# All API

struct ncclUniqueId

  Public Members

  char internal[NCCL_UNIQUE_ID_BYTES]

file nccl.h
  #include <hip/hip_runtime.h>
  #include <hip/hip_fp16.h>

  Defines

  NCCL_MAJOR
  NCCL_MINOR
  NCCL_PATCH
  NCCL_SUFFIX
  NCCL_VERSION_CODE
  NCCL_VERSION(X, Y, Z)
  RCCL_BFLOAT16
  RCCL_GATHER_SCATTER
  RCCL_ALLTOALLV
  RCCL_MULTIRANKPERGPU
  NCCL_UNIQUE_ID_BYTES

  Typedefs

  typedef struct ncclComm *ncclComm_t

    Opaque handle to communicator.

  Enums

  enum ncclResult_t

    Error type.

    Values:

    enumerator ncclSuccess
    enumerator ncclUnhandledCudaError
    enumerator ncclSystemError
    enumerator ncclInternalError
    enumerator ncclInvalidArgument
    enumerator ncclInvalidUsage
    enumerator ncclRemoteError
    enumerator ncclNumResults
  enum ncclRedOp_dummy_t

    Reduction operation selector.

    Values:

    enumerator ncclNumOps_dummy
  enum ncclRedOp_t

    Values:

    enumerator ncclSum
    enumerator ncclProd
    enumerator ncclMax
    enumerator ncclMin
    enumerator ncclAvg
    enumerator ncclNumOps
    enumerator ncclMaxRedOp
  enum ncclDataType_t

    Data types.

    Values:

    enumerator ncclInt8
    enumerator ncclChar
    enumerator ncclUint8
    enumerator ncclInt32
    enumerator ncclInt
    enumerator ncclUint32
    enumerator ncclInt64
    enumerator ncclUint64
    enumerator ncclFloat16
    enumerator ncclHalf
    enumerator ncclFloat32
    enumerator ncclFloat
    enumerator ncclFloat64
    enumerator ncclDouble
    enumerator ncclBfloat16
    enumerator ncclNumTypes
  enum ncclScalarResidence_t

    Values:

    enumerator ncclScalarDevice
    enumerator ncclScalarHostImmediate

  Functions

  ncclResult_t ncclGetVersion(int *version)

    Return the NCCL_VERSION_CODE of the NCCL library in the supplied integer.

    This integer is coded with the MAJOR, MINOR and PATCH level of the NCCL library

  ncclResult_t ncclGetUniqueId(ncclUniqueId *uniqueId)

    Generates an ID for ncclCommInitRank.

    Generates an ID to be used in ncclCommInitRank. ncclGetUniqueId should be called once and the Id should be distributed to all ranks in the communicator before calling ncclCommInitRank.

    Parameters:

      uniqueId – [in] ncclUniqueId* pointer to uniqueId

  ncclResult_t ncclCommInitRank(ncclComm_t *comm, int nranks, ncclUniqueId commId, int rank)

    Creates a new communicator (multi thread/process version).

    rank must be between 0 and nranks-1 and unique within a communicator clique. Each rank is associated to a CUDA device, which has to be set before calling ncclCommInitRank. ncclCommInitRank implicitly syncronizes with other ranks, so it must be called by different threads/processes or use ncclGroupStart/ncclGroupEnd.

    Parameters:

      comm – [in] ncclComm_t* communicator struct pointer

  ncclResult_t ncclCommInitRankMulti(ncclComm_t *comm, int nranks, ncclUniqueId commId, int rank, int virtualId)

    Creates a new communicator (multi thread/process version) allowing multiple ranks per device.

    rank must be between 0 and nranks-1 and unique within a communicator clique. Each rank is associated to a HIP device, which has to be set before calling ncclCommInitRankMulti. Since this version of the function allows multiple ranks to utilize the same HIP device, a unique virtualId per device has to be provided by each calling rank. ncclCommInitRankMulti implicitly syncronizes with other ranks, so it must be called by different threads/processes or use ncclGroupStart/ncclGroupEnd.

