Deadlock

A loop in the graph can indicate deadlock. This is where every process is waiting to receive from the preceding process in the loop. For synchronous communications, such as with MPI_Send, this is a common problem.

For other types of communication it can be the case, with MPI_Send that messages get stuck, for example in an O/S buffer, and the send part of the communication is complete but the receive has not started. If the loop persists after playing the processes and interrupting them again, this indicates a deadlock is likely.