SlackenedMemoryDependenceEnforcementCombining放松记忆依赖执法相结合.ppt

6/20/2006 Slackened Memory Dependence Enforcement, Alok Garg, ISCA 2006 Slackened Memory Dependence Enforcement: Combining Opportunistic Forwarding with Decoupled Verification Alok Garg, M. W. Rashid, and Michael Huang Department of Electrical Computer Engineering University of Rochester Motivation Out-of-order execution needs efficient memory dependence enforcement logic Conventional approach – complex, hard to scale Tightly coupled forwarding and enforcement We use two decoupled components to simplify the task Opportunistic forwarding using L0 cache Verification against in-order re-execution Slackened memory dependence enforcement (SMDE) LSQ: complex hard to scale Needs priority CAMs Forwarding from LSQ on timing critical path Serialized with address translation Design further complicated by Coherence and consistency considerations Corner cases: e.g., partial overlap of operands Highlights of prior work Two-level load store queue [sethumadhavan03], [akkary03], [baugh04], [roth04], [torres05], [gandhi05] Reducing search frequency using clever filtering and prediction mechanism [park03], [sethumadhavan03] Memory dependence prediction [moshovos.isca97], [moshovos.micro97], [sha05], [stone05] Value based re-execution [cain04], [roth04], [sha05] (more detailed contrast in paper) Outline Overview of SMDE Optional performance optimizations Evaluation Conclusion Overview of SMDE Decoupled execution LSQ: competing requirements Front-end execution: little mem dependence enforcement Back-end execution: detect violations (mem access only) Memory B/W: naturally handled Why it works – two perspectives Back-end execution is the only one required Totally in-order, preserving dependence Any front-end execution is OK L0 effectively a slow but accurate value predictor Front-end execution correct most of the time Common case: 99% of loads happen at right time Speculation is on timing of load store pairs Two-level LSQ speculate on the scope of stores Relatively expensive repl