    Parameters:

      comm – [in] ncclComm_t* communicator struct pointer

  ncclResult_t ncclCommInitAll(ncclComm_t *comm, int ndev, const int *devlist)

    Creates a clique of communicators (single process version).

    This is a convenience function to create a single-process communicator clique. Returns an array of ndev newly initialized communicators in comm. comm should be pre-allocated with size at least ndev*sizeof(ncclComm_t). If devlist is NULL, the first ndev HIP devices are used. Order of devlist defines user-order of processors within the communicator.

  ncclResult_t ncclCommDestroy(ncclComm_t comm)

    Frees resources associated with communicator object, but waits for any operations that might still be running on the device.

  ncclResult_t ncclCommAbort(ncclComm_t comm)

    Frees resources associated with communicator object and aborts any operations that might still be running on the device.

  const char *ncclGetErrorString(ncclResult_t result)

    Returns a string for each error code.

  const char *ncclGetLastError(ncclComm_t comm)

    Returns a human-readable message of the last error that occurred. comm is currently unused and can be set to NULL.

  ncclResult_t ncclCommGetAsyncError(ncclComm_t comm, ncclResult_t *asyncError)
  ncclResult_t ncclCommCount(const ncclComm_t comm, int *count)

    Gets the number of ranks in the communicator clique.

  ncclResult_t ncclCommCuDevice(const ncclComm_t comm, int *device)

    Returns the rocm device number associated with the communicator.

  ncclResult_t ncclCommUserRank(const ncclComm_t comm, int *rank)

    Returns the user-ordered “rank” associated with the communicator.

  ncclResult_t ncclRedOpCreatePreMulSum(ncclRedOp_t *op, void *scalar, ncclDataType_t datatype, ncclScalarResidence_t residence, ncclComm_t comm)
  ncclResult_t pncclRedOpCreatePreMulSum(ncclRedOp_t *op, void *scalar, ncclDataType_t datatype, ncclScalarResidence_t residence, ncclComm_t comm)
  ncclResult_t ncclRedOpDestroy(ncclRedOp_t op, ncclComm_t comm)
  ncclResult_t pncclRedOpDestroy(ncclRedOp_t op, ncclComm_t comm)
  ncclResult_t ncclReduce(const void *sendbuff, void *recvbuff, size_t count, ncclDataType_t datatype, ncclRedOp_t op, int root, ncclComm_t comm, hipStream_t stream)

    Reduce.

    Reduces data arrays of length count in sendbuff into recvbuff using op operation. recvbuff may be NULL on all calls except for root device. root is the rank (not the CUDA device) where data will reside after the operation is complete.

    In-place operation will happen if sendbuff == recvbuff.

  ncclResult_t ncclBcast(void *buff, size_t count, ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream)

    (deprecated) Broadcast (in-place)

    Copies count values from root to all other devices. root is the rank (not the CUDA device) where data resides before the operation is started.

    This operation is implicitely in place.

  ncclResult_t ncclBroadcast(const void *sendbuff, void *recvbuff, size_t count, ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream)

    Broadcast.

    Copies count values from root to all other devices. root is the rank (not the HIP device) where data resides before the operation is started.

    In-place operation will happen if sendbuff == recvbuff.

  ncclResult_t ncclAllReduce(const void *sendbuff, void *recvbuff, size_t count, ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm, hipStream_t stream)

    All-Reduce.

    Reduces data arrays of length count in sendbuff using op operation, and leaves identical copies of result on each recvbuff.

    In-place operation will happen if sendbuff == recvbuff.

  ncclResult_t ncclReduceScatter(const void *sendbuff, void *recvbuff, size_t recvcount, ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm, hipStream_t stream)

    Reduce-Scatter.

