pynccl_wrapper.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project

# 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 dataclasses import dataclass
from typing import Any, Optional

import torch
from torch.distributed import ReduceOp

from vllm import envs
from vllm.logger import init_logger
from vllm.platforms import current_platform
from vllm.utils import find_nccl_library

logger = init_logger(__name__)

# === 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
ncclComm_t = ctypes.c_void_p
ncclWindow_t = ctypes.c_void_p


class ncclUniqueId(ctypes.Structure):
    _fields_ = [("internal", ctypes.c_byte * 128)]


cudaStream_t = ctypes.c_void_p
buffer_type = ctypes.c_void_p

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}")


@dataclass
class Function:
    name: str
    restype: Any
    argtypes: list[Any]


class NCCLLibrary:
    exported_functions = [
        # const char* ncclGetErrorString(ncclResult_t result)
        Function("ncclGetErrorString", ctypes.c_char_p, [ncclResult_t]),
        # ncclResult_t  ncclGetVersion(int *version);
        Function("ncclGetVersion", ncclResult_t,
                 [ctypes.POINTER(ctypes.c_int)]),
        # ncclResult_t ncclGetUniqueId(ncclUniqueId* uniqueId);
        Function("ncclGetUniqueId", ncclResult_t,
                 [ctypes.POINTER(ncclUniqueId)]),
        # 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
        Function("ncclCommInitRank", ncclResult_t, [
            ctypes.POINTER(ncclComm_t), ctypes.c_int, ncclUniqueId,
            ctypes.c_int
        ]),
        # ncclResult_t  ncclAllReduce(
        #   const void* sendbuff, void* recvbuff, size_t count,
        #   ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm,
        #   cudaStream_t stream);
        # note that cudaStream_t is a pointer type, so the last argument
        # is a pointer
        Function("ncclAllReduce", ncclResult_t, [
            buffer_type, buffer_type, ctypes.c_size_t, ncclDataType_t,
            ncclRedOp_t, ncclComm_t, cudaStream_t
        ]),

        # ncclResult_t  ncclReduce(
        #   const void* sendbuff, void* recvbuff, size_t count,
        #   ncclDataType_t datatype, ncclRedOp_t op, int root,
        #   ncclComm_t comm,  cudaStream_t stream);
        # note that cudaStream_t is a pointer type, so the last argument
        # is a pointer
        Function("ncclReduce", ncclResult_t, [
            buffer_type, buffer_type, ctypes.c_size_t, ncclDataType_t,
            ncclRedOp_t, ctypes.c_int, ncclComm_t, cudaStream_t
        ]),

        # ncclResult_t  ncclAllGather(
        #   const void* sendbuff, void* recvbuff, size_t count,
        #   ncclDataType_t datatype, ncclComm_t comm,
        #   cudaStream_t stream);
        # note that cudaStream_t is a pointer type, so the last argument
        # is a pointer
        Function("ncclAllGather", ncclResult_t, [
            buffer_type, buffer_type, ctypes.c_size_t, ncclDataType_t,
            ncclComm_t, cudaStream_t
        ]),

        # ncclResult_t  ncclReduceScatter(
        #   const void* sendbuff, void* recvbuff, size_t count,
        #   ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm,
        #   cudaStream_t stream);
        # note that cudaStream_t is a pointer type, so the last argument
        # is a pointer
        Function("ncclReduceScatter", ncclResult_t, [
            buffer_type, buffer_type, ctypes.c_size_t, ncclDataType_t,
            ncclRedOp_t, ncclComm_t, cudaStream_t
        ]),

        # ncclResult_t  ncclSend(
        #   const void* sendbuff, size_t count, ncclDataType_t datatype,
        #   int dest, ncclComm_t comm, cudaStream_t stream);
        Function("ncclSend", ncclResult_t, [
            buffer_type, ctypes.c_size_t, ncclDataType_t, ctypes.c_int,
            ncclComm_t, cudaStream_t
        ]),

        # ncclResult_t  ncclRecv(
        #   void* recvbuff, size_t count, ncclDataType_t datatype,
        #   int src, ncclComm_t comm, cudaStream_t stream);
        Function("ncclRecv", ncclResult_t, [
            buffer_type, ctypes.c_size_t, ncclDataType_t, ctypes.c_int,
            ncclComm_t, cudaStream_t
        ]),

        # ncclResult_t ncclBroadcast(
        #   const void* sendbuff, void* recvbuff, size_t count,
        #   ncclDataType_t datatype, int root, ncclComm_t comm,
        #   cudaStream_t stream);
        Function("ncclBroadcast", ncclResult_t, [
            buffer_type, buffer_type, ctypes.c_size_t, ncclDataType_t,
            ctypes.c_int, ncclComm_t, cudaStream_t
        ]),

