According to one embodiment, a controller includes a state detecting unit, a calculating unit, and a determining unit. The state detecting unit detects an idle state in which indicates there are no process that can execute on a processing device capable of performing one or more processes. The calculating unit calculates a resuming time, which indicates a time length until the next process starts, when the state detecting unit detects the idle state. The determining unit determines an operation mode of the processing device on the basis of the resuming time calculated by the calculating unit.