1. A method for migrating workload from a source server computer system to at least one of a plurality of candidate target server computer systems, comprising: determining at least one candidate workload unit for migration from said source server computer system; for each said candidate workload unit, generating a respective projected performance optimization representation for each of said plurality of candidate target server computer systems, each respective projected performance optimization representation representing a respective projected performance optimization obtainable by migrating the respective candidate workload unit to the respective candidate target server computer system, the respective projected performance optimization being dependent at least in part on a projected processor-memory affinity resulting from migrating the respective candidate workload unit to the respective candidate target server computer system, wherein said projected processor-memory affinity comprises at least one parameter projecting an extent to which data required for executing a process or thread on a processor of a non-uniform memory access computer system is held in a main memory portion of the non-uniform memory access computer system which is physically closest to the processor executing the process or thread; selecting a respective target server computer system to which each of at least one said candidate workload unit is migrated from among said plurality of candidate target server computer systems using the projected performance optimization representation; and migrating each at least one candidate workload unit to the respective target server computer system selected by said selecting at least one respective target server computer system to which each of at least one said candidate workload unit is migrated.