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 projected relative performance optimization obtainable by migrating the respective candidate workload unit to the respective candidate target server computer system, the projected relative 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; 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.