    Reduces data in sendbuff using op operation and leaves reduced result scattered over the devices so that recvbuff on rank i will contain the i-th block of the result. Assumes sendcount is equal to nranks*recvcount, which means that sendbuff should have a size of at least nranks*recvcount elements.

    In-place operations will happen if recvbuff == sendbuff + rank * recvcount.

  ncclResult_t ncclAllGather(const void *sendbuff, void *recvbuff, size_t sendcount, ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream)

    All-Gather.

    Each device gathers sendcount values from other GPUs into recvbuff, receiving data from rank i at offset i*sendcount. Assumes recvcount is equal to nranks*sendcount, which means that recvbuff should have a size of at least nranks*sendcount elements.

    In-place operations will happen if sendbuff == recvbuff + rank * sendcount.

  ncclResult_t ncclSend(const void *sendbuff, size_t count, ncclDataType_t datatype, int peer, ncclComm_t comm, hipStream_t stream)

    Send.

    Send data from sendbuff to rank peer. Rank peer needs to call ncclRecv with the same datatype and the same count from this rank.

    This operation is blocking for the GPU. If multiple ncclSend and ncclRecv operations need to progress concurrently to complete, they must be fused within a ncclGroupStart/ ncclGroupEnd section.

  ncclResult_t ncclRecv(void *recvbuff, size_t count, ncclDataType_t datatype, int peer, ncclComm_t comm, hipStream_t stream)

    Receive.

    Receive data from rank peer into recvbuff. Rank peer needs to call ncclSend with the same datatype and the same count to this rank.

    This operation is blocking for the GPU. If multiple ncclSend and ncclRecv operations need to progress concurrently to complete, they must be fused within a ncclGroupStart/ ncclGroupEnd section.

  ncclResult_t ncclGather(const void *sendbuff, void *recvbuff, size_t sendcount, ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream)

    Gather.

    Root device gathers sendcount values from other GPUs into recvbuff, receiving data from rank i at offset i*sendcount.

    Assumes recvcount is equal to nranks*sendcount, which means that recvbuff should have a size of at least nranks*sendcount elements.

    In-place operations will happen if sendbuff == recvbuff + rank * sendcount.

  ncclResult_t ncclScatter(const void *sendbuff, void *recvbuff, size_t recvcount, ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream)

    Scatter.

    Scattered over the devices so that recvbuff on rank i will contain the i-th block of the data on root.

    Assumes sendcount is equal to nranks*recvcount, which means that sendbuff should have a size of at least nranks*recvcount elements.

    In-place operations will happen if recvbuff == sendbuff + rank * recvcount.

  ncclResult_t ncclAllToAll(const void *sendbuff, void *recvbuff, size_t count, ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream)

    All-To-All.

    Device (i) send (j)th block of data to device (j) and be placed as (i)th block. Each block for sending/receiving has count elements, which means that recvbuff and sendbuff should have a size of nranks*count elements.

    In-place operation will happen if sendbuff == recvbuff.

  ncclResult_t ncclAllToAllv(const void *sendbuff, const size_t sendcounts[], const size_t sdispls[], void *recvbuff, const size_t recvcounts[], const size_t rdispls[], ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream)

    All-To-Allv.

    Device (i) sends sendcounts[j] of data from offset sdispls[j] to device (j). In the same time, device (i) receives recvcounts[j] of data from device (j) to be placed at rdispls[j].

    sendcounts, sdispls, recvcounts and rdispls are all measured in the units of datatype, not bytes.

    In-place operation will happen if sendbuff == recvbuff.

  ncclResult_t ncclGroupStart()

    Group Start.

    Start a group call. All calls to NCCL until ncclGroupEnd will be fused into a single NCCL operation. Nothing will be started on the CUDA stream until ncclGroupEnd.

  ncclResult_t ncclGroupEnd()

    Group End.

    End a group call. Start a fused NCCL operation consisting of all calls since ncclGroupStart. Operations on the CUDA stream depending on the NCCL operations need to be called after ncclGroupEnd.