A Lock-Free Approach to Object Sharing in Real-Time Systems

Srikanth Ramamurthy, Ph.D. 1997

University of North Carolina at Chapel Hill
Department of Computer Science
Chapel Hill, NC
August, 1997


This work aims to establish the viability of lock-free object sharing in uniprocessor real-time systems. Naive usage of conventional lock-based object-sharing schemes in real-time systems leads to unbounded priority inversion. A priority inversion occurs when a task is blocked by a lower-priority task that is inside a critical section. Mechanisms that bound priority inversion usually entail kernel overhead that is sometimes excessive.

We propose that lock-free objects offer an attractive alternative to lock-based schemes because they eliminate priority inversion and its associated problems. On the surface, lock-free objects may seem to be unsuitable for hard real-time systems because accesses to such objects are not guaranteed to complete in bounded time. Nonetheless, we present scheduling conditions that demonstrate the applicability of lock-free objects in hard real-time systems. Our scheduling conditions are applicable to schemes such as rate-monotonic scheduling and earliest-deadline- first scheduling.

Previously known lock-free constructions are targeted towards asynchronous systems; such constructions require hardware support for strong synchronization primitives such as compare-and-swap. We show that constructions for uniprocessor real-time systems can be significantly simplified -- and the need for strong primitives eliminated -- by exploiting certain characteristics of real-time scheduling schemes.

Under lock-based schemes, a task can perform operations on many shared objects simultaneously via nested critical sections. For example, using nested critical sections, a task can atomically dequeue an element from one shared queue and enqueue that element in another shared queue. In order to achieve the level of functionality provided by nested critical sections, we provide a lock-free framework that is based on a multi-word compare-and-swap primitive and that supports multi-object accesses | the lock-free counterpart to nested critical sections. Because multi-word primitives are not provided in hardware, they have to be implemented in software. We provide a time-optimal implementation of the multi-word compare-and-swap primitive.

Finally, we present a formal comparison of the various object-sharing schemes based on scheduling conditions, followed by results from a set of simulation experiments that we conducted. Also, as empirical proof of the viability of lock-free objects in practical systems, we present results from a set of experiments conducted on a desktop videoconferencing system.

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Last revised Tue Jul 19 22:55:08 EDT 1998 by jeffay at cs.unc.edu.