As the name implies, collective communications refers to those MPI functions involving all the processors within the defined communicator group. A collective operation requires that all processes within the communicator group call the same collective communication function with matching arguments. The size of data sent must exactly match the size of data received . In collective communications they must be the same. Except for synchronization routines, MPI collective communication functions are not synchronizing as set by the MPI standards.
Collective communications exist in blocking mode only.
Blocking here means that a process will block until its role in the collective communication is complete, no matter what the completion status is of the other processes participating in the communications. Collective operations do not use the tag field. They are matched according to the order they are executed. Then they all proceed. All calling processes must specify the same root process.