Rewind the trace, and replay it again. Observe that for this program messages appear initially over the whole body of the matrix, but eventually they all fill just the last column of the matrix. The fact that it is only the last column that is filled means that all messages are gathered by process rank 7, which in this case must have been made responsible for coordinating the work of the Jacobi iterator. Another push button in the VT View Selector window called Connectivity Graph displays a similar information. But this time processes are represented by dots placed on the perimeter of a circle.
Arcs connecting the dots then appear, as messages begin to fly between processes.
As processes perform various actions, e.g., No Communications , or MPI Blocking Send the dots that correspond to those processes change colours. You can stop the trace replay, at any stage, and left-click on any of the dots, to see what it has been up to at that instance. You can also select Source Code in the VT View Selector panel.