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Efficient Cache Coherency for Multicore Processors: The Scalable Commutativity Rule

This paper discusses a scalable approach to managing pages of varying sizes in a VM system using superpages to overcome TLB limitations. It introduces the MESI Coherency protocol with four cache line states and common-sense transitions. The text explores snoop-based cache behaviors and optimizations with store buffers and invalidation queues. It highlights the use of consistency requirements like store barriers and read barriers to ensure efficient cache coherence.

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Efficient Cache Coherency for Multicore Processors: The Scalable Commutativity Rule

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  1. The Scalable Commutativity Rule:Designing Scalable Software for Multicore Processors A. T. Clements, M. F. Kaashoek, N.Zeldovich,R. T. Morris, and E. Kohler† MIT CSAIL and †Harvard

  2. Paper Highlights • Presents an general efficient mechanism to manage pages of different sizes in a VM system • Superpages • Objective is to address the limitations of extant translation lookaside buffers (TLB).

  3. MESI Coherency protocol (I) • Write-through cache and memory consistency protocol • Name refers to four possible states for cache lines • Modified (locked and dirty) • Exclusive (locked and clean) • Shared (unlocked and clean) • Invalid • Common-sense transitions between states

  4. Snooping • Any cache that hold a line in the modified statemust intercept and service all attempted read accesses to that line from other caches • Any cache that holds a line in the shared state must act on invalidate or request-for-ownership broadcasts from other caches • A cache that holds a line in the Exclusive state must snoop all read requests to that line from all other caches, and move the line to shared state if needed

  5. Optimizations • Store buffers allow caches to write into invalid lines • Invalidation queues contain invalidation requests that have not yet been acted upon • Weaken consistency and require barriers • Store barriers flush the store buffers • Read barrier flush the invalidation queue

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