Most existing general-purpose operating systems provide rich collections of tools and applications, but lack the support for fine-grain concurrency that would ease the construction of applications dealing with asynchrony and allow the exploitation of multiprocessors. Multiple threads (program counters sharing an address space) provide such support, but at the cost of placing additional constraints on the design of operating system interfaces. In this paper we present several guidelines that we used to design the multithreaded operating system interface of the Topaz system built at DEC's Systems Research Center. We show how we used these guidelines to evolve the Topaz interface from the 4.2BSD UNIX [7,12] system interface. We believe the guidelines will be useful for adding multithreading to other operating systems.
One implementation of Topaz runs as the native operating system on SRC's Firefly multiprocessor [18] and allows concurrent execution on separate processors of multiple threads within the same address space. A second implementation of Topaz is layered on 4.2BSD UNIX; it uses multiprogramming techniques to create multiple threads within a single UNIX process. Both implementations make it convenient to compose single-threaded UNIX programs and multithreaded programs using the standard UNIX process composition mechanisms.
Topaz is an extension of the architecture of an existing system rather than an entirely new design because of the dual role it plays at SRC. Topaz serves both as the base for research into distributed systems and multiprocessing and also as the support for SRC's current computing needs (mainly document preparation, electronic mail, and software development). When experimental software can be put into everyday use on the same system that runs existing tools and applications, it is easier to get relevant feedback on that software.
There were several reasons for choosing UNIX in particular as an architectural starting point. The machine-independence of UNIX left the way open for future work at SRC on processor design. UNIX also offered a large set of tools and composition mechanisms, and a framework for exchanging ideas about software throughout the research community.
Section 2 gives a brief overview of Topaz, to set the stage for the rest of the paper. Sections 3 and 4 constitute the heart of the paper: our guidelines for multithreaded interfaces and our use of those guidelines in designing the Topaz operating system interface. Section 5 draws some conclusions about the approach taken in Topaz.