1. A method for allocating resource units of an allocatable resource among a plurality of clients, the method comprising: for each client, evaluating a metric that is a function of at least a usage-based factor, the usage-based factor being a function of a first measure of resource units allocated to the client that the client is actively using and a second measure of resource units allocated to the client that the client is not actively using; and reclaiming and reallocating a resource unit from one of the clients when the metric for that client meets a predetermined criterion.