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 corresponding to the last error that occurred in NCCL. Note: The error is not cleared by calling this function. Please note that the string returned by ncclGetLastError could be unrelated to the current call and can be a result of previously launched asynchronous operations, if any.


const char* ncclGetErrorString(ncclResult_t result)

Returns a human-readable string corresponding to the passed error code.


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 used within 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 ncclCommSplit(ncclComm_t comm, int color, int key, ncclComm_t* newcomm, ncclConfig_t* config)

The ncclCommSplit function creates a set of new communicators from an existing one. Ranks which pass the same color value will be part of the same group; color must be a non-negative value. If it is passed as NCCL_SPLIT_NOCOLOR, it means that the rank will not be part of any group, therefore returning NULL as newcomm. The value of key will determine the rank order, and the smaller key means the smaller rank in new communicator. If keys are equal between ranks, then the rank in the original communicator will be used to order ranks. If the new communicator needs to have a special configuration, it can be passed as config, otherwise setting config to NULL will make the new communicator inherit the original communicator’s configuration. When split, there should not be any outstanding NCCL operations on the comm. Otherwise, it might cause a deadlock.


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 becomes ncclSuccess before calling ncclCommDestroy. In all cases, the communicator should no longer be accessed after ncclCommDestroy returns. It is recommended that users call ncclCommFinalize and then ncclCommDestroy. This function is an intra-node collective call, which all ranks on the same node should call to avoid a hang.


ncclResult_t ncclCommAbort(ncclComm_t comm)

ncclCommAbort frees resources that are allocated to a communicator object comm and aborts any uncompleted operations before destroying the communicator. All active ranks are required to call this function in order to abort the NCCL communicator successfully. For more use cases, please check Fault Tolerance.


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 the return code of any NCCL function 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 all the states become ncclSuccess before calling another 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 caller in the NCCL communicator comm.


ncclResult_t ncclCommRegister(const ncclComm_t comm, void* buff, size_t size, void** handle)

Registers the buffer buff with size under communicator comm for zero-copy communication; handle is returned for future deregistration. See buff and size requirements and more instructions in User Buffer Registration.


ncclResult_t ncclCommDeregister(const ncclComm_t comm, void* handle)

Deregister buffer represented by handle under communicator comm.


ncclResult_t ncclMemAlloc(void **ptr, size_t size)

Allocate a GPU buffer with size. Allocated buffer head address will be returned by ptr, and the actual allocated size can be larger than requested because of the buffer granularity requirements from all types of NCCL optimizations.


ncclResult_t ncclMemFree(void *ptr)

Free memory allocated by ncclMemAlloc().