1. A computer system having a plurality of system resources including processors, memory and I/O circuitry, the computer system comprising: an interconnection mechanism which electrically interconnects the processors, memory and I/O circuitry so that each processor has electrical access to all of the memory and at least some of the I/O circuitry; a plurality of partitions, each partition including at least one processor, some memory and some I/O circuitry; at least one operating system instance running in each partition wherein a first pair of operating system instances share a first subset of the plurality of system resources and a second pair of operating system instances share a second subset of the plurality of system resources different from the first subset no of operating system resources.