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 graphics engines, 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 the workload index.