so the first argument
# is a pointer to a pointer
_c_ncclCommInitRank = nccl.ncclCommInitRank
_c_ncclCommInitRank.restype = ctypes.c_int
_c_ncclCommInitRank.argtypes = [
ctypes.POINTER(ctypes.c_void_p), ctypes.c_int, NcclUniqueId, ctypes.c_int
]
ncclDataType_t = ctypes.c_int
class ncclDataTypeEnum:
ncclInt8 = 0
ncclChar = 0
ncclUint8 = 1
ncclInt32 = 2
ncclInt = 2
ncclUint32 = 3
ncclInt64 = 4
ncclUint64 = 5
ncclFloat16 = 6
ncclHalf = 6
ncclFloat32 = 7
ncclFloat = 7
ncclFloat64 = 8
ncclDouble = 8
ncclBfloat16 = 9
ncclNumTypes = 10
@classmethod
def from_torch(cls, dtype: torch.dtype) -> int:
if dtype == torch.int8:
return cls.ncclInt8
if dtype == torch.uint8:
return cls.ncclUint8
if dtype == torch.int32:
return cls.ncclInt32
if dtype == torch.int64:
return cls.ncclInt64
if dtype == torch.float16:
return cls.ncclFloat16
if dtype == torch.float32:
return cls.ncclFloat32
if dtype == torch.float64:
return cls.ncclFloat64
if dtype == torch.bfloat16:
return cls.ncclBfloat16
raise ValueError(f"Unsupported dtype: {dtype}")
ncclRedOp_t = ctypes.c_int
class ncclRedOpTypeEnum:
ncclSum = 0
ncclProd = 1
ncclMax = 2
ncclMin = 3
ncclAvg = 4
ncclNumOps = 5
@classmethod
def from_torch(cls, op: ReduceOp) -> int:
if op == ReduceOp.SUM:
return cls.ncclSum
if op == ReduceOp.PRODUCT:
return cls.ncclProd
if op == ReduceOp.MAX:
return cls.ncclMax
if op == ReduceOp.MIN:
return cls.ncclMin
if op == ReduceOp.AVG:
return cls.ncclAvg
raise ValueError(f"Unsupported op: {op}")
# equivalent to c declaration:
# ncclResult_t ncclAllReduce(
# const void* sendbuff, void* recvbuff, size_t count,
# ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm,
# udaStream_t stream);
# note that cudaStream_t is a pointer type, so the last argument is a pointer
_c_ncclAllReduce = nccl.ncclAllReduce
_c_ncclAllReduce.restype = ctypes.c_int
_c_ncclAllReduce.argtypes = [
ctypes.c_void_p, ctypes.c_void_p, ctypes.c_size_t, ncclRedOp_t,
ncclDataType_t, ctypes.c_void_p, ctypes.c_void_p
]
# be cautious! this is a collective call, it will block until all
# processes in the communicator have called this function.
# because Python object destruction can happen in random order,
# it is better not to call it at all.
# equivalent to c declaration:
# ncclResult_t ncclCommDestroy(ncclComm_t comm);
_c_ncclCommDestroy = nccl.ncclCommDestroy
_c_ncclCommDestroy.restype = ctypes.c_int
_c_ncclCommDestroy.argtypes = [ctypes.c_void_p]
class NCCLCommunicator:
def __init__(
self,
group: Optional[ProcessGroup] = None,
device: Optional[Union[int, str, torch.device]] = None,
):
"""
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
# note: this rank is the rank in the 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))
ranks = dist.get_process_group_ranks(group)
# arg `src` in `broadcast` is the global rank
dist.broadcast(tensor, src=ranks[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()
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
# `torch.cuda.device` is a context manager that changes the
# current cuda device to the specified one
with torch.cuda.device(device):
NCCL_CHECK(
_c_ncclCommInitRank(ctypes.byref(self.comm), self.world_size,
self.unique_id, self.rank))
self.stream = torch.cuda.Stream()
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
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)))