RelaxReplay: record and replay for relaxed-consistency multiprocessors

Abstract

Record and Deterministic Replay (RnR) of multithreaded programs on relaxed-consistency multiprocessors has been a long-standing problem. While there are designs that work for Total Store Ordering (TSO), finding a general solution that is able to record the access reordering allowed by any relaxed-consistency model has proved challenging. This paper presents the first complete solution for hard-ware-assisted memory race recording that works for any relaxed-consistency model of current processors. With the scheme, called RelaxReplay, we can build an RnR system for any relaxed-consistency model and coherence protocol. RelaxReplay's core innovation is a new way of capturing memory access reordering. Each memory instruction goes through a post-completion in-order counting step that detects any reordering, and efficiently records it. We evaluate RelaxReplay with simulations of an 8-core release-consistent multicore running SPLASH-2 programs. We observe that RelaxReplay induces negligible overhead during recording. In addition, the average size of the log produced is comparable to the log sizes reported for existing solutions, and still very small compared to the memory bandwidth of modern machines. Finally, deterministic replay is efficient and needs minimal hardware support.