        # 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.
        # ncclResult_t  ncclCommDestroy(ncclComm_t comm);
        Function("ncclCommDestroy", ncclResult_t, [ncclComm_t]),
        # ncclResult_t ncclGroupStart();
        Function("ncclGroupStart", ncclResult_t, []),
        # ncclResult_t ncclGroupEnd();
        Function("ncclGroupEnd", ncclResult_t, []),
        # ncclResult_t ncclCommWindowRegister(
        #   ncclComm_t comm, void* buff, size_t size,
        #   ncclWindow_t* win, int winFlags);
        Function(
            "ncclCommWindowRegister",
            ncclResult_t,
            [
                ncclComm_t,
                buffer_type,
                ctypes.c_size_t,
                ctypes.POINTER(ncclWindow_t),
                ctypes.c_int,
            ],
        ),
        # ncclResult_t ncclCommWindowDeregister(
        #   ncclComm_t comm, ncclWindow_t win);
        Function("ncclCommWindowDeregister", ncclResult_t,
                 [ncclComm_t, ncclWindow_t]),
    ]

    # class attribute to store the mapping from the path to the library
    # to avoid loading the same library multiple times
    path_to_library_cache: dict[str, Any] = {}

    # class attribute to store the mapping from library path
    #  to the corresponding dictionary
    path_to_dict_mapping: dict[str, dict[str, Any]] = {}

    def __init__(self, so_file: Optional[str] = None):

        so_file = so_file or find_nccl_library()

        try:
            if so_file not in NCCLLibrary.path_to_dict_mapping:
                lib = ctypes.CDLL(so_file)
                NCCLLibrary.path_to_library_cache[so_file] = lib
            self.lib = NCCLLibrary.path_to_library_cache[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. "
                "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

        if so_file not in NCCLLibrary.path_to_dict_mapping:
            _funcs: dict[str, Any] = {}
            for func in NCCLLibrary.exported_functions:
                try:
                    f = getattr(self.lib, func.name)
                    f.restype = func.restype
                    f.argtypes = func.argtypes
                    _funcs[func.name] = f
                except AttributeError:
                    if func.name in [
                            "ncclCommWindowRegister",
                            "ncclCommWindowDeregister"
                    ]:
                        if envs.VLLM_USE_NCCL_SYMM_MEM:
                            logger.warning_once(
                                "The symbol %s is not found in the NCCL "
                                "library %s. To enable VLLM_USE_NCCL_SYMM_MEM "
                                " please update your NCCL version to >= "
                                "2.27.03.", func.name, so_file)
                        if current_platform.is_rocm():
                            # Having an exception here on ROCm platform is
                            # not allowed during graph capturing
                            continue
                    raise
            NCCLLibrary.path_to_dict_mapping[so_file] = _funcs
        self._funcs = NCCLLibrary.path_to_dict_mapping[so_file]

    def ncclGetErrorString(self, result: ncclResult_t) -> str:
        return self._funcs["ncclGetErrorString"](result).decode("utf-8")

    def NCCL_CHECK(self, result: ncclResult_t) -> None:
        if result != 0:
            error_str = self.ncclGetErrorString(result)
            raise RuntimeError(f"NCCL error: {error_str}")

    def ncclGetRawVersion(self) -> int:
        version = ctypes.c_int()
        self.NCCL_CHECK(self._funcs["ncclGetVersion"](ctypes.byref(version)))
        # something like 21903
        return version.value

    def ncclGetVersion(self) -> str:
        version_str = str(self.ncclGetRawVersion())
        # something like 21903 --> "2.19.3"
        major = version_str[0].lstrip("0")
        minor = version_str[1:3].lstrip("0")
        patch = version_str[3:].lstrip("0")
        return f"{major}.{minor}.{patch}"

    def ncclGetUniqueId(self) -> ncclUniqueId:
        unique_id = ncclUniqueId()
        self.NCCL_CHECK(self._funcs["ncclGetUniqueId"](
            ctypes.byref(unique_id)))
        return unique_id

    def unique_id_from_bytes(self, data: bytes) -> ncclUniqueId:
        if len(data) != 128:
            raise ValueError(
                f"Expected 128 bytes for ncclUniqueId, got {len(data)} bytes")
        unique_id = ncclUniqueId()
        ctypes.memmove(ctypes.addressof(unique_id.internal), data, 128)
        return unique_id

    def ncclCommInitRank(self, world_size: int, unique_id: ncclUniqueId,
                         rank: int) -> ncclComm_t:
        comm = ncclComm_t()
        self.NCCL_CHECK(self._funcs["ncclCommInitRank"](ctypes.byref(comm),
                                                        world_size, unique_id,
                                                        rank))
        return comm

