We describe an operating system architecture that securely multiplexes machine resources while permitting an unprecedented degree of application-specific customization of traditional operating system abstractions. By abstracting physical hardware resources, traditional operating systems have significantly limited the performance, flexibility, and functionality of applications. The exokernel architecture removes these limitations by allowing untrusted software to implement traditional operating system abstractions entirely at application-level. We have implemented a prototype exokernel-based system that includes Aegis, an exokernel, and ExOS, an untrusted application-level operating system. Aegis defines the low-level interface to machine resources. Applications can allocate and use machine resources, efficiently handle events, and participate in resource revocation. Measurements show that most primitive Aegis operations are 10–100 times faster than Ultrix,a mature monolithic UNIX operating system. ExOS implements processes, virtual memory, and inter-process communication abstractions entirely within a library. Measurements show that ExOS’s application-level virtual memory and IPC primitives are 5–50 times faster than Ultrix’s primitives. These results demonstrate that the exokernel operating system design is practical and offers an excellent combination of performance and flexibility. 1

The exokernel operating system architecture safely gives untrusted software efficient control over hardware and software resources by separating management from protection. This paper describes an exokernel system that allows specialized applications to achieve high performance without sacrificing the performance of unmodified UNIX programs. It evaluates the exokernel architecture by measuring end-to-end application performance on Xok, an exokernel for Intel x86-based computers, and by comparing Xok’s performance to the performance of two widely-used 4.4BSD UNIX systems (Free-BSD and OpenBSD). The results show that common unmodified UNIX applications can enjoy the benefits of exokernels: applications either perform comparably on Xok/ExOS and the BSD UNIXes, or perform significantly better. In addition, the results show that customized applications can benefit substantially from control over their resources (e.g., a factor of eight for a Web server). This paper also describes insights about the exokernel approach gained through building three different exokernel systems, and presents novel approaches to resource multiplexing. 1

We introduce server operating systems, which are sets of abstractions and runtime support for specialized, highperformance server applications. We have designed and are implementing a prototype server OS with support for aggressive specialization, direct device-to-device access, an event-driven organization, and dynamic compiler-assisted ILP. Using this server OS, we have constructed an HTTP server that outperforms servers running on a conventional OS by more than an order of magnitude and that can safely timeshare the hardware platform with other applications. 1 Introduction Servers, the foundations of the client/server model of computing, are rapidly becoming more critical. If we are to realize the promise of global information access, widely distributed computing and even high-performance local area computing, system implementors must construct a variety of server applications (whether centralized or not) that can support large numbers of active clients. Ideally, the development an...

The defining tragedy of the operating systems community has been the definition of an operating system as software that both multiplexes and abstracts the hardware is based on the assumption that it is possible both to define abstractions that are appropriate for all areas and to implement them to perform efficiently in all situations. We believe that the fallacy of this quixotic goal is self-evident, and that the operating system problems of the last two decades (poor performance, poor reliability, poor adaptability, and in exibility) can be traced back to it. The solution we propose is simple: complete elimination of operating system abstractions by lowering the operating system interface to the hardware level.

To provide modularity and performance, operating system kernels should have only minimal embedded functionality. Today's operating systems are large, inefficient and, most importantly, inflexible. In our view, most operating system performance and flexibility problems can be eliminated simply by pushing the operating system interface lower. Our goal is to put abstractions traditionally implemented by the kernel out into user-space, where user-level libraries and servers abstract the exposed hardware resources. To achieve this goal, we have defined a new operating system structure, exokernel, that safely exports the resources defined by the underlying hardware. To enable applications to benefit from full hardware functionality and performance, they are allowed to download additions to the supervisor-mode execution environment. To guarantee that these extensions are safe, techniques such as code inspection, inlined cross-domain procedure calls, and secure languages are used. To test and ...

This paper describes how Xok /ExOS's kernel mechanisms and library operating system organization achieve this flexibility, and retrospectively shares our experiences and lessons learned (both positive and negative). It also describes how we used this flexibility to build and specialize three network data services: the Cheetah HTTP server, the webswamp Web benchmarking tool, and an application-level TCP forwarder. Overall measurements show large performance improvements relative to similar services built on conventional interfaces, in each case reaching the maximum possible end-to-end performance for the experimental platform. For example, Cheetah provides factor of 2--4 increases in throughput compared to highly tuned socket-based implementations and factor of 3--8 increases compared to conventional systems. Webswamp can offer loads that are two to eight times heavier. The TCP forwarder provides 50--300% higher throughput while also providing end-to-end TCP semantics that cannot be achieved with POSIX sockets. With more detailed measurements and profiling, these overall performance improvements are also broken down and attributed to the specific specializations described, providing server writers with insights into where to focus their optimization efforts

The Design and Implementation of a Prototype Exokernel Operating System

Traditional operating system architectures are not able to cope with the demands of ubiquitous computing. These demands include mobility of code and running applications, precise resource control, robustness, and user-friendly failure handling. Furthermore, traditional operating systems were designed for desktop or server use and contain functionality that is ballast for a ubiquitous system. One example for such ballast is the file system. We describe JX, our own operating system architecture, that has the desired properties, follows a microkernel approach, and structures the operating system as a set of components. 1

The exokernel operating system architecture safely gives untrusted software efficient control over hardware and software resources by separating management from protection. This paper describes an exokernel system that allows specialized applications to achieve high performance without sacrificing the performance of unmodified UNIX programs. It evaluates the exokernel architecture by measuring end-to-end application performance on Xok, an exokernel for Intel x86-based computers, and by comparing Xok's performance to the performance of two widely-used 4.4BSD UNIX systems (FreeBSD and OpenBSD). The results show that common unmodified UNIX applications can enjoy the benefits of exokernels: applications either perform comparably on Xok/ExOS and the BSD UNIXes, or perform significantly better. In addition, the results show that customized applications can benefit substantially from control over their resources (e.g., a factor of eight for a Web server). This paper also describes insights ab...