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 teh 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 significantly 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 different implementation of restartable atomic sequences for the Mach 3.0 and Taos operating systems. These systems' thread management packages rely on atomic operations to implement higher-level mutual exclusion facilties. We show that improving the performance of low-level atomic operations, and therefore mutual exclusion mechanisms, improves application perfomance.

*David D. Redell and John R. Ellis are currently with Digital Equipment Corporation, Palo Alto, CA.