Hard Constant Bandwidth Server: Comprehensive formulation and critical scenarios

The Constant Bandwidth Server (CBS) is one of the most used algorithms for implementing resource reservation upon deadline-based schedulers. Although many CBS variants are available in the literature, no proper formalization has been proposed for the CBS in the context of hard reservations, where it is essential to guarantee a bounded-delay service across applications. Existing formulations are affected by a problem that can expose the system to dangerous deadline misses in the presence of blocking. This paper analyzes such a problem and presents a comprehensive and consistent formulation of the CBS for hard reservation scenarios. An overview of the contexts in which a hard CBS can be applied is also provided, focusing on the impact that previous formulations can have on schedulability, when used in conjunction with specific resource sharing protocols or other scheduling mechanisms that may cause a server to block.