The Unexpected messages portion of the
Message Queue Window shows information for retrieved and enqueued messages that are not yet matched with a receive operation.
Some MPI libraries, such as MPICH, only retrieve messages that have already been received as a side effect of calls to functions such as
MPI_Recv() or
MPI_Iprobe(). (In other words, while some versions of MPI may know about the message, the message may not yet be in a queue.) This means that TotalView can’t list a message until after the destination process makes a call that retrieves it.