Adjusting clock frequency

One aspect of the disclosure provides a computer system. In one embodiment, the computer system comprises a clock generator, at least one processor, and a clock frequency controller. The clock generator is configured to provide a clock signal at a clock frequency. The at least one processor is configured to receive the clock signal and to operate at a speed dependent on the clock frequency. The clock frequency controller is configured to receive efficiency information indicating a current efficiency of the at least one processor. The clock frequency controller is further configured to receive a request from the processor for a target number of processor instructions to be handled in a particular time period. The clock frequency controller is further configured to output a frequency control signal to the clock generator for controlling the clock frequency in dependence thereon.