A multithreaded microprocessor includes a thread scheduler and voltage-frequency scheduler (VFS). The thread scheduler uses application-specified QoS requirements, which include required instruction completion rates, and instruction completion information from execution units to schedule the priorities of the threads at which the thread scheduler issues instructions to the execution units. Concurrently, the VFS uses the instruction completion and QoS information to calculate an aggregate utilization of the microprocessor by all the active threads when it is time to scale the voltage-frequency. The aggregate utilization is an effective measure of the amount of work left to be performed relative to the rate requirements. The VFS scales the voltage-frequency based on the aggregate utilization.