Figure 8 demonstrates the phenomenon that viewing a parallel execution with tools like Jumpshot almost always reveals something unexpected. In this case we were conducting a simple test of a medium-sized Beowulf cluster with the ringtest program of Section Using Jumpshot . Here the program is being run on 64 processors, with one process on each. State display has been turned off to better show the paths of the messages. What is interesting is that the messages consistently take just a little longer to get from process 31 to 32 and from 47 to 48. We conjecture that what we are seeing is the hierarchical structure of the Fast Ethernet switch at Caltech, which is connecting groups of 16 machines. We don't know why the jump from 15 to 16 is not as noticeable as the others.