    def ncclAllReduce(self, sendbuff: buffer_type, recvbuff: buffer_type,
                      count: int, datatype: int, op: int, comm: ncclComm_t,
                      stream: cudaStream_t) -> None:
        # `datatype` actually should be `ncclDataType_t`
        # and `op` should be `ncclRedOp_t`
        # both are aliases of `ctypes.c_int`
        # when we pass int to a function, it will be converted to `ctypes.c_int`
        # by ctypes automatically
        self.NCCL_CHECK(self._funcs["ncclAllReduce"](sendbuff, recvbuff, count,
                                                     datatype, op, comm,
                                                     stream))

    def ncclReduce(self, sendbuff: buffer_type, recvbuff: buffer_type,
                   count: int, datatype: int, op: int, root: int,
                   comm: ncclComm_t, stream: cudaStream_t) -> None:
        # `datatype` actually should be `ncclDataType_t`
        # and `op` should be `ncclRedOp_t`
        # both are aliases of `ctypes.c_int`
        # when we pass int to a function, it will be converted to `ctypes.c_int`
        # by ctypes automatically
        self.NCCL_CHECK(self._funcs["ncclReduce"](sendbuff, recvbuff, count,
                                                  datatype, op, root, comm,
                                                  stream))

    def ncclReduceScatter(self, sendbuff: buffer_type, recvbuff: buffer_type,
                          count: int, datatype: int, op: int, comm: ncclComm_t,
                          stream: cudaStream_t) -> None:
        # `datatype` actually should be `ncclDataType_t`
        # and `op` should be `ncclRedOp_t`
        # both are aliases of `ctypes.c_int`
        # when we pass int to a function, it will be converted to `ctypes.c_int`
        # by ctypes automatically
        self.NCCL_CHECK(self._funcs["ncclReduceScatter"](sendbuff, recvbuff,
                                                         count, datatype, op,
                                                         comm, stream))

    def ncclAllGather(self, sendbuff: buffer_type, recvbuff: buffer_type,
                      count: int, datatype: int, comm: ncclComm_t,
                      stream: cudaStream_t) -> None:
        # `datatype` actually should be `ncclDataType_t`
        # which is an aliases of `ctypes.c_int`
        # when we pass int to a function, it will be converted to `ctypes.c_int`
        # by ctypes automatically
        self.NCCL_CHECK(self._funcs["ncclAllGather"](sendbuff, recvbuff, count,
                                                     datatype, comm, stream))

    def ncclSend(self, sendbuff: buffer_type, count: int, datatype: int,
                 dest: int, comm: ncclComm_t, stream: cudaStream_t) -> None:
        self.NCCL_CHECK(self._funcs["ncclSend"](sendbuff, count, datatype,
                                                dest, comm, stream))

    def ncclRecv(self, recvbuff: buffer_type, count: int, datatype: int,
                 src: int, comm: ncclComm_t, stream: cudaStream_t) -> None:
        self.NCCL_CHECK(self._funcs["ncclRecv"](recvbuff, count, datatype, src,
                                                comm, stream))

    def ncclBroadcast(self, sendbuff: buffer_type, recvbuff: buffer_type,
                      count: int, datatype: int, root: int, comm: ncclComm_t,
                      stream: cudaStream_t) -> None:
        self.NCCL_CHECK(self._funcs["ncclBroadcast"](sendbuff, recvbuff, count,
                                                     datatype, root, comm,
                                                     stream))

    def ncclCommDestroy(self, comm: ncclComm_t) -> None:
        self.NCCL_CHECK(self._funcs["ncclCommDestroy"](comm))

    def ncclGroupStart(self) -> None:
        self.NCCL_CHECK(self._funcs["ncclGroupStart"]())

    def ncclGroupEnd(self) -> None:
        self.NCCL_CHECK(self._funcs["ncclGroupEnd"]())

    def ncclCommWindowRegister(self, comm: ncclComm_t, buff: buffer_type,
                               size: int, win_flags: int) -> ncclWindow_t:
        window = ncclWindow_t()
        self.NCCL_CHECK(self._funcs["ncclCommWindowRegister"](
            comm, buff, size, ctypes.byref(window), win_flags))
        return window

    def ncclCommWindowDeregister(self, comm: ncclComm_t,
                                 window: ncclWindow_t) -> None:
        self.NCCL_CHECK(self._funcs["ncclCommWindowDeregister"](comm, window))


__all__ = [
    "NCCLLibrary", "ncclDataTypeEnum", "ncclRedOpTypeEnum", "ncclUniqueId",
    "ncclComm_t", "cudaStream_t", "buffer_type"
]