The above graph shows HINT run on a unscalable parallel computer. It plots logarithmic time on x-axis and logarithmic QUIPS on y-axis. There is initial overhead to run the program for more than one node and it increases with the number of nodes. Non-scalability of the computer is determined by the fact that the QUIPS decreases with the increase in number of nodes. This is because the processors share a single memory bus, which saturates as nodes are added. There are other things that can limit scalability, of course.
Scalable Parallel Computer