Amdahl's law
Both Amdahl's law and Gustafson's law assume that the running time of the serial part of the program is independent of the number of processors. Amdahl's law assumes that the entire problem is of fixed size so that the total amount of work to be done in parallel is also "independent of the number of processors", whereas Gustafson's law assumes that the total amount of work to be done in parallel "varies linearly with the number of processors".