WeeFence: toward making fences free in TSO

Yuelu Duan, A. Muzahid, J. Torrellas
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引用次数: 32

Abstract

Although fences are designed for low-overhead concurrency coordination, they can be expensive in current machines. If fences were largely free, faster fine-grained concurrent algorithms could be devised, and compilers could guarantee Sequential Consistency (SC) at little cost. In this paper, we present WeeFence (or WFence for short), a fence that is very cheap because it allows post-fence accesses to skip it. Such accesses can typically complete and retire before the pre-fence writes have drained from the write buffer. Only when an incorrect reordering of accesses is about to happen, does the hardware stall to prevent it. In the paper, we present the WFence design for TSO, and compare it to a conventional fence with speculation for 8-processor multicore simulations. We run parallel kernels that contain explicit fences and parallel applications that do not. For the kernels, WFence eliminates nearly all of the fence stall, reducing the kernels' execution time by an average of 11%. For the applications, a conservative compiler algorithm places fences in the code to guarantee SC. In this case, on average, WFences reduce the resulting fence overhead from 38% of the applications' execution time to 2% (in a centralized WFence design), or from 36% to 5% (in a distributed WFence design).
WeeFence:在TSO实现围栏免费
尽管篱笆是为低开销的并发性协调而设计的,但在当前的机器中,它们的成本可能很高。如果栅栏在很大程度上是免费的,就可以设计出更快的细粒度并发算法,并且编译器可以以很小的成本保证顺序一致性(sequence Consistency, SC)。在本文中,我们介绍了WeeFence(或简称WFence),这是一种非常便宜的围栏,因为它允许围栏后访问跳过它。这样的访问通常可以在pre-fence写入从写缓冲区中耗尽之前完成并退出。只有当不正确的访问重新排序即将发生时,硬件才会停止以防止它发生。在本文中,我们提出了用于TSO的WFence设计,并将其与传统的围栏进行了比较,并推测了8处理器多核仿真。我们运行包含显式围栏的并行内核和不包含围栏的并行应用程序。对于内核来说,WFence消除了几乎所有的fence停顿,将内核的执行时间平均减少了11%。对于应用程序,一个保守的编译器算法在代码中放置栅栏以保证SC。在这种情况下,平均而言,WFence将最终产生的栅栏开销从应用程序执行时间的38%减少到2%(在集中式WFence设计中),或从36%减少到5%(在分布式WFence设计中)。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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