1. A method for managing energy consumption of a multiprocessor system comprising two or more processors, the multiprocessor system executing at least one application, said method comprising, for each application: a first phase for off-line characterization of a variation of a potential rate of parallelism of execution of the tasks by the two or more processors as a function of time, the off-line characterization being based on a global dynamic task scheduling with worst-case execution times of the tasks, the first phase comprising: receiving a chart of tasks for each of the at least one application; a task labeling phase that associates subsets of the tasks into labels, and a storing phase that stores execution start times of the labels and a number of processors necessary in the multiprocessor system at each start time; and a second phase for on-line detection and exploitation of potential inactivity intervals, the second phase comprising: an idle mode that predicts a duration of the potential inactivity intervals, between two consecutive labels that reduce a rate of parallelism, during which a processor of the multiprocessor system is inactive, and a processor energy consumption scaling technique that assigns a slack time to the next task allocated on the processor to reduce the execution speed of the processor during the next task by lowering the voltage and/or frequency of the processor, wherein the second phase executes a detection and an exploitation of time excesses produced by the tasks during actual execution relative to the execution times of the worst-case execution times.