Scale-up experiments evaluate the performance of the system with respect to the number of processors for a fixed workload per processor. We keep the workload per processor constant and measure the runtime as additional processors are added. Ideally the runtime measured this way would remain constant, as for example, doubling the number of processors would double the amount of data to be processed by the system as a whole but there would be twice the number of processors to do it. A constant runtime would be indicated by a horizontal line on a graph of runtime against the number of processors.