Consider a parallel program with n processes and a synchronization granularity z. Consider also two multiprocessors: a multiprocessor with q processors and run-time reallocation of processes to processors, and a multiprocessor with k processors and no run-time reallocation. There is an inter processor communication delay of t time units for the system with no run-time reallocation. In this paper we define a function g(n,k,q,t,z) such that the minimum completion time for all programs with n processes and a granularity z is at most g(n,k,q,t,z) times longer using the system with no reallocation and k processors compared to using the system with q processors and run-time reallocation. We assume optimal allocation and scheduling of processes to processors. The function g(n,k,q,t,z) is optimal in the sense that there is at least one program, with n processes and a granularity z, such that the ratio is exactly g(n,k,q,t,z). We also validate our results using measurements on distributed and multiprocessor Sun/Solaris environments.