Theoretical aspects of proportional share schedulers have received considerable attention recently. We contribute practical considerations on how to retrofit such schedulers into mainstream time-sharing systems. In particular, we propose /reserv, a uniform API for hierarchical proportional resource sharing. The central idea in /reserv is associating resource reservations with references to shared objects (and not with the objects themselves). We discuss in detail the implementation of /reserv and several proportional share schedulers on FreeBSD; the modified system is called Eclipse/BSD. Our experiments demonstrate that the proposed modifications allow selected applications to isolate their (or their clients') performance from CPU, disk, or network overloads caused by other applications. This capability is increasingly important for soft real-time, multimedia, Web, and distributed client-server applications.