In addition to an untuned performance, inefficient resource management in hinders any attempt to offer Quality of Service in scavenging grids. In this case, Best-Effort mechanisms are synonym to unreliability. Evidently it would be impossible, with undedicated resources, to offer any deterministic warranty (if based on a classic reservation of resources) to service access. Although the usage of scavenging grids for real-time execution remains a challenge, new "Service Qualities" may be proposed to compatiblize applications' preferences versus system oscillations. Categories and Subject Descriptors C.2.4 [Distributed Systems]: Distributed applications;