Demand bound server: Generalized resource reservation for hard real-time systems

Servers have been proposed to implement resource reservations on shared resources. Such reservations isolate the temporal behavior of tasks sharing the shared resources, thereby providing performance guarantees to tasks independent of other tasks. In existing work, resource reservation has been synonymous to utilization (also called bandwidth) on the resource, i.e., we can reserve only a constant fraction of the resource utilization via a server. Such reservation schemes are not suited to serve interrupt-like tasks: tasks that occur seldom but require quick service or tasks with jitter. With this motivation, we present a generalized server algorithm, called Demand Bound Server (DBS), whose offered service is characterized by the demand bound function (dbf) of the task it serves. We show that schedulability of DBS tightly follows that of EDF, and if schedulable a DBS provides a performance guarantee as requested by the dbf of the task. We present an implementation of DBS when the dbf is a shifted-periodic curve and characterize its overhead. We also present efficient composition operations on DBS that widen the class of implemented servers to tightly serve tasks arising in most practical settings.