1. A voltage-frequency profile calculator ( 17 ) for determining a voltage and/or frequency profile for a processing resource for a predetermined task, the calculator comprising: means ( 30 ) for determining a number of metrics characterising a distribution of cycle counts for completing said task using said resource; means ( 31 ) for determining a voltage or frequency profile for a next said task dependent on said metrics; wherein the voltage or frequency profile comprises two or more phases each having a different voltage or frequency function over its duration.