1. A graphics processing unit, comprising: at least one graphics engine, performing a video decoding function or a graphics rendering function according to a graphics command from a driver software; and an engine manager, coupled to each of the graphics engines, recording a workload index of each of the at least one graphics engine, and adjusting a work ability of one of or more of the at least one graphics engine according to an adjustment command from the driver software, wherein the driver software provides the adjustment command according to a management policy and the workload index of each of the at least one graphics engine, wherein the management policy comprises: setting that each of the at least one graphics engine has a first work ability corresponding to a time-limited power supply; and setting that each of the at least one graphics engine has a second work ability corresponding to a permanent power supply, wherein the first work ability of each of the at least one graphics engine is lower than the second work ability of the same graphics engine.