With μ-kernel based systems becoming more and more prevalent, the demand for extensible resource management raises - and with it the demand for flexible thread scheduling. In this paper, we investigate the benefits and costs of a μ-kernel that exports scheduling from the kernel to user level. A key idea of our approach is to involve the user level whenever the μ-kernel encounters a situation that is ambiguous with respect to scheduling, and to permit the kernel to resolve the ambiguity based on user decisions. A further key aspect is that we rely on a generic, protection domain neutral interface between kernel and applications. For evaluation, we have developed a hierarchical user level scheduling architecture for the L4 μ-kernel, and a virtualiza-tion environment running on its top. Our environment supports Linux 2.6.9 guest operating systems on IA-32 processors. Experiments indicate an application overhead between 0 and 10 percent compared to a pure in-kernel scheduler solution, but also demonstrate that our architecture enables effective and accurate user-directed scheduling.
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