1. A computer system having a plurality of partitions, comprising: a pay-per-use (PPU) system including at least one computing resource available for use in the computer system on a chargeable basis; a resource management system for receiving a computing resource request from one of the partitions and allocating computing resources amongst the partitions; wherein the resource management system is arranged to allocate a computing resource from the PPU system to the requesting partition according to the received request without charge by the PPU system, and to allocate a corresponding computing resource to the PPU system from another of the partitions.