According to an embodiment, a control system includes a detector, an estimating unit, a determining unit, and a controller. The detector detects an idle state. The estimating unit estimates an idle period. When the idle state is detected, the determining unit determines whether a first power consumption when writeback of data which needs to be written back to a main storage device is performed and supply of power to a cache memory is stopped, is larger than a second power consumption when writeback of the data is not performed and supply of power is continued for the idle period. The controller stops the supply of power to the cache memory when the first power consumption is determined to be smaller than the second power consumption and continues the supply of power when the first power consumption is determined to be larger than the second power consumption.