Lightweight Remote Procedure Call (LRPC) is a communication facility designed and optimized for communication between protection domains on the same machine. In contemporary small-kernel operating systems, existing RPC systems incur an unnecessarily high cost when used for the type of communication that predominates-between protection domains on the same machine. This cost leads system designers to coalesce weakly related subsystems into the same protection domain, trading safety for performance. By reducing the overhead of same-machine communication, LRPC encourages both safety and performance. LRPC combines the control transfer and communication model of capability systems with the programming semantics and large-grained protection model of RPC. LRPC achieves a factor-of-three performance improvement over more traditional approaches based on independent threads exchanging messages, reducing the cost of same-machine communication to nearly the lower bound imposed by conventional hardware. LRPC has been integrated into the Taos operating system of the DEC SRC Firefly multiprocessor workstation.

In teaching operating systems at an undergraduate level,itisvery important toprovide a project that is realistic enough to show how real operating systems work, yet simple enough that the students can understand and modify it in signi cant ways. A number of these instructional systems have been created over the last two decades, but recent changes in hardware and software design, along with the increasing power of available computational resources, have changed the basis for many of the tradeo s made by these systems. We have implemented an instructional operating system, called Nachos, and designed a series of assignments to go with it. Our system includes CPU and device simulators, and runs as a regular UNIX process. Nachos illustrates and takes advantage of modern OS technology, such as threads and remote procedure calls, recent hardware advances, such as RISC's and the prevalence of memory hierarchies, and modern software design techniques, such as objectoriented programming and distributed computing. We have used Nachos in the undergraduate operating systems class at Berkeley, with positive results. Nachos is freely available, and we would like to see it widely used for undergraduate instruction. 1

Multiprocessors have been accepted as vehicles for improved computing speeds, cost/performance, and enhanced reliability or availability. However, the added performance requirements of user programs and functional capabilities of parallel hardware introduce new challenges to operating system design and implementation. This paper reviews research and commercial developments in multiprocessor operating system kernels from the late 1970's to the early 1990's. The paper first discusses some common operating system structuring techniques and examines the advantages and disadvantages of using each technique. It then identifies some of the major design goals and key issues in multiprocessor operating systems. Issues and solution approaches are illustrated by review of a variety of research or commercial multiprocessor operating system kernels. College of Computing Georgia Institute of Technology Atlanta, Georgia 30332--0280 Contents 1 Introduction 1 2 Structuring an Operating System 4 2....