FLEX: A Tool for Building E cient and Flexible Systems

Modern operating systems must support a wide variety of services for a diverse set of users. Designers of these systems face a tradeoo between functionality and performance. Systems like Mach provide a set of general abstractions and attempt to handle every situation, which can lead to poor performance for common cases. Other systems, such as Unix, provide a small set of abstractions that can be made very eecient, at the expense of functionality. We are implementing a exible system building tool, FLEX, that allows us to support a powerful operating systems interface eeciently by constructing specialized module implementations at runtime. FLEX improves the performance of existing systems by optimizing interprocess communications paths and relocating servers and clients to reduce communications overhead. These facilities improve the performance of Unix system calls on Mach from 20-400%. Furthermore, FLEX can dynamically extend the kernel in a controlled fashion, which gives user programs access to privileged data and devices not envisioned by the original operating system implementor. 1 Overview Operating systems consist of a collection of interrelated entities that provide a variety of services. Beyond this general description, the speciics of individual operating systems varies widely depending on the decisions made by each system's designers. We believe that there is no \optimal" organization, because the \optimal" design is dependent on a particular system's resources and the needs of its clients. No single operating system structure can perform optimally in a wide variety of environments. The most important aspect of an operating system from the user's point of view is its application programming interface deenition. As long as the operating system eeciently supports its interface and protection requirements, it is irrelevant whether this is provided via functions found at known locations in the user address space (e.g., MS-DOS), via traps to the kernel (e.g., Unix), via remote procedure calls (RPCs) to server processes (e.g., much of Mach 3.0), or any combination of these options. To exploit the exibility that this perspective provides, we are developing the FLEX system building tool. FLEX is a coarse-grained system building service that allows systems to be dynamically constructed using the implementation most appropriate to a given situation. In this context, a \system" is an entity that interacts with the operating system, plus the operating system itself. FLEX provides functionality to an operating system similar to, but more powerful than, that provided to a programming system by a linker. FLEX …