Communicator Creation and Management Functions

The following functions are public APIs exposed by NCCL to create and manage the collective communication operations.


const char* ncclGetLastError(ncclComm_t comm)

Returns a human-readable string of the last error that occurred in NCCL. Note: The error is not cleared by calling this function. The comm argument is currently unused and can be set to NULL.


ncclResult_t ncclGetVersion(int* version)

The ncclGetVersion function returns the version number of the currently linked NCCL library. The NCCL version number is returned in version and encoded as an integer which includes the NCCL_MAJOR, NCCL_MINOR and NCCL_PATCH levels. The version number returned will be the same as the NCCL_VERSION_CODE defined in nccl.h. NCCL version numbers can be compared using the supplied macro; NCCL_VERSION(MAJOR,MINOR,PATCH)


ncclResult_t ncclGetUniqueId(ncclUniqueId* uniqueId)

Generates an Id to be used in ncclCommInitRank. ncclGetUniqueId should be called once when creating a communicator and the Id should be distributed to all ranks in the communicator before calling ncclCommInitRank. uniqueId should point to a ncclUniqueId object allocated by the user.


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 synchronizes with other ranks, hence it must be called by different threads/processes or use ncclGroupStart/ncclGroupEnd.


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

Creates a clique of communicators (single process version) in a blocking way. This is a convenience function to create a single-process communicator clique. Returns an array of ndev newly initialized communicators in comms. comms should be pre-allocated with size at least ndev*sizeof(ncclComm_t). devlist defines the CUDA devices associated with each rank. If devlist is NULL, the first ndev CUDA devices are used, in order.


ncclResult_t ncclCommInitRankConfig(ncclComm_t* comm, int nranks, ncclUniqueId commId, int rank, ncclConfig_t* config)

This function works the same way as ncclCommInitRank but accepts a configuration argument of extra attributes for the communicator. If config is passed as NULL, the communicator will have the default behavior, as if ncclCommInitRank was called.

See the Creating a communication with options section for details on configuration options.


ncclResult_t ncclCommFinalize(ncclComm_t comm)

Finalize a communicator object comm. When the communicator is marked as nonblocking, ncclCommFinalize is a nonblocking function. Successful return from it will set communicator state as ncclInProgress and indicates the communicator is under finalization where all uncompleted operations and the network-related resources are being flushed and freed. Once all NCCL operations are complete, the communicator will transition to the ncclSuccess state. Users can query that state with ncclCommGetAsyncError.


ncclResult_t ncclCommDestroy(ncclComm_t comm)

Destroy a communicator object comm. ncclCommDestroy only frees the local resources that are allocated to the communicator object comm if ncclCommFinalize was previously called on the communicator; otherwise, ncclCommDestroy will call ncclCommFinalize internally. If ncclCommFinalize is called by users, users should guarantee that the state of the communicator become ncclSuccess before calling ncclCommDestroy. In all cases, the communicators should no longer be accessed after ncclCommDestroy returns. It is recommended that user call ncclCommFinalize and then ncclCommDestroy.


ncclResult_t ncclCommAbort(ncclComm_t comm)

Frees resources that are allocated to a communicator object comm. Will abort any uncompleted operations before destroying the communicator.


ncclResult_t ncclCommGetAsyncError(ncclComm_t comm, ncclResult_t* asyncError)

Queries the progress and potential errors of asynchronous NCCL operations. Operations which do not require a stream argument (e.g. ncclCommFinalize) can be considered complete as soon as the function returns ncclSuccess; operations with a stream argument (e.g. ncclAllReduce) will return ncclSuccess as soon as the operation is posted on the stream but may also report errors through ncclCommGetAsyncError() until they are completed. If return code of any NCCL functions is ncclInProgress, it means the operation is in the process of being enqueued in the background, and users must query the states of the communicators until the all states become ncclSuccess before calling next NCCL function. Before the states change into ncclSuccess, users are not allowed to issue CUDA kernel to the streams being used by NCCL. If there has been an error on the communicator, user should destroy the communicator with ncclCommAbort(). If an error occurs on the communicator, nothing can be assumed about the completion or correctness of operations enqueued on that communicator.


ncclResult_t ncclCommCount(const ncclComm_t comm, int* count)

Returns in count the number of ranks in the NCCL communicator comm.


ncclResult_t ncclCommCuDevice(const ncclComm_t comm, int* device)

Returns in device the CUDA device associated with the NCCL communicator comm.


ncclResult_t ncclCommUserRank(const ncclComm_t comm, int* rank)

Returns in rank the rank of the NCCL communicator comm.