A shared data structure is lock-free if its operations do not require mutual exclusion. If one process is interrupted in the middle of an operation, other processes will not be prevented from operating on that object. In highly concurrent systems, lock-free data structures avoid common problems associated with conventional locking techniques, including priority inversion, convoying, and difficulty of avoiding deadlock. This paper introduces transactional memory, a new multiprocessor architecture intended to make lock-free synchronization as efficient (and easy to use) as conventional techniques based on mutual exclusion. Transactional memory allows programmers to define customized read-modify-write operations that apply to multiple, independently-chosen words of memory. It is implemented by straightforward extensions to any multiprocessor cache-coherence protocol. Simulation results show that transactional memory matches or outperforms the best known locking techniques for simple benchmarks, even in the absence of priority inversion, convoying, and deadlock.

Serialization of threads due to critical sections is a fundamental bottleneck to achieving high performance in multithreaded programs. Dynamically, such serialization may be unnecessary because these critical sections could have safely executed concurrently without locks. Current processors cannot fully exploit such parallelism because they do not have mechanisms to dynamically detect such false inter-thread dependences. We propose Speculative Lock Elision (SLE), a novel micro-architectural technique to remove dynamically unnecessary lock-induced serialization and enable highly concurrent multithreaded execution. The key insight is that locks do not always have to be acquired for a correct execution. Synchronization instructions are predicted as being unnecessary and elided. This allows multiple threads to concurrently execute critical sections protected by the same lock. Misspeculation due to inter-thread data conflicts is detected using existing cache mechanisms and rollback is used for recovery. Successful speculative elision is validated and committed without acquiring the lock. SLE can be implemented entirely in microarchitecture without instruction set support and without system-level modifications, is transparent to programmers, and requires only trivial additional hardware support. SLE can provide programmers a fast path to writing correct high-performance multithreaded programs.

This paper is motivated by the difficulty in writing correct high-performance programs. Writing shared-memory multithreaded programs imposes a complex trade-off between programming ease and performance, largely due to subtleties in coordinating access to shared data. To ensure correctness programmers often rely on conservative locking at the expense of performance. The resulting serialization of threads is a performance bottleneck. Locks also interact poorly with thread scheduling and faults, resulting in poor system performance.

Non-blocking synchronization has significant advantages over blocking synchronization: however, it has not been used to a significant degree in practice. We designed and implemented a multiprocessor operating system kernel and run-time library for high-performance, reliability and modularity. We used nonblocking synchronization, not because it was an objective in itself, but because it became the approach of choice. It was an attractive approach because of the synergy between other structuring techniques we used to achieve our primary goals and the benefits of non-blocking synchronization. This paper

Most multiprocessors are multiprogrammed in order to achieve acceptable response time and to increase their uti-lization. Unfortunately, inopportune preemption may significantly degrade the performance of synchronized parallel applications. To address this problem, researchers have developed two principal strategies for concurrent, atomic update of shared data structures: (1) preemption-safe locking and (2) non-blocking (lock-free) algorithms. Preemption-safe locking requires kernel support. Non-blocking algorithms generally require a universal atomic primitive such as compare-and-swap orload-linked/store-conditional, and are widely regarded as inefficient. We evaluate the performance of preemption-safe lock-based and non-blocking implementations of important data structures—queues, stacks, heaps, and counters—including non-blocking and lock-based queue algorithms of our own, in micro-benchmarks and real applications on a 12-processor SGI Challenge multiprocessor. Our results indicate that our non-blocking queue consistently outperforms the best known alternatives, and that data-structure-specific non-blocking algorithms, which exist for queues, stacks, and counters, can work extremely well. Not only do they outperform preemption-safe lock-based algorithms on multiprogrammed machines, they also outperform ordinary locks on dedicated machines. At the same time, since general-purpose non-blocking techniques do not yet appear to be practical, preemption-safe locks remain the preferred alternative for complex data structures: they outperform

Concurrency management is a basic requirement for interprocess communication in any multitasking system. This usually takes the form of lock-based or other blocking algorithms. In real-time and/or time-sensitive systems, the less-predictable timing behavior of lock-based mechanisms and the additional task-execution dependency make synchronization undesirable. Recent research has provided non-blocking and wait-free algorithms for interprocess communication, particularly in the domain of single-writer, multiple-reader semantics, but these algorithms typically incur high costs in terms of computation or space complexity, or both. In this paper, we propose a general transformation mechanism that takes advantage of temporal characteristics of the system to reduce both time and space overheads of current single-writer, multiple-reader algorithms. We show a 17--66% execution time reduction along with a 14-- 70% memory space reduction when three wait-free algorithms are improved by applying our transformation. We present three new algorithms for wait-free, single-writer, multiple-reader communication along with detailed performance evaluation of nine algorithms under various experimental conditions.

Abstract. Lock-free shared data structures in the setting of distributed computing have received a fair amount of attention. Major motivations of lock-free data structures include increasing fault tolerance of a (possibly heterogeneous) system and getting rid of the problems associated with critical sections such as priority inversion and deadlock. For parallel computers with closely-coupled processors and shared memory, these issues are no longer major concerns. While many of the results are applicable especially when the model used is shared memory multiprocessors, no prior studies have considered improving the performance of a parallel implementation by way of lock-free programming. As a matter of fact, often times in practice lock free data structures in a distributed setting do not perform as well as those that use locks. As the data structures and algorithms for parallel computing are often drastically different from those in distributed computing, it is possible that lock-free programs perform better. In this paper we compare the similarity and difference of lock-free programming in both distributed and parallel computing environments and explore the possibility of adapting lock-free programming to parallel computing to improve performances. Lock-free programming also provides a new way of simulating PRAM and asynchronous PRAM algorithms on current parallel machines.

This thesis is motivated by the difficulty in writing correct high-performance programs. Writing shared-memory multithreaded programs imposes a complex trade-off between programming ease and performance, largely due to subtleties in coordinating access to shared data. To ensure correctness programmers often rely on conservative locking at the expense of performance. The resulting serialization of threads is a performance bottleneck. Locks also interact poorly with thread scheduling and faults, resulting in poor system performance.

moss @ cs.umass.edu A shared data structure is lock-free if its operations do not require mutual exclusion. If one process is interrupted in the middle of an operation, other processes will not be prevented from operating on that object. In highly concurrent systems, lock-free data structures avoid common problems associated with conventional locking techniques, including priority inversion, convoying, and difficulty of avoiding deadlock. This paper introduces transactional memory, a new multiprocessor architecture intended to make lock-free synchronization as efficient (and easy to use) as conventional techniques based on mutual exclusion. Transactional memory allows programmers to define customized read-modify-write operations that apply to multiple, independently-chosen words of memory. It is implemented by straightforward extensions to any multiprocessor cache-coherence protocol. Simulation results show that transactional memory matches or outperforms the best known locking techniques for simple benchmarks, even in the absence of priority inversion, convoying, and deadlock. 1