From: manish (manish_mittal_at_hotmail.com)
Date: Tue Jan 20 2004 - 22:53:06 PST
A traditional operating system has several big disadvantages. It attempts to provide a complete features needed by all applications which is almost impossible and is inefficient. Its high-level abstractions hide information from applications and limit the functionality of applications. The Exokernel system described in this paper addresses the issue of providing application-level management of physical resources for more flexibility and better OS performance. In this architecture, a minimal kernel called Exokernel securely multiplexes available hardware resources, and enables virtual memory and interprocess communication to be implemented entirely at application level. The paper evaluates the exokernel architecture by measuring end-to-end application performance on Xok/ExOS, an exokernel system for Intel x86-based computers. This paper then covers the issues that arise in its design, explains the authors' experimental methodology, presents the implementation and summarizes with performance measurements of Aegis and ExOS.
One of the design goals of Exokernel is to separate protection from management. The system exports hardware resources rather than emulating them, which allows an efficient and simple implementation. An Exokernel employs three techniques to export resources securely: using secure bindings between applications and machine resources, enabling applications to participate in a resource revocation protocol and allowing exokernel to break secure bindings of uncooperative application by force. The Exokernel enables protected sharing using following mechanisms such as software regions, hierarchically-named capabilities, wakeup predicates and critical sections.
In the end, author lists the advantages of Xok system supported by the performance test results. The Tests show how the performance of a web server application (Cheetah) can be improved by using extensible XIO library. Based on the results of their experiments, the authors demonstrate that Cheetah, that uses exokernal features, performs up to eight times faster than the best UNIX HTTP server on the same hardware. The Aegis and ExOS also demonstrate that low-level secure multiplexing and library operating systems can offer excellent performance.
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