This paper describes a new operating system kernel, called the x-kernel, that provides an explicit architecture for constructing and composing network protocols. Our experience implementing and evaluating several protocols in the x-kernel shows that this architecture is both general enough to accommodate a wide range of protocols, yet efficient enough to perform competitively with less structured operating systems. 1 Introduction Network software is at the heart of any distributed system. It manages the communication hardware that connects the processors in the system and it defines abstractions through which processes running on those processors exchange messages. Network software is extremely complex: it must hide the details of the underlying hardware, recover from transmission failures, ensure that messages are delivered to the application processes in the appropriate order, and manage the encoding and decoding of data. To help manage this complexity, network software is divi...

Threads are the vehicle,for concurrency in many approaches to parallel programming. Threads separate the notion of a sequential execution stream from the other aspects of traditional UNIX-like processes, such as address spaces and I/O descriptors. The objective of this separation is to make the expression and control of parallelism sufficiently cheap that the programmer or compiler can exploit even fine-grained parallelism with acceptable overhead. Threads can be supported either by the operating system kernel or by user-level library code in the application address space, but neither approach has been fully satisfactory. This paper addresses this dilemma. First, we argue that the performance of kernel threads is inherently worse than that of user-level threads, rather than this being an artifact of existing implementations; we thus argue that managing par- allelism at the user level is essential to high-performance parallel computing. Next, we argue that the lack of system integration exhibited by user-level threads is a consequence of the lack of kernel support for user-level threads provided by contemporary multiprocessor operating systems; we thus argue that kernel threads or processes, as currently conceived, are the wrong abstraction on which to support user- level management of parallelism. Finally, we describe the design, implementation, and performance of a new kernel interface and user-level thread package that together provide the same functionality as kernel threads without compromis- ing the performance and flexibility advantages of user-level management of parallelism.

This paper describes the motivation, architecture and performance of SPIN, an extensible operating system. SPIN provides an extension infrastructure, together with a core set of extensible services, that allow applications to safely change the operating system's interface and implementation. Extensions allow an application to specialize the underlying operating system in order to achieve a particular level of performance and functionality. SPIN uses language and link-time mechanisms to inexpensively export ne-grained interfaces to operating system services. Extensions are written in a type safe language, and are dynamically linked into the operating system kernel. This approach o ers extensions rapid access to system services, while protecting the operating system code executing within the kernel address space. SPIN and its extensions are written in Modula-3 and run on DEC Alpha workstations. 1

The V distributed System was developed at Stanford University as part of a research project to explore issues in distributed systems. Aspects of the design suggest important directions for the design of future operating systems and communication systems.

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.

The problem of cache coherence in shared-memory multiprocessors has been addressed using two basic approaches: directory schemes and snoopy cache schemes. Directory schemes have been given less attention in the past several years, while snoopy cache methods have become extremely popular. Directory schemes for cache coherence are potentially attractive in large multiprocessor systems that are beyond the scaling limits of the snoopy cache schemes. Slight modifications to directory schemes can make them competitive in performance with snoopy cache schemes for small multiprocessors. Trace driven simulation, using data collected from several real multiprocessor applications, is used to compare the performance of standard directory schemes, modifications to these schemes, and snoopy cache protocols. 1 Introduction In the past several years, shared-memory multiprocessors have gained wide-spread attention due to the simplicity of the shared-memory parallel programming model. However, allowing...

Microprocessor-based shared-memory multiprocessors are becoming widely available and promise to provide cost-effective high-performance computing. This paper describes a programming system called Amber which permits a single application program to use a homogeneous network of multiprocessors in a uniform way, making the network appear to the application as an integrated, non-uniform memory access, shared-memory multiprocessor. This simplifies the development of applications and allows compute-intensive parallel programs to effectively harness the potential of multiple nodes. Amber programs are written using an object-oriented subset of the C++ programming language, supplemented with primitives for managing concurrency and distribution. Amber provides a network-wide shared-object virtual memory in which coherence is provided by hardware means for locally-executing threads, and by software means for remote accesses. Amber runs on top of the Topaz operating system on a network of DEC SRC ...

Caches enhance the performance of multiprocessors by reducing network tra#c and average memory access latency. However, cache-based systems must address the problem of cache coherence. We propose the LimitLESS directory protocol to solve this problem. The LimitLESS scheme uses a combination of hardware and software techniques to realize the performance of a full-map directory with the memory overhead of a limited directory. This protocol is supported by Alewife, a large-scale multiprocessor. We describe the architectural interfaces needed to implement the LimitLESS directory, and evaluate its performance through simulations of the Alewife machine.

Memory Management Units (MMUs) are traditionally used by operating systems to implement disk-paged virtual memory. Some operating systems allow user programs to specify the protection level (inaccessible, readonly. read-write) of pages, and allow user programs t.o handle protection violations. bur. these mechanisms are not. always robust, efficient, or well-mat. ched to the needs of applications.

this paper we do not describe any formal notations or rules for propositional connectives. Instead, we use English keywords, like "if" and "then", and informal reasoning. 4 \Delta E. Wobber et al. --- Conjunctions of principals. We write A B for the conjunction of A and B. If both A says S and B says S then (A B) says S as well. --- Principals quoting principals. We write B j A for B quoting A. If B says A says