General-purpose operating systems are ill-equipped to meet the quality of service (QoS) requirements of complex real-time applications. Consequently, many classes of real-time applications have either been carefully developed to compensate for inadequate system support, or they have been developed to run on special purpose systems. This work focuses on a safe extension architecture for general purpose systems, to allow applications to customize the behavior of the system for their individual needs. Using Linux as the basis for our work, we enable application programmers to safely incorporate `service extensions' into the kernel, so that application-specific QoS guarantees can be provided. We introduce the notion of `QoS safety', which is concerned with meeting the QoS constraints of applications while maintaining system integrity. 

Our safe extension architecture supports the dynamic-linking of code into the address space of the kernel, to affect service management decisions. Using this approach, we are implementing several service managers  (such as CPU and network service managers), that adapts the scheduling of various real-time tasks and network flows, to ensure their service constraints are met even when there are  run-time changes in resource demands. Extensions are written in a type-safe language, to monitor and adapt resource usage on behalf of specific applications. By embedding code inside the kernel, finer-grained management of system resources can be achieved. Experimental results show that safe kernel extensions can lead to fewer service violations (and, hence, better qualities of service) for real-time tasks, compared to user-level methods that monitor and adapt system resources.

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