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 being a numerical figure of merit 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, wherein said generating a respective projected performance optimization representation for each of said plurality of candidate target server computer systems comprises determining a respective optimum workload unit configuration for each of said plurality of candidate target server computer systems, wherein, for at least one candidate target server computer system, the respective optimum workload unit configuration alters the configuration of one or more existing workload units in the respective candidate target server computer system, wherein for at least one candidate target server computer system, said determining a respective optimum workload unit configuration comprises: (a) generating a respective projected performance optimization representation for each of a plurality of hypothetical configurations of the respective candidate target server computer system, 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 configured according to the respective hypothetical configuration, the projected performance optimization representation 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 configured according to the respective hypothetical configuration; and (b) determining a respective optimum workload unit configuration for the respective candidate target server computer system by selecting one of the plurality of hypothetical configurations of the respective target server computer system according to the projected performance optimization representations for the hypothetical configurations of the respective target server computer system; for each of a plurality of said candidate workload units, generating a respective current performance optimization representation, each respective current performance optimization representation representing a relative performance optimization of the corresponding candidate workload unit as currently configured in the source server computer system; selecting at least one said candidate workload unit for migration from said source server computer system using the current performance optimization representation generated by said generating a respective current performance optimization representation; 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; 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; and if said 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 selects a first target server computer system for which the respective optimum workload unit configuration alters the configuration of one or more existing workload units, then altering at least one configuration parameter of at least one existing workload unit in the first target server computer system according to respective optimum workload unit configuration before migrating the corresponding candidate workload unit selected for migration to the first target server computer system; wherein said source server computer system and said candidate target server computer systems are server systems within a cloud computing environment for providing cloud computing services to a plurality of clients; wherein said 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 is performed in an administrative system which manages said source server computer system and said plurality of candidate target server computer systems, and wherein said for each said candidate workload unit, generating a respective projected performance optimization representation for each of said plurality of candidate target server computer systems is performed respectively in each candidate target server computer system and the respective results are communicated to said administrative system.