One function, the barrier, serves to synchronize processes without passing data. MPI provides the following collective communication functions. In each case, each row of boxes represents data locations in one process. Thus, in the broadcast, initially just the first process contains the item Ao , but after the broadcast all processes contain it.
Figure gives a pictorial representation of the global communication functions.
Some of these functions, such as broadcast or gather, have a single origin or a single receiving process. Such a process is called the root . A major simplification is that collective functions come in blocking versions only. Thus, within each intragroup communication domain, collective calls are matched strictly according to the order of execution. A final simplification of collective functions concerns modes.