Fast Mutual Exclusion for Uniprocessors Brian

In this paper we describe restartable atomic sequences, an optimistic mechanism for implementing simple atomic operations (such as Test-And-Set) on a uniprocessor. A thread that is suspended within a restartable atomic sequence is resumed by the operating system at the beginning of the sequence, rather than at the point of suspension. This guarantees that the thread eventually executes the sequence atomically. A restartable atomic sequence has signi cantly less overhead than other software-based synchronization mechanisms, such as kernel emulation or software reservation. Consequently, it is an attractive alternative for use on uniprocessors that do not support atomic operations. Even on processors that do support atomic operations in hardware, restartable atomic sequences can have lower overhead. We describe di erent implementations of restartable atomic sequences for the Mach 3.0 and Taos operating systems. These systems' thread management packages This research was sponsored in part by The Defense Advanced Research Projects Agency, Information Science and Technology O ce, under the title \Research on Parallel Computing", ARPA Order No. 7330, issued by DARPA/CMO under Contract MDA972-90-C-0035, by the Open Software Foundation (OSF), and by a grant from the Digital Equipment Corporation (DEC). Bershad was partially supported by a National Science Foundation Presidential Young Investigator Award. The views and conclusions contained in this document are those of the authors and should not be interpretedas representing the o cial policies, either expressed or implied, of DARPA, OSF, DEC, the NSF, or the U.S. government. rely on atomic operations to implement higher-level mutual exclusion facilities. We show that improving the performance of low-level atomic operations, and therefore mutual exclusion mechanisms, improves application performance.