Pacifier: Record and replay for relaxed-consistency multiprocessors with distributed directory protocol

Record and Deterministic Replay (R&R) of multithreaded programs on relaxed-consistency multiprocessors with distributed directory protocol has been a long-standing open problem. The independently developed RelaxReplay [8] solves the problem by assuming write atomicity. This paper proposes Pacifier, the first R&R scheme to provide a solution without assuming write atomicity. R&R for relaxed-consistency multiprocessors needs to detect, record and replay Sequential Consistency Violations (SCV). Pacifier has two key components: (i) Relog, a general memory reordering logging and replay mechanism that can reproduce SCVs in relaxed memory models, and (ii) Granule, an SCV detection scheme in the record phase with good precision, that indicates whether to record with Relog. We show that Pacifier is a sweet spot in the design space with a reasonable trade-off between hardware and log overhead. An evaluation with simulations of 16, 32 and 64 processors with Release Consistency (RC) running SPLASH-2 applications indicates that Pacifier incurs 3.9% ~ 16% larger logs. The slowdown of Pacifier during replay is 10.1% ~ 30.5% compared to native execution.