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 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

Recently, there has been an effort to specify an IEEE standard for portable operating systems for open systems, called POSIX. One part of it, the POSIX 1003.4a threads extension (Pthreads for short) [12], describes the interface for light-weight threads that rely on shared memory and have a smaller context frame than processes. This paper describes and evaluates the design and implementation of a library of Pthreads calls that is solely based on UNIX. It shows that a library implementation is feasible and can result in good performance. This work can also be used as a comparison of the performance of other implementations, or as a prototyping, testing, and debugging system in the regular UNIX environment. Finally, some problems with the Pthreads standard are identified.

This white paper describes Scout, a new operating system being designed for systems connected to the National Information Infrastructure (NII). Scout provides a communication-oriented software architecture for building operating system code that is specialized for the different systems that we expect to be available on the NII. It includes an explicit path abstraction that both facilitates effective resource management and permits optimizations of the critical path that I/O data follows. These path-enabled optimizations, along with the application of advanced compiler techniques, result in a system that has both predictable and scalable performance. June 17, 1994 Department of Computer Science The University of Arizona Tucson, AZ 1 Introduction As the National Information Infrastructure (NII) evolves, and digital computer networks become ubiquitous, communication will play an increasingly important role in computer systems. In fact, a recent report on the NII rejects the term "compu...

Lock-free data structures implement concurrent objects without the use of mutual exclusion. This approach can avoid performance problems due to unpredictable delays while processes are within critical sections. Although universal methods are known that give lock-free data structures for any abstract data type, the overhead of these methods makes them inefficient when compared to conventional techniques using mutual exclusion, such as spin locks. We give lock-free data structures and algorithms for implementing a shared singly-linked list, allowing concurrent traversal, insertion, and deletion by any number of processes. We also show how the basic data structure can be used as a building block for other lock-free data structures. Our algorithms use the single word Compare-and-Swap synchronization primitive to implement the linked list directly, avoiding the overhead of universal methods, and are thus a practical alternative to using spin locks. 1 Introduction A concurrent object is an...

Application domains such as multimedia, databases, and parallel computing, require operating system services with high performance and high functionality. Existing operating systems provide fixed interfaces and implementations to system services and resources. This makes them inappropriate for applications whose resource demands and usage patterns are poorly matched by the services provided. The SPIN operating system enables system services to be defined in an application-specific fashion, through an extensible microkernel. It offers applications fine-grained control over a machine's logical and physical resources through run-time adaptation of the system to application requirements. 1

threads, library, register promotion, compiler optimization, garbage collection In many environments, multi-threaded code is written in a language that was originally designed without thread support (e.g. C), to which a library of threading primitives was subsequently added. There appears to be a general understanding that this is not the right approach. We provide specific arguments that a pure library approach, in which the compiler is designed independently of threading issues, cannot guarantee correctness of the resulting code. We first review why the approach almost works, and then examine some of the surprising behavior it may entail. We further illustrate that there are very simple cases in which a pure library-based approach seems incapable of expressing an efficient parallel algorithm. Our discussion takes place in the context of C with Pthreads, since it is commonly used, reasonably well specified, and does not attempt to ensure type-safety, which would entail even stronger constraints. The issues we raise are not specific to that context.

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.

This paper describes the architecture and performance of the JX operating system. JX is both an operating system completely written in Java and a runtime system for Java applications.

Efficient implementation of monitors and exceptions is crucial for the performance of Java. One implementation of threads showed a factor of 30 difference in run time on some benchmark programs. This article describes an efficient implementation of monitors for Java as used in the CACAO just-in-time compiler. With this implementation the thread overhead is less than 40% for typical application programs and can be completely eliminated for some applications. This article also gives the implementation details of the new exception handling scheme in CACAO. The new approach reduces the size of the generated native code by a half and allows null pointers to be checked by hardware. By using these techniques, the CACAO system has become the fastest JavaVM implementation for the Alpha processor.