Communicator Operations

Each of the communicator functions operates simultaneously with each of the noncommunicator functions, regardless of what the parameters are and of whether the functions are on the same or different communicators. However, if you are using multiple instances of the same communicator function on the same communicator, where all parameters are the same, it cannot be determined which threads belong to which resultant communicator. Therefore, when concurrent threads issue such calls, you must assure that the calls are synchronized in such a way that threads in different processes participating in the same communicator operation are grouped together. Do this either by using a different base communicator for each call or by making the calls in single-thread mode before actually using them within the separate threads.

Please note also these special situations:

• If you are using multiple instances of the same function with differing parameters and multiple threads, you must use different communicators. You must not use multiple instances of the same function on the same communicator with other differing parameters.

• When using splits with multiple instances of the same function with the same parameters, but with different threads at the split, you must use different communicators.

For example, suppose you wish to produce several communicators in different sets of threads by performing MPI_Comm_split on some base communicator. To ensure proper, thread-safe operation, you should replicate the base communicator via MPI_Comm_dup (in the root thread or in one thread) and then perform MPI_Comm_split on the resulting duplicate communicators.

• Do not free a communicator in one thread if it is still being used by